Revised Draft- Subject to Chance 

44 

APPENDIX A 

 FOLSOM DAM SAFETY AND FLOOD DAMAGE REDUCTION PROJECT 

 HABITAT EVALUATION PROCEDURES 

 FEBRUARY 2007 
Revised Draft- Subject to Chance 

45 

 INTRODUCTION 

The U.S. Army Corps of Engineers (Corps) and the U.S. Bureau of Reclamation (Reclamation) 

seek to significantly reduce the risk of flooding along the main stem of the American River in the 

Sacramento area while meeting dam safety and public safety objectives. The project is 

authorized by the Corps’ American River Watershed Investigation, Folsom Dam Modification 

project under section 101 (a) (6) of the Water Resources Development Act (WRDA) of 1999 and 

the Bureau’s Dam Safety Program (static, earthquake, etc) (Reclamation 2006). Modifications to 

the existing authorities were made in the Energy and Water Appropriations Act of 2006, which 

directed the Secretary of the Army and the Secretary of the Interior to collaborate on authorized 

activities to maximize flood damage reduction improvements and address dam safety needs at 

Folsom Dam and Reservoir as one Joint Federal Project. 

This application of Habitat Evaluation Procedures (HEP) is intended to provide a quantification 

of the impacts on fish and wildlife resources associated with Folsom Dam Safety and Flood 

Damage Reduction (Folsom DS/FDR). Any dam raise or spillway construction measure would 

be a major modification and would allow Folsom Dam to pass the probable maximum flood 

(PMF) volume without failure and meet Reclamation’s Dam Safety Program. 

PROJECT AREA 

The project area is in the American River watershed, and would affect lands around Folsom 

Reservoir, and along the North and South Forks of the American River, which are impounded by 

Folsom Dam (Figure 1 and Figure 2). The project could also directly affect the Mormon Island 

Preserve located just downstream of Mormon Island Auxiliary Dam (MIAD) and the lower 

American River--the river's reach downstream of Folsom Dam (Figure 3). 

The American River is the second largest tributary to the Sacramento River. The three forks 

(north, middle, and south) of the river originate in the Sierra Nevada Mountains at an elevation 

of about 10,400 feet (mean sea level), and generally flow in a southwesterly direction. The 

Middle Fork joins the North Fork near the City of Auburn, just upstream of Folsom Reservoir; 

the North Fork then joins the South Fork just upstream of Folsom Dam. All three forks of the 

American River above Folsom Reservoir are nationally popular areas for whitewater sports, and 

the reach of the South Fork from Coloma to the reservoir is the State's most popular whitewater 

rafting run. 
Folsom Dam Rd Folsom Lake

 Mormon Island Auxiliary Dam and Morman Island Preserve Prepared by the US Fish and Wildlife Service, Sacreamento Fish andWildlife Office, Flood and Waterway Planning Branch; Sept. 18, 2006 This map is for illustrative purposes only. The US Fish and Wildlife Service shall not be held liable for improper or incorrect use of the data described and /or contained herein. Figure 3

Momon Island Auxiliary Dam

Momon Island Preserve

Green Val

Empire Ranch Golf Course

! !!!

Draft- Subject to Change 48
Revised Draft- Subject to Change 49

Folsom Dam, located near the city of Folsom, is a multi-purpose dam built by the Corps in 1955, 

and operated by Reclamation. It is the largest of about 20 dams in the American River watershed 

and, except for Nimbus Dam, is the furthest downstream. Five reservoirs in the upper American 

River watershed (Loon Lake, Ice House, Union Valley, French Meadows, and Hell Hole) 

represent 90% of the existing storage capacity upstream of Folsom Reservoir. 

The main dam is a 345-foot high concrete gravity dam across the American River channel. 

Associated with Folsom Dam is a series of auxiliary dams and dikes which span topographic 

lows; these structures are needed to contain the reservoir. Mormon Island Dam is the largest of 

these structures, and is located on the southeast end of the reservoir. Folsom Reservoir blocks 

about 20 miles of the North Fork and 10 miles of the South Fork, and has a total storage capacity 

of 974,000 acre-feet, which fills the reservoir to an elevation of 466 feet above mean sea level 

(msl). 

Reclamation operates Folsom Dam as an integrated component of the Central Valley Project. 

The dam's primary purposes have been to: provide flood control; provide instream flows; 

manage Sacramento-San Joaquin Delta water quality; produce hydropower; provide recreation; 

and more recently, protection and restoration of the region’s fish and wildlife resources. 

PROJECT DESCRIPTION 

The Folsom DS/FDR project includes measures to remedy dam safety issues associated with 

seismic, static, and hydrologic concerns, and to provide increased flood damage protection. 

These measures include several different options to remedy the various issues at the Folsom 

facilities. The Folsom Facilities to be addressed by one or more of the engineering options 

include the main concrete dam, the right and left wing dams, Mormon Island Auxiliary Dam 

(MIAD), and eight dikes (1 through 8). The concrete dam and earthen wing dams serve to 

impound water associated with the main stem of the American River. MIAD serves to dam 

water within an historic river channel, while the earthen dikes serve to contain water at low spots 

in the topography during periods when the reservoir is full or nearly full. 

The improvements would be designed so that they could be constructed and operated without 

affecting ongoing water conservation and hydropower operations. The plan would maintain the 

current Folsom Dam design flood control release of 115,000 cubic feet per second (cfs) and an 

emergency release of 160,000 cfs. Four scales of enlargement alternatives were developed using 

maximum flood control pool elevations of 468, 486.5, 489.5 and 499.5 feet msl. 

Several constraints were imposed on plan formulation for Folsom DS/FDR project, these are: 

o dam raise measures are solely for flood control as stipulated in section 566 of WRDA 

1999; 
Revised Draft- Subject to Change 50

o dam raise measures are to avoid disruptions to the normal operation of Folsom Dam for 

water supply, hydropower, and flood control; 

o no loss of flood protection from existing flood damage reduction projects is permitted; 

o minimize disturbance of habitat for threatened and endangered species. 

The no action alternative serves as the base against which the proposed flood protection and 

Dam Safety alternatives will be evaluated to determine effectiveness and to identify effects that 

would result from them. Several actions that are currently authorized are expected to be 

completed prior to implementation of any Folsom DS/FDR project. Therefore, the effects and 

benefits associated with these actions are part of the no-action condition. See the accompanying 

Fish and Wildlife Coordination Act report for a complete description of the no action condition. 

A complete project description can be seen in the March 2007 Folsom DS/FDR FEIR/EIR. 

Alternative 1 – No Dam Raise/Minimal Embankment Raise, Fuseplug Spillway 

Under Alternative 1, there would be no raise to the concrete structure with minimal 

modifications to the existing spillway. A large auxiliary spillway would be constructed adjacent 

to the left wing dam to address hydrologic and flood control concerns. Some of the earthen 

structures would be raised to address hydrologic concerns, but not to increase the flood storage 

capacity of the reservoir since this alternative is a Dam Safety only alternative. 

Alternative 2 – 4-foot Dam and Embankment Raise 

Alternative 2 incorporates a 4-foot dam raise with a fuseplug auxiliary spillway and gatecontrolled tunnel spillway for better hydrologic control of large flood events. Under this 

alternative, there could be a 4-foot raise to the concrete structure with some modifications to the 

existing spillway gates. An auxiliary spillway with a chute or a tunnel would be constructed to 

address hydrologic and flood control concerns. All of the earthen structures could be raised to 

address hydrologic concerns and to provide additional flood storage capacity. 

Alternative 3; Preferred Alternative- Joint Auxiliary Spillway, 3.5-foot Parapet Wall Raise 

Under the Preferred Alternative a smaller six-submerged tainter gate (six gate) auxiliary spillway 

would be constructed to address both Dam Safety and Flood Damage Reduction objectives 

including hydrologic and flood control concerns. Construction of the six gate auxiliary spillway 

would increase project discharge capacity. The 3.5-foot raise, in conjunction with modification 

and/or replacement of the three emergency spillway gates and the six-gate auxiliary spillway, 

would only serve as additional freeboard for the Folsom facilities. Once construction is 

completed the raise would not exceed the existing take line for a 200-year design event and there 

would be an anticipated lower maximum water surface elevation. The 3.5-foot raise, 

modification and/or replacement of the three emergency spillway gates and the six-gate auxiliary 

spillway, have been identified by the Corps as their Selected Plan within the Corps’ Post 

Authorization Change report. The remaining elements of Alternative 3 are Dam Safety 

Modification as revised above. 
Revised Draft- Subject to Change 51

A tentative schedule showing the sequencing of construction for the preferred alternative is 

shown in Table 1. 

Table 1 

 Folsom DS/FDR Project Phase Sequencing

Activity 

ID 

Folsom Facility Construction Period 

1 Auxiliary Spillway Excavation Phase 1 September 2007 to March 2009 

2 Right and Left Wing Dam Static Modifications February 2008 to March 2009 

3 Mormon Island Jet Grouting July 2008 to December 2009 

4 Auxiliary Spillway Excavation Phase 2 September 2010 to January 

2014 

5 Dike 5 Static Modifications September 2009 to May 2010 

6 Mormon Island Seismic Overlay June 2015 to April 2017 

7 Dike 4 and 6 Static Modifications September 2017 to April 2018 

8a Pier Tendon Installation at Main Dam January 2014 to March 2015 

8b Spillway Pier Wraps & Braces August 2016 to April 2018 

8c Spillway Gate Repairs January 2018 to August 2020 

9 Auxiliary Spillway Approach Channel Excavation 

and Gate Structure Construction 

September 2011 to December 

2014 

10 Raise of all Folsom Facilities September 2018 to September 

2019 

Alternative 4 – 7-foot Dam and Embankment Raise 

Alternative 4 contains many of the same elements as Alternative 3 with the exception of a 7-foot 

raise that could result in increased reservoir flood storage during large flood events. Under this 

alternative all Folsom Facilities and earthen structures would be raised 7 feet. A smaller foursubmerged tainter gate (four gate) auxiliary spillway would be constructed to address hydrologic 

and flood control concerns. 

Alternative 5 – 17-foot Dam and Embankment Raise 

Alternative 5 was specifically developed as an alternative that would address both Dam Safety 

and Flood Damage Reduction requirements without the construction of an auxiliary spillway. 

Under this alternative all Folsom Facilities could be raised 17 feet which would increase 

reservoir storage capacity to control large flood events. 
Revised Draft- Subject to Change 52

METHODOLOGY 

HEP is a methodology developed by the Fish and Wildlife Service (Service) and other State and 

Federal resource and water development agencies which can be used to document the quality and 

quantity of available habitat for selected fish and wildlife species. HEP provides information for 

two general types of habitat comparisons: (1) the relative value of different areas at the same 

point in time; and (2) the relative value of the same areas at future points in time. By combining 

the two types of comparisons, the impacts of proposed or anticipated land-use and water-use 

changes on habitat can be quantified. In a similar manner, any mitigation needs (in terms of 

acreage) for the project can also be quantified, provided a mitigation plan has been developed for 

specific alternative mitigation sites. 

A HEP application is based on the assumption that the value of a habitat for selected species or 

the value of a community can be described in a model which produces a Habitat Suitability Index 

(HSI). This HSI value (from 0.0 to 1.0) is multiplied by the area of available habitat to obtain 

Habitat Units (HUs). The HUs and Average Annual Habitat Units (AAHUs) over the life of the 

project are then used in the comparisons described above. 

The reliability of a HEP application and the significance of HUs are directly dependent on the 

ability of the user to assign a well-defined and accurate HSI to the selected evaluation elements 

or communities. Also, a user must be able to identify and measure the area of each distinct 

habitat being utilized by fish and wildlife species within the project area. Both the HSIs and the 

habitat acreage must also be reasonably estimable at various future points in time. The HEP 

team, comprised of Corps, Reclamation and Service staff, determined that these HEP criteria 

could be met, or at least reasonably approximated, for the Folsom DS/FRD project. Thus HEP 

was considered an appropriate analytical tool to analyze impacts of the proposed project 

alternatives1

. Further the HEP team determined that HSI values for habitats impacted by the 

Folsom DS/FRD project would be taken from the American River Watershed Investigation, 

Folsom Bridge (Bridge) project, the American River Watershed Investigation Long-Term 

Evaluation (Long-Term) and the American River Watershed Investigation Folsom Dam 

Modification (MODS) project. HSI values for oak/grey pine woodland and seasonal wetland 

habitats were used from the data collected in Reach 1 and riparian woodland habitat HSI values 

were used from data collected in Reach 3 in 2005, from the Bridge project. Chaparral HSI 

values were taken from Long-Term data, collected in 2000 for the inundation impacts and the 

direct impacts for chaparral HSI values were taken from MODS data, collected in 2004, for the 

staging, borrow and construction use areas. 

GENERAL HEP ASSUMPTIONS

Some general assumptions are necessary to use HEP and Habitat Suitability Index (HSI) Models 

in the impact assessment: 

 

1

 For further information on HEP see ESM 100-104 which is available from the Service’s Sacramento Fish and Wildlife Office. 
Revised Draft- Subject to Change 53

Use of HEP:

1. HEP is the preferred method to evaluate the impacts of the proposed project on fish 

and/or wildlife resources. 

2. HEP is a suitable methodology for quantifying project-induced impacts to fish and 

wildlife habitats. 

3. Quality and quantity of fish and wildlife habitat can generally be numerically described 

using the indices derived from the HSI models and associated habitat units. 

4. The HEP assessment is applicable to the habitat types being evaluated. 

Use of HSI Models

5. HSI models are hypotheses based on available data. 

6. HSI models are conceptual models and may not measure all ecological factors that affect 

the quality of a given cover-type for the evaluation species (e.g. vulnerability to 

predation). In some cases, assumptions may need to be made by the HEP Team and 

incorporated into the analysis to account for loss of those factors not reflected by the 

model. 

The additional HEP field work for the project was completed by staff from the Service’s 

Sacramento Fish and Wildlife Office, the Corps (Sacramento District) and Reclamation and 

occurred during May 2006 and included vegetation mapping around the Folsom Reservoir. Six 

cover-types would be permanently impacted by the project including oak woodland, oak 

savannah, blue oak/grey pine woodland, riparian woodland, seasonal wetland, annual grassland 

and other2. These cover-types were mapped by the HEP Team on aerial photographs in the field 

then digitized into ArcGIS. Using the project footprint supplied by Reclamation and the Corps 

acreages were quantified using GIS. The cover-types and acreage affected by the proposed work 

is summarized in Table 2 and Table 3. 

 

2. “Other” encompasses those areas which do not fall within the other cover-types such as gravel and paved roads, parking areas, buildings, bare 

ground, riprap, etc. 
Revised Draft- Subject to Change 54

Table 2. Summary of Cover-Types, Acres Impacted, and Compensation Recommended for the 

Alternatives Compared to the Preferred Alternative for the Construction of the 

Folsom DS/FRD Project, California. 

Folsom DS/FRD Project 

Alternative 3 (Preferred) 1 2 4 5 

Cover-Type Impacted Acres: 

Compensation 

Needed 

Difference from 

the Preferred 

Alternative 

Impacted Acres 

Difference from 

the Preferred 

Alternative 

Impacted Acres 

Difference from 

the Preferred 

Alternative 

Impacted Acres 

Difference from 

the Preferred 

Alternative 

Impacted Acres 

Oak/grey 

pine 

woodland 

52.4 : 64.5 0.39 0.39 0.70 -1.07 

Riparian 

woodland 42.7 : 48.0 -0.28 -0.62 -0.15 -1.66 

Chaparral 0.7 : 0.8 0 0 0 -0.21 

Seasonal 

wetland 1.2 : 4.7 0 0 0 0 

Total 97.0 : 117.9 

Table 3. Preliminary Summary of Cover-Types, Impacted Acres and Compensation 

Recommended for the Inundation and Construction at Dikes 1-3 of the Folsom 

Reservoir for the Folsom Dam Raise Alternatives 3.5, 4.0, 7.0, or 17 feet as part of 

the Folsom DS/FDR Project, California. 

Folsom Dam Raise Alternatives 

 3.5-ft Raise 4-ft Raise 7-ft Raise 17-ft Raise 

 (Preferred) 

Cover Type Impacted Acres: 

Compensation Needed 

Impacted Acres: 

Compensation Needed 

Impacted Acres: 

Compensation Needed 

Impacted Acres: 

Compensation Needed 

Oak/Grey Pine 

woodland 781.5 : 939.4 820.2 : 985.8 935.1 : 1,123.8 1,331.8 : 1,600.1 

Riparian 

woodland* 45.47 : 0.02 48.68 : 0.02 56.5 : 0.02 48.68 : 0.02 

Chaparral 32.2 : 34.1 34.3 : 36.3 40.8 : 43.2 34.3 : 36.3 

Seasonal 

wetland* 0.58 : 0.0 0.58 : 0.0 0.58 : 0.0 0.58 : 0.0 

Total 859.8 : 973.5 903.8 : 995.12 1,033 : 1,167 1,415.4 : 1,636.4 

*No permanent impacts to riparian woodland and seasonal wetland are expected from the short inundation that would occur 

from a raise component of the Folsom DS/FDR project. Acres shown are from the construction at Dikes 1-3. 

Eleven HSI models were used in this HEP application to quantify project impacts. A summary 

of the models applied for each cover-type is also included in Table 4. The western gray squirrel 

and plain titmouse models were selected to evaluate the oak woodland, and oak/grey pine 

woodland cover-types. These species were chosen because they utilize this cover-type for 
Revised Draft- Subject to Change 55

Table 4. HEP Cover-types, proposed HSI models, and model variables for the Folsom DS/FDR 

Project, California. 

COVER-TYPE PROPOSED HSI MODELSHSI MODEL VARIABLES

Western gray 

squirrel 

V1 - Canopy closure of mast-producing species>5m tall 

V2 - Density of leaf litter layer 

V3 - Tree canopy cover 

V4 - Den site availability per acre 

(1) Oak 

woodland 

Plain titmouse V1 - Tree diameter 

V2 - Trees per acre 

V3 - % composition of tree species that are oaks 

Yellow warbler V1 - % deciduous shrub crown cover 

V2 – Average height of deciduous shrub canopy 

V3 - % deciduous shrub canopy comprised of hydrophytic shrubs 

Northern oriole V1 - Average height of deciduous tree shrub 

V2 – % deciduous tree crown cover 

V3 – Stand width 

(2) Riparian 

woodland 

Western fence 

lizard 

V1 - % ground cover 

V2 – Average size of ground cover objects 

V3 – Structural diversity/interspersion 

V4 - % canopy cover 

Great egret 

(feeding) 

V1 - Percentage of area with water 10-23 cm deep 

V2 - Percentage of submerged or emergent vegetation cover in zone 10-23 cm deep 

California vole V1 - Height of herbaceous vegetation 

V2 - Percent cover of herbaceous vegetation 

V3 - Soil type 

V4 - Presence of logs and other types of cover 

(3) Seasonal 

wetlands 

Red-winged 

blackbird 

V1 - Predominance of narrow or broadleaf monocots 

V2 - Water presence throughout the year 

V3 - Presence or absence of carp 

V4 - Presence or absence of damselflies or dragonflies 

V5 - Mix of herbaceous vegetation 

V6 - Suitability of foraging substrate 

Bobcat V1 - % shrub cover 

V2 - % herbaceous cover 

V3 – degree of patchiness 

V4 – rock outcroppings 

Wrentit V1 - % shrub cover 

V2 - % shrub cover <5 feet 

(4) Chaparral 

California thrasher V1 – Presence of low shrub openings 

V2 – Shrub/seedling cover 

(5) Annual 

grassland 

No HEP proposed; disturbed areas will be reseeded after construction is complete.
Revised Draft- Subject to Change 56

nesting and foraging. The western fence lizard, yellow warbler, and northern oriole models were 

chosen to evaluate the project impacts to the riparian woodland cover-type. These species were 

selected because the bird species utilize the riparian tree canopy provided by the cover-type for 

nesting and foraging. For analysis purposes these two cover types were treated as one because 

the same models were chosen by the HEP Team. The western fence lizard utilizes the ground 

component of the cover-type including rocks boulders, and downed wood for shelter and 

foraging. 

The red-winged blackbird, great egret (feeding) and California vole models were selected for 

evaluating impacts to the seasonal wetland cover-type because these species forage, nest, or 

inhabit this cover-type. 

The bobcat, wrentit and California thrasher models were selected for evaluating impacts to the 

chaparral cover-type because these species forage, nest, or inhabit this cover-type. 

The annual grassland and “other” cover-types were not included in the HEP analysis because 

they do not currently provide significant habitat for wildlife species or the conditions (habitat 

values) after the completion of work are expected to be similar to pre-project conditions. 

The cover-type designations and HSI models were also selected in part to be consistent with 

previous impact analyses completed for the American River Watershed Investigation Folsom 

Dam Modification project which is occurring concurrently with the Folsom Bridge project. 

More information on the HEP for those projects can be found in the Service’s Fish and Wildlife 

Coordination Act Report for those projects. 

RESULTS AND DISCUSSION 

This HEP analyzed the potential impacts of the proposed Folsom DS/FDR project. Impact areas 

were divided into five components to facilitate possible design changes and subsequent impact 

analyses as the planning process proceeds toward selection of a construction alternative. The 

components are: (1) the construction footprint of the spillway alternatives; (2) impacts 

associated with Safety of Dams construction at dikes 4 thru 8, both wing dams, and MIAD; (3) 

impacts from borrow and stockpile; (4) impacts associated with the Flood Damage Reduction 

construction as dikes 1 thru 3; and (5) the potential impacts to vegetation in the new reservoir 

inundation zone. 

The HEP does not address potential impacts to aquatic resources at Folsom Reservoir during 

construction, nor are potential lower American River fishery impacts addressed for the 

construction period or subsequent reservoir operation. 
Revised Draft- Subject to Change 57

Construction Impacts 

The impacts and mitigation recommended for the Preferred Alternative for the Folsom DS/FDR 

project is summarized in Table 5. A specific compensation site was not analyzed in this HEP 

application. Instead a typical site was developed, and assumptions were made that the site would 

be an annual grassland area without existing woody vegetation for a baseline condition. For the 

riparian and seasonal wetland cover-types, a critical assumption was made that any site selected 

for compensation would require the appropriate hydrology to support these cover-types. 

Folsom Reservoir Inundation

Between 811.74 and 1,323.35 acres could be affected by enlarging Folsom Dam, depending on 

which dam raise alternative is selected. Some of these lands are already developed or otherwise 

disturbed habitat which provides little or no value for wildlife species, and some support 

vegetation that is tolerant of flooding. Table 5 summarizes the acreages of each habitat which 

provides value for wildlife and is expected to receive inundation over the life of the project. 

Inundation effects around Folsom Reservoir would occur in large part by the frequency, timing, 

and duration of flooding. Studies to date indicate that predicting the effects of inundation on 

vegetation is not straightforward. The raising of Folsom Dam would have potential for at least 

two significant impacts on vegetation: (1) changes in vegetation composition caused by 

inundation affecting survival and reproduction of vegetation within the zone between current and 

proposed maximum reservoir levels; and (2) effects of inundation on soil erosion and slippage, 

especially on steep slopes as are found along the upper reservoir and the forks of the American 

River. 

The vegetation types exposed to flooding are not, in general, highly tolerant of flooding. With 

the exception of riparian and riverine habitats, natural flooding does not occur in the areas which 

would be flooded by raising Folsom Dam. Studies of the effects of inundation on blue oaks 

(1975 in USFWS 1980; MWA-JSA 1994) have found that blue oaks can survive some flooding, 

but may be sensitive to periods of inundation of as little as 7 days. It is not clear from these 

studies, however, at what time of year flooding occurred, and the ability of vegetation to tolerate 

inundation depends on the time of year. For example, deciduous trees, such as oaks, tend to be 

much more sensitive to flooding during their period of active growth (i.e., in the spring), while 

winter-dormant plants appear to be more tolerant of flooding (USFWS 1980). Folsom Reservoir 

can reasonably be expected to fill during a major spring flood event, when oaks are actively 

growing. The absence of blue oaks within the current inundation zone of Folsom Reservoir and 

other foothill impoundments indicate that blue oaks cannot tolerate the flooding regime existing 

there. Further, evergreen species, including grey pines and live oaks, occur commonly around 

the reservoir, and tend to be more sensitive to inundation than deciduous trees such as blue oaks 

(MWA-JSA 1994). 

The other factor which could affect vegetation is erosion of the saturated soil in the new 

inundation area during a flood event from the water being drawn down or wind driven wave 

wash during a major storm event. Slopes in the Folsom Reservoir area are generally between 5 

and 25% (USACE 2001). Slopes in the Mooney Ridge area in the northwestern corner of the 
Revised Draft- Subject to Change 58

Table 5. Alternative 3, Preferred- Summary of Cover-Types, Acres Impacted, Net Change 

in Average Annual Habitat Units With- and Without-Project, and Compensation Recommended 

for the Direct Impacts and Inundation Impacts of Construction and Raise of the Folsom DS/FDR 

Project, California.

Folsom Dam 

Auxiliary Spillway and Dike Construction

Cover-Type Acres 

Impacted 

AAHUs 

W/O Project 

AAHUs 

W/ Project 

Net Change 

in AAHUs 

Compensation 

Needed 

Construction, Haul 

Rds, Borrow &

Stockpile 

Oak - grey pine 

woodland 

Riparian woodland 

Seasonal wetland 

Chaparral 

35.29 

39.08 

0.89 

0.26 

0.07 

0.13 

0.00 

0.04 

16.23 

30.09 

0.18 

0.15 

-16.16 

-19.96 

-0.18 

-0.10 

42.37 

43.88 

3.56 

0.27 

Dikes 4-8, 

Wing Dams 

& MIAD 

Oak - grey pine 

woodland 

Riparian woodland 

Seasonal wetland 

Chaparral 

16.04 

1.93 

0.28 

0.26 

7.38 

1.49 

0.06 

0.15 

0.04 

0.01 

0.00 

0.04 

-7.34 

-1.48 

-0.06 

-0.10 

20.75 

2.19 

1.12 

0.28 

Spillway 

(Six-Gate) 

Oak - grey pine 

woodland 

Riparian woodland 

Seasonal wetland 

Chaparral 

1.07 

1.66 

0 

0.21 

0.49 

1.28 

0 

0.12 

0.00 

0.01 

0 

0.03 

-0.49 

-1.27 

0 

-0.08 

1.38 

1.88 

0 

0.22 

Raise- 0 feet 

(Inundation) 

Oak - grey pine 

woodland 

Riparian woodland 

Seasonal wetland 

Chaparral 

773.08 

45.45 

0.58 

32.22 

355.62 

35.00 

0.12 

23.20 

1.57 

35.00 

0.12 

5.24 

-354.04 

0.00 

0.00 

-17.96 

928.23 

0 

0 

34.08 

3Dikes 1-3 

Raise 

Oak - grey pine 

woodland 

Riparian woodland 

Seasonal wetland 

Chaparral 

8.46 

0.02 

0 

0 

3.89 

0.02 

0 

0 

0.02 

0.54 

0 

0 

-3.87 

-0.02 

0 

0 

11.16 

0.02 

0 

0 

 

3

 Construction at Dike 1-3 is dependent on the implementation of the raise component of the Folsom DS/FDR 

project. Impact acres for this component are preliminary in this document. 
Revised Draft- Subject to Change 59

reservoir and the shoreline just west of the South Fork of the American River exceed 30% 

(USACE 2001). It is likely that during a major flood event some, or all, of the soil on steep 

slopes would experience some erosion. The extent of erosion and its effect on vegetation would 

be difficult to predict. 

Assuming a worst case scenario that over the life of the project all of the existing vegetation 

(except riparian and seasonal wetlands) in the inundation zone would be lost, a mitigation need 

was developed for each cover-type using the HEP results. Statistically, there is a relatively small 

chance of complete inundation coupled with total loss of vegetation. However, it is reasonable 

to expect some impacts, especially at the lower zones due to the potential for more frequent 

inundation, over the life of the project. 

Given the uncertainties on effects of inundation on vegetation and soil erosion, the HEP Team 

decided to recommend that a monitoring and adaptive management program be developed to 

monitor vegetation around the reservoir over the life of the project. Baseline conditions would 

be managed and updated at intervals (10 years). After major flood events (those which encroach 

above the existing maximum flood pool elevation), vegetation would be surveyed and damages 

attributable to inundation would be mitigated as deemed appropriate using the best management 

practices at the time (replanting on site would be the first priority). 
Revised Draft- Subject to Change 

60 

DATA ANALYSIS AND ASSUMPTIONS 

FOLSOM BRIDGE PROJECT

REACH 1 EAST NATOMA STREET TO PARKING LOT NEAR SOUTH END OF DAM 

PA 1 - Future Without Project (Impact Area)

OAK WOODLAND

WESTERN GRAY SQUIRREL

TY 0 - Baseline (measured) 

 V1 - % canopy closure of trees and shrubs that produce hard mast (65%) 

 V2 - Density of leaf litter layer (M) 

 V3 - % tree cover (61%) 

 V4 - Den site availability (53) 

 HSI Food = (V1 x V2)1⁄2

 HSI Cover/Reproduction = (V3 x V4)1⁄2

 

 HSI = 0.46 (lowest of values) 

TY 1 V1 - no change from TY 0 

 V2 - no change from TY 0 

 V3 - no change from TY 0 

 V4 - no change from TY 0 

 HSI = 0.46 

TY 60 V1 - no change from TY 1 

 V2 - no change from TY 1 

 V3 - no change from TY 1 

 V4 - no change from TY 1 

 HSI = 0.46 

PLAIN TITMOUSE

TY 0 - Baseline (measured) 

 V1 - dbh 

 V2 - Number trees/acre 

 V3 - % trees that are oaks 

 HSI = V1 +V2 + V3

 3 

 HSI = 0 .65 

TY 1 V1 - no change from TY 0 

 V2 - no change from TY 0 

 V3 - no change from TY 0 

 HSI = 0.65 
Revised Draft- Subject to Change 61

TY 60 V1 - no change from TY 0 

 V2 - no change from TY 0 

 V3 - no change from TY 0 

 HSI = 0.65 

PA 2 - Future With Project (Impact Area)

Assume: 1. All vegetation removed from temporary and permanent impact zones in year 1 

 2. temporary easement areas will not be replanted with woody vegetation 

WESTERN GRAY SQUIRREL

TY 0 - Baseline (measured) HSI = 0.46 

TY 1 - V1 - no trees SI = 0 

 V2 - low leaf litter SI = 0.2 

 V3 - no trees SI = 0 

 V4 - no den sites SI = 0 

 HSI Food = (V1 x V2)1⁄2 HSI Cover/Reproduction = (V3 x V4)1⁄2

 = (0 x 0.2)1⁄2

 = (0 x 0)1⁄2

 = 0 = 0 

 HSI = 0 

TY 60- V1 - no change from TY 1 

 V2 - no change from TY 1 

 V3 - no change from TY 1 

 V4 - no change from TY 1 

 HSI = 0 

TY 100 no change from TY60 

PLAIN TITMOUSE

TY 0 - Baseline (measured) HSI = 0.65 

TY 1 - V1 - no trees SI = 0.2 

 V2 - no trees SI = 0 

 V3 - no trees SI = 0 

HSI = V1+ V2 + V3 = 0.2 = 0.06 

 3 3 

TY 60 - V1 - no change from TY 1 

 V2 - no change from TY 1 

 V3 - no change from TY 1 

 HSI = .06 

TY 100 – no change from TY60 
Revised Draft- Subject to Change 62

MP 1 - Management Area - Future Without Project (Compensation Site)

Assume: 1. Annual grassland area selected for conversion to oak woodland. 

WESTERN GRAY SQUIRREL

TY 0 - Baseline (estimated) 

V1 - % canopy closure of trees and shrubs that produce hard mast (no trees) SI = 0 

 V2 - Density of leaf litter (low) SI = 0.2 

 V3 - Den site availability (no trees) SI = 0

 HSI Food = (V1 x V2)1⁄2 HSI Cover/Reproduction = (V3 x V4)1⁄2

 = (0 x 0.2)1⁄2 = (0 x 0)1⁄2

 = 0 = 0 

 HSI = 0 

TY 1 - V1 - no change from TY 0 

 V2 - no change from TY 0 

 V3 - no change from TY 0 

 V4 - no change from TY 0 

 HSI = 0 

TY 15 - no change from TY 1 HSI = 0 

TY 60 - no change from TY 15 

TY 100- no change from TY TY60 

PLAIN TITMOUSE

TY 0 - Baseline (estimated) 

 V1 - dbh (0) SI = 0.2 

 V2 - Number trees/acre (0) SI = 0 

 V3 - % trees that are oaks (0) SI = 0 

 HSI = V1 +V2 +V3 = 0.2 + 0 + 0 = .06 

 3 3 

TY 1 - V1 - no change from TY 0 

 V2 - no change from TY 0 

 V3 - no change from TY 0 

 HSI = .06 

TY 15 - no change from TY 1 HSI = .06 

TY 60 - no change from TY 15 HSI = .06 

TY 100- no change from TY 60 
Revised Draft- Subject to Change 63

MP 2 - Management Area - Future With Project (Compensation Site)

Assume: 

1. Acquire lands (currently annual grasslands) 

2. Annual grassland area prepared for planting in TY 1 , provide access and maintenance roads 

3. Plant 100% blue and live oak trees (4"x4"x14" tree pots) at a density of 400 trees/acre and 

cover crop 

4. Moderate management intensity (assume 1.5 inches dbh after 10 yrs; 90 percent survival). 

5. Watering, weed, pest control for minimum of 3 years and remedial actions as necessary to ensure plant 

establishment. 

6. Assume maximum growth rate of 12"/year 

7. Develop O&M manual 

8. TY 51 values equal values measured for impact zone 

WESTERN GRAY SQUIRREL

TY 0 - Baseline (estimated) HSI = 0 

TY 1 - V1 - tree species planted /no mast SI = 0 

 V2 - low SI = 0.2 

 V3 - 0 (no trees) SI = 0 

 V4 - 0 (no trees) SI = 0 

 HSI = 0 

TY 15 - V1 - oak trees reach 16ft. high 8% SI = 0.15 

 V2 - low SI = 0.2 

 V3 - 8% SI = 0.15 

 V4 - 0 SI = 0 

 HSI Food = (V1 x V2)1⁄2

 HSI Cover/Reproduction = (V3 x V4)1⁄2

 = (0.15 x 0.2)1⁄2

 = (0.15 x 0)1⁄2

 = .17 = 0 

 HSI = 0 

TY60 V1 - 40% SI = 0.8 

 V2 - medium SI = 0.8 

 V3 - 53% SI = 1.0 

 V4 - 24/ac SI = 1.0 

 HSI Food = (V1 x V2)1⁄2

 HSI Cover/Reproduction = (V3 x V4)1⁄2

 = (0.8 x 0.2)1⁄2 = (1.0 x 1.0)1⁄2

 = 0.40 = 1.0 

 HSI = 0.40 

TY 100 V1 - 60% SI = 1.0 

 V2 - high SI = 1.0 

 V3 - 53% SI = 1.0 

 V4 - 24/ac SI = 1.0 

 HSI Food = (V1 x V2)1⁄2 HSI Cover/Reproduction = (V3 x V4)1⁄2

 = (1.0 x 1.0)1⁄2

 = (1.0 x 1.0)1⁄2

 = 1.0 = 1.0 

 HSI = 1.0 
Revised Draft- Subject to Change 64

PLAIN TITMOUSE

TY 0 - Baseline (estimated) 

 HSI = .06 

TY 1 - V1 - tree species planted (oak) (0 dbh) SI = 0.2 

 V2 - 400 (100% < 16 ft tall; no trees) SI = 0 

 V3 - 100% (no trees) SI = 0 

 HSI = V1 + V2 + V3 = 0.2 + 0 + 0 = 0 .06 

 3 3 

TY 15 - V1 - oak trees reach 16 ft. high (dbh = 1.75) SI = 0.2 

 V2 - > 100 tree/ac SI = 1.0 

 V3 - 100% SI = 1.0 

 HSI = 0.2 + 1.0 + 1.0 = 0.73 

 3 

TY 60 - V1 - 13 dbh SI = 0.6 

 V2 - > 100 tree/ac SI = 1.0 

 V3 - 100% SI = 1.0 

 HSI = 0.6 + 1.0 + 1.0 = 0 .86 

 3 

TY 100- no change from TY60 
Revised Draft- Subject to Change 65

PA 1 - Future Without Project (Impact Area)

SEASONAL WETLAND

GREAT EGRET 

TY 0 – Baseline (measured) 

 V1 - % area with water 4-9 inches deep 

 V2 - % of substrate in zone 4-9 inches deep with sub- and emergent vegetation 

 

 HSI = V1 + V2 = 0.23 

 2 

TY 1 – no change from baseline HSI = 0.23 

TY 60 – no change from baseline HSI = 0.23 

TY 100- no change from baseline 

 

RED-WINGED BLACKBIRD 

TY 0 – Baseline (measured) 

 V6 quality of foraging areas within 620 feet of suitable nest areas 

 Condition C wetland HSI = (0.1 x V6)1⁄2

 = 0.2 

TY 1 – no change from baseline HSI = 0.2 

TY 60 – no change from baseline HSI = 0.2 

TY 100 – no change from baseline 

CALIFORNIA VOLE 

TY 0 – Baseline (measured) 

 V1 – Height herbaceous vegetation 

 V2 - % herbaceous cover 

 V3 – Soil type 

 HSI = V1 + V2 + V3 = 0.76 

 3 

TY 1 – no change from baseline HSI = 0.76 

TY 60 – no change from baseline HSI = 0.76 

TY 100- no change from baseline 
Revised Draft- Subject to Change 66

PA 2 - Future With Project (Impact Area)

Assume: 1. All vegetation removed from temporary and permanent impact zones in year 1 

 2. temporary easement areas will not be replanted with woody vegetation 

 3. existing drainages culverted under roads 

GREAT EGRET 

TY 0 – Baseline (measured) 

 V1 - % area with water 4-9 inches deep 

 V2 - % of substrate in zone 4-9 inches deep with sub- and emergent vegetation 

 

 HSI = V1 + V2 = 0.23 

 2 

TY 1 – V1 – 0 SI = 0 

 V2 - 0 SI = 0.1 

 HSI = 0 + 0.1 = 0.05 

 2 

TY 60 – no change from TY 1 HSI = 0.05 

TY 100 no change from TY60 

RED-WINGED BLACKBIRD 

TY 0 – Baseline (measured) 

 V6 quality of foraging areas within 620 feet of suitable nest areas 

 Condition C wetland HSI = (0.1 x V6)1⁄2

 = 0.2 

TY 1 – no change from baseline HSI = 0 

TY 60 – no change from baseline TY 1 HSI = 0 

TY 100 – no change from baseline 

 

CALIFORNIA VOLE 

TY 0 – Baseline (measured) 

 V1 – Height herbaceous vegetation 

 V2 - % herbaceous cover 

 V3 – Soil type 

 HSI = V1 + V2 + V3 = 0.76 

 3 
Revised Draft- Subject to Change 67

TY 1 – V1 – 0 SI = 0 

 V2 – 0 SI = 0 

 V3 – not silty or loamy ; not friable SI = 0.2 

 HSI = 0 + 0 + 0.2 = 0.06 

 3 

TY 60 – no change from TY 1 HSI = 0.06 

TY 100 – no change from TY60 

MP 1 - Future Without Project (Compensation Area)

Assumption: 1. Annual grassland area will be converted to wetlands 

GREAT EGRET 

TY 0 - Baseline (measured) 

 V1 - % of area with water 4-9 inches deep (0) SI = 0 

 V2 - % of area 4-9 deep with emergent/submergent vegetation (0) SI = .1 

 

 HSI = V1 + V2 = 0 + 0.1 = .05 

 2 2 

TY 1 no change from TY 0 

TY 4 no change from TY 1 

TY 60 no change from TY 4 

TY 100 no change from TY 60 

CALIFORNIA VOLE

TY 0 - Baseline (estimated) 

 V1 - Height of herbaceous vegetation (> 6in.) SI = 1.0 

 V2 - % cover of herbaceous vegetation (80%) SI = 6.7 

 V3 - soil type (mod. friable) SI = 0.5 

TY 1 - V1 - no change from TY 0 

 V2 - no change from TY 0 

 V3 - no change from TY 0 

 HSI = V1+ V2 + V3 = 1.0 + 0.7 + 0.5 = .73 

 3 3 

TY 4 - V1 - no change from TY 1 

TY 60 - V1 - no change from TY 4 

TY 100- no change from TY 60 

RED-WINGED BLACKBIRD

TY 0 - Baseline (estimated) - upland area unsuitable for species HSI = 0 
Revised Draft- Subject to Change 68

TY 1 - no change from TY 0 

TY 4 - no change from TY 1 

TY 60 - no change from TY 4 

TY 100 – no change from TY 60 

MP 2 - Future With Project (Compensation Site)

Assumption: 1. Acquire annual grassland area 

2. Portion of wetland area will have permanent water 

 3. Wetland will be designed to provide equal mix of open water and emergent vegetation 

 4. Carp will not be stocked 

 5. Site baseline is a Condition C wetland. 

 6. Site is minimum of 1-acre in size and access and maintenance roads are provided. 

 7. 40% of area designed for summer conditions of water 4-9 in deep 

8. Plant appropriate wetland plant species, provide pest control and maintenance as needed for 

minimum of 3 years or until wetland is established.

9. Cover crop planted on all disturbed non-wetland areas. 

GREAT EGRET

TY 0 - Baseline (estimated) 

 V1 - % of area with water 4-9 inches deep (0) SI = 0 

 V2 - % of area with water 4-9 deep with emergent/submergent vegetation SI = 0.1 

 HSI = V1 + V2 = 0 + 0.1 = .05 

 2 2 

TY 1 - V1 - 40% SI = 0.4 

 V2 - 5% SI = 0.2 

 HSI = 0.4 + 0.2 = 0.6 = .30 

 2 2 

TY 4 - V1 - 40% SI = 0.4 

 V2 - 40% - 60% SI = 1.0 

 HSI = 0.4 + 1.0 = .70 

 2 

TY 60 - no change from TY 4 HSI = .70 

TY 100 no change from TY 60 

CALIFORNIA VOLE

TY 0 - Baseline (estimated) 

 V1 - Height of herbaceous vegetation ( > 6 in. ) SI = 1.0 

 V2 - % cover of herbaceous vegetation (80%) SI = 0.7 

 V3 - soil type (mod friable) SI = 0.5 
Revised Draft- Subject to Change 69

 HSI = V1 + V2 + V3 = 1.0 + 0.7 + 0.5 = .73 

 3 3 

TY 1 - V1 - > 6 in SI = 1.0 

 V2 - 90% SI = 0.85 

 V3 - no change fro baseline SI = 0.5 

 HSI = 1.0 + 0.85 + 0.5 = .78 

 3 

TY 4 - V1 - no change from TY 1 SI = 1.0 

 V2 - 100% SI = 0 

 V3 - no change from TY 1 SI = 0.5 

 HSI = 1.0 + 0.85 + 0.5 = .78 

 3 

TY 60- no change from TY 4 

TY 100 –no change from TY 60 

RED-WINGED BLACKBIRD

TY 0 - Baseline (estimated) - upland area unsuitable for species 

 HSI = 0 

TY 1 - V1 - Emergent vegetation is old/new growth monocot (other) SI = 0.1 

 V2 - Water present throughout year (yes) SI = 1.0 

 V3 - Carp presence (absent) SI = 1.0 

 V4 - larvae of dragonflies/damselflies presence (yes) SI = 1.0 

 V5 - vegetation density (sparse first year) SI = 0.1 

 HSI = (V1 + V2 + V3 + V4 + V5)1⁄2 = (0.1 x 1.0 x 1.0 x 1.0 x 0.1)1⁄2 = 0.1 

TY 4 - V1 - old/new growth monocots SI = 1.0 

 V2 - no change SI = 1.0 

 V3 - no change SI = 1.0 

 V4 - no change SI = 1.0 

 V5 - 50% SI = 1.0 

 HSI = (1.0 x 1.0 x 1.0 x 1.0 x 1.0)1⁄2

 = 1.0 

TY 60 - no change from TY 4 HSI = 1.0 

TY 100- no change from TY 60 
Revised Draft- Subject to Change 70

AMERICAN RIVER WATERSHED INVESTIGATION 

FOLSOM BRIDGE PROJECT

REACH 3 - FOLSOM PRISON ACCESS ROAD TO SOUTH END OF BRIDGE 

RIPARIAN

YELLOW WARBLER 

TY 0 – Baseline (measured) 

 V1 - % deciduous shrub crown cover 

 V2 - average height of deciduous shrub canopy 

 V3 - % deciduous shrub canopy comprised of hydrophytic shrubs 

 HSI = (V1 x V2 x V3)1⁄3

TY 1 – no change from baseline HSI = 0.22 

TY 60 – no change from baseline HSI = 0.22 

TY 100 – no change from baseline 

NORTHERN ORIOLE 

TY 0 – Baseline (measured) 

 V1 - average height of deciduous tree canopy 

 V2 - % deciduous tree crown cover 

 V3 – stand width 

 HSI = (V1 x V2 x V3)1⁄3

TY 1 – no change from baseline HSI = 0.77 

TY 58 – no change from baseline HSI = 0.77 

TY 100 – no change from baseline 

WESTERN FENCE LIZARD 

TY 0 – Baseline (measured) 

 V1 - % ground cover 

 V2 - average size of ground cover objects 

 V3 - structural diversity/interspersion 

 V4 - % canopy cover 

 CI = (2V1 x V2 x V3)1⁄3 

 TI = (V1 x V4) 1⁄2

 

HSI = (CI x TI)1⁄2 = 0.63 (average of transects) 

 

TY 1 – no change from baseline HSI = 0.63 
Revised Draft- Subject to Change 71

TY 60 – no change from baseline HSI = 0.63 

TY 100 – no change from basline 

PA 2 - Future With Project (Impact Area)

Assume: 1. All vegetation removed from temporary and permanent impact zones in year 1. 

 2. Temporary easement areas will not be replanted with woody vegetation. 

YELLOW WARBLER 

TY 0 – Baseline (measured) 

 V1 - % deciduous shrub crown cover 

 V2 - average height of deciduous shrub canopy 

 V3 - % deciduous shrub canopy comprised of hydrophytic shrubs 

 HSI = (V1 x V2 x V3)1⁄3

TY 1 – V1 – no shrubs SI = 0 

 V2 – no shrubs SI = 0 

 V3 - no shrubs SI = 0 

 HSI = (V1 x V2 x V3)1⁄3 = 0 

TY 60 – V1 – no shrubs SI = 0 

 V2 – no shrubs SI = 0 

 V3 - no shrubs SI = 0 

 HSI = (V1 x V2 x V3)1⁄3

 = 0 

TY 100- no change from TY 60 

NORTHERN ORIOLE 

TY 0 – Baseline (measured) 

 V1 - average height of deciduous tree canopy 

 V2 - % deciduous tree crown cover 

 V3 – stand width 

 HSI = (V1 x V2 x V3)1⁄3

TY 1 - V1 – no trees SI = 0 

 V2 – no trees SI = 0 

 V3 – no trees SI = 0 

 HSI = (V1 x V2 x V3)1⁄3 = 0 
Revised Draft- Subject to Change 72

TY 60 – V1 – no trees SI = 0 

 V2 – no trees SI = 0 

 V3 – no trees SI = 0 

 HSI = (V1 x V2 x V3)1⁄3 = 0 

TY100 - no change from TY 60 

WESTERN FENCE LIZARD 

TY 0 – Baseline (measured) 

 V1 - % ground cover 

 V2 - average size of ground cover objects 

 V3 - structural diversity/interspersion 

 V4 - % canopy cover 

 CI = (2V1 x V2 x V3)1⁄3 

 TI = (V1 x V4) 1⁄2

 

HSI = (CI x TI)1⁄2 = 0.63 (average of transects) 

 

TY 1 – V1 – no ground cover SI = 0 

 V2 – no cover objects SI = 0 

V3 – A SI = 0.1 

V4 – no canopy cover SI = 1.0 

 CI = (2V1 x V2 x V3)1⁄3 = 0 

 TI = (V1 x V4) 1⁄2 = 0 

 

HSI = (CI x TI)1⁄2 = 0 

TY 60 – no change from TY 1 

TY100 - no change from TY 60 

MP 1 – Management Area – Future Without the Project (Compensation Site)

Assume: 1. Existing riparian river bank upstream of Rossmoor Bar can be enhanced by planting riparian species 

(south side of river). 

YELLOW WARBLER 

TY 0 – Baseline (measured) 

 V1 - % deciduous shrub crown cover (0) SI = 0 

 V2 - average height of deciduous shrub canopy (5 ft) SI = 0.82 

 V3 - % deciduous shrub canopy comprised of hydrophytic shrubs (0) SI = 0 

 HSI = (V1 x V2 x V3)1⁄3

 = 0 
Revised Draft- Subject to Change 73

TY 1 – no change from baseline HSI = 0 

TY 15 – no change from baseline HSI = 0 

TY 30 – no change from baseline HSI = 0 

TY 60 – no change from baseline HSI = 0 

TY100 - no change from TY 60 

NORTHERN ORIOLE 

TY 0 – Baseline (measured) 

 V1 - average height of deciduous tree canopy (27 ft) SI = 0.77 

 V2 - % deciduous tree crown cover (0) SI = 0 

 V3 – stand width (1) SI = 0.2 

 HSI = (V1 x V2 x V3)1⁄3

 = 0 

TY 1 – no change from baseline HSI = 0 

TY 15 – no change from baseline HSI = 0 

TY 30 – no change from baseline HSI = 0 

TY 60 – no change from baseline HSI = 0 

TY100 - no change from TY 60 

WESTERN FENCE LIZARD 

TY 0 – Baseline (measured) 

 V1 - % ground cover (0) SI = 0 

 V2 - average size of ground cover objects (< 1 ft) SI = 0.2 

 V3 - structural diversity/interspersion (A) SI = 0.1 

 V4 - % canopy cover (0) SI = 1.0 

 CI = (2V1 x V2 x V3)1⁄3

 = 0 

 TI = (V1 x V4) 1⁄2 = 0 

 

HSI = (CI x TI)1⁄2 = 0 

TY 1 – no change from baseline HSI = 0 

TY 15 – no change from baseline HSI = 0 

TY 30 – no change from baseline HSI = 0 

TY 60 – no change from baseline HSI = 0 

TY100 - no change from TY 60 
Revised Draft- Subject to Change 74

MP 2 – Management Area – Future With Project (Compensation Site)

 

Assume: 

1. Acquire lands. 

2. Watering, weed and pest management for a minimum of 3 years and remedial actions as necessary to ensure 

plant establishment. 

3. Willow species and cottonwoods (80% of woody plantings will be planted near the mean summer water surface 

elevation and less water tolerant plants (oaks, etc) will be planted higher on the bank. 

4. The site will extend no more than 25 feet up the bank from mean summer water surface elevation 

5. Assume average growth rate of 24 inches/year for willows and cottonwood trees.. 

YELLOW WARBLER 

TY 0 – Baseline (measured) 

 V1 - % deciduous shrub crown cover (0) SI = 0 

 V2 - average height of deciduous shrub canopy (5 ft) SI = 0.82 

 V3 - % deciduous shrub canopy comprised of hydrophytic shrubs (0) SI = 0 

 HSI = (V1 x V2 x V3)1⁄3

 = 0 

TY 1 – V1 - % deciduous shrub crown cover (5%) SI = 0.15 

 V2 - average height of deciduous shrub canopy (1 ft) SI = 0.17 

 V3 - % deciduous shrub canopy comprised of hydrophytic shrubs (80%) SI = 0.80 

 HSI = (0.15 x 0.17 x 0.80)1⁄2 = 0.14 

TY 15 – V1 - % deciduous shrub crown cover (75%) SI = 1.0 

 V2 - average height of deciduous shrub canopy (5ft) SI = 0.82 

 V3 - % deciduous shrub canopy comprised of hydrophytic shrubs (80%) SI = 0.80 

 HSI = (1.0 x 0.82 x 0.80)1⁄2 = 0.81 

TY 30 – V1 - % deciduous shrub crown cover (75%) SI = 1.0 

 V2 - average height of deciduous shrub canopy (5ft) SI = 0.82 

 V3 - % deciduous shrub canopy comprised of hydrophytic shrubs (80%) SI = 0.80 

 HSI = (1.0 x 0.82 x 0.80)1⁄2 = 0.81 

TY 60 – no change from TY 30 

TY100 - no change from TY 60 

NORTHERN ORIOLE 

TY 0 – Baseline (measured) 

 V1 - average height of deciduous tree canopy (27 ft) SI = 0.77 

 V2 - % deciduous tree crown cover (0) SI = 0 

 V3 – stand width (1) SI = 0.2 
Revised Draft- Subject to Change 75

 HSI = (V1 x V2 x V3)1⁄3 = 0 

TY 1 – V1 - average height of deciduous tree canopy (27 ft) SI = 0.77 

V2 - % deciduous tree crown cover (0) SI = 0 

 V3 – stand width (< 300 ft) SI = 0.5 

 HSI = (V1 x V2 x V3)1⁄3

 = 0 

TY 15 – V1 - average height of deciduous tree canopy (16 ft) SI = 0.77 

V2 - % deciduous tree crown cover (25%) SI = 1.0

 V3 – stand width (< 300 ft) SI = 0.5 

 HSI = (0.77 x 1.0 x 0.5) 1⁄3 = 0.54 

TY 30 – V1 - average height of deciduous tree canopy (40 ft) SI = 1.0 

V2 - % deciduous tree crown cover (50%) SI = 1.0

 V3 – stand width (< 300 ft) SI = 0.5 

 HSI = (1.0 x 1.0 x 0.5)1⁄3 = 0.79 

TY 60 - V1 - average height of deciduous tree canopy (>40 ft) SI = 1.0 

V2 - % deciduous tree crown cover (75%) SI = 0.9

 V3 – stand width (< 300 ft) SI = 0.5 

 HSI = (1.0 x 0.9 x 0.5)1⁄3 = 0.77 

TY 100- no change from TY 60 

WESTERN FENCE LIZARD 

TY 0 – Baseline (measured) 

 V1 - % ground cover (0) SI = 0 

 V2 - average size of ground cover objects (< 1 ft) SI = 0.2 

 V3 - structural diversity/interspersion (A) SI = 0.1 

 V4 - % canopy cover (0) SI = 1.0 

 CI = (2V1 x V2 x V3)1⁄3 = 0 

 TI = (V1 x V4) 1⁄2 = 0 

 

HSI = (CI x TI)1⁄2 = 0 

TY 1 – V1 - % ground cover (0) SI = 0 

 V2 - average size of ground cover objects (< 1 ft) SI = 0.2 

 V3 - structural diversity/interspersion (A) SI = 0.1 

 V4 - % canopy cover (0) SI = 1.0 

 CI = (2V1 x V2 x V3)1⁄3 = 0 

 TI = (V1 x V4) 1⁄2 = 0 

 HSI = (CI x TI)1⁄2 =0 
Revised Draft- Subject to Change 76

TY 15 – V1 - % ground cover (5%) SI = 0 

 V2 - average size of ground cover objects (≤ 1 ft) SI = 0.2 

 V3 - structural diversity/interspersion (A) SI = 0.1 

 V4 - % canopy cover (40%) SI = 1.0 

 CI = (2V1 x V2 x V3)1⁄3 = 0 

 TI = (V1 x V4) 1⁄2 = 0 

 

HSI = (CI x TI)1⁄2 = 0 

TY 30 – V1 - % ground cover (25%) SI = 1.0 

 V2 - average size of ground cover objects (2 ft) SI = 0.8 

 V3 - structural diversity/interspersion (C) SI = 1.0 

 V4 - % canopy cover (75%) SI = 0.33 

 CI = (2V1 x V2 x V3)1⁄3

 = 1.16 (1.0) 

 TI = (V1 x V4) 1⁄2 = 0.57 

 

HSI = (CI x TI)1⁄2 = 0.75 

TY 60 – V1 - % ground cover (50%) SI = 1.0 

 V2 - average size of ground cover objects (2 ft) SI = 0.8 

 V3 - structural diversity/interspersion (C) SI = 1.0 

 V4 - % canopy cover (75%) SI = 0.33 

 CI = (2V1 x V2 x V3)1⁄3 = 1.16 (1.0) 

 TI = (V1 x V4) 1⁄2 = 0.57 

 

HSI = (CI x TI)1⁄2 = 0.75 

TY100 - no change from TY 60 
Revised Draft- Subject to Change 77

AMERICAN RIVER WATERSHED INVESTIGATION 

FOLSOM DAM OUTLET MODIFICATION PROJECT

PA 1 - Future Without Project (Impact Area)

CHAPARRAL

BOBCAT 

TY 0 – Baseline (measured) 

 V1 - % shrub cover 

 V2 - % herbaceous cover 

 V3 - degree of patchiness 

 V4 – rock outcroppings 

 HSI = V1 + V2 +V3 +2V4 = 0.56 

 5 

TY 1 V1 – no change from TY 0 

 V2 - no change from TY 0 

 V3 - no change from TY 0 

 V4 – no change from TY 0 

 HIS = 0.56 

TY 60 V1 – no change from TY 1 

 V2 - no change from TY 1 

 V3 - no change from TY 1 

 V4 – no change from TY 1 

 HSI = 0.56 

TY100 - no change from TY 60 

WRENTIT 

TY 0 – Baseline (measured) 

 V1 - % shrub cover 

 V2 - % shrub cover ≤ 5 feet(19%) 

 HSI = (V1 x V2)1⁄2 = 0.34 

TY 1 V1 – no change from TY 0 

 V2 - no change from TY 0 

 HSI = (V1 x V2)1⁄2 = 0.34 

TY 60 V1 – no change from TY 1 

 V2 - no change from TY 1 
Revised Draft- Subject to Change 78

 HSI = (V1 x V2)1⁄2 = 0.34 

TY100 - no change from TY 60 

CALIFORNIA THRASHER 

TY 0 – Baseline (measured) 

 V1 – Presence of low shrub openings SI=1.0 

 V2 - Shrub/seedling cover SI=1.0 

 HSI = (V1 x V22

)

1⁄3

 = 1.0 

TY 1 - V1 – no change from TY 0 

 V2 - no change from TY 0 

TY 60- V1 – no change from TY 1 

 V2 - no change from TY 1 

TY100 - no change from TY 60 

PA 2 - Future With Project (Impact Area)

Assume: 1. All vegetation removed from temporary and permanent impact zones in year 1 

 2. Temporary easement areas will not be replanted with woody vegetation 

BOBCAT 

TY 0 – Baseline (measured) 

 V1 - % shrub cover 

 V2 - % herbaceous cover 

 V3 - degree of patchiness 

 V4 – rock outcroppings 

 HSI = V1 + V2 +V3 +2V4 = 0.56 

 5 

 

TY 1 V1 – no shrub cover SI = 0.2 

 V2 - no herbaceous cover SI = 0.2 

 V3 – patchiness (1) SI = 0.2 

 V4 – no rock outcroppings SI = 0.1 

 HSI = 0.2 + 0.2 + 0.2 + 0.2 = 0.16 

 5 

TY 60 V1 – no change from TY 1 

 V2 - no change from TY 1 

 V3 - no change from TY 1 
Revised Draft- Subject to Change 79

 V4 – no change from TY 1 

 HSI = 0.16 

TY100 - no change from TY 60 

WRENTIT 

TY 0 - V1 - % shrub cover 

 V2 - % shrub cover ≤ 5 feet 

 HSI = (V1 x V2)1⁄2 = 0.34 

TY 1 V1 – no shrub cover SI = 0 

 V2 - no shrubs SI = 0 

 HSI = (0 x 0)1⁄2 = 0 

TY 60 V1 – no change from TY 1 

 V2 - no change from TY 1 

 HSI = 0 

TY100 - no change from TY 60 

CALIFORNIA THRASHER 

TY 0 – Baseline (measured) 

 V1 – Presence of low shrub openings 

 V2 - Shrub/seedling cover 

 HSI = (V1 x V22

)

1⁄3

 = 0.34 

TY 1 - V1 – no shrubs SI = 0 

 V2 - no shrubs/seedlings SI = 0 

 HSI = (0 x 02)

1⁄3

 = 0 

TY 60- V1 – no change from TY 1 

 V2 - no change from TY 1 

TY100 - no change from TY 60 

PA 3 - Future Without Project (Inundation Area)

CHAPARRAL
Revised Draft- Subject to Change 80

BOBCAT 

TY 0 – Baseline (measured) 

 V1 - % shrub cover SI=1.0 

 V2 - % herbaceous cover SI=0.98 

 V3 - degree of patchiness SI= 0.6 

 V4 – rock outcroppings SI=1.0 

 HSI = V1 + V2 +V3 +2V4 = 0.72 

 5 

TY 1 V1 – no change from TY 0 

 V2 - no change from TY 0 

 V3 - no change from TY 0 

 V4 – no change from TY 0 

 HIS = 0.72 

TY 60 V1 – no change from TY 1 

 V2 - no change from TY 1 

 V3 - no change from TY 1 

 V4 – no change from TY 1 

 HSI = 0.72 

TY100 - no change from TY 60 

WRENTIT 

TY 0 – Baseline (measured) 

 V1 - % shrub cover SI=0.40 

 V2 - % shrub cover ≤ 5 feet(19%) SI=0.09 

 HSI = (V1 x V2)1⁄2 = 0.19 

TY 1 V1 – no change from TY 0 

 V2 - no change from TY 0 

 HSI = (V1 x V2)1⁄2 = 0.19 

TY 60 V1 – no change from TY 1 

 V2 - no change from TY 1 

 HSI = (V1 x V2)1⁄2 = 0.19 

TY100 - no change from TY 60 
Revised Draft- Subject to Change 81

CALIFORNIA THRASHER 

TY 0 – Baseline (measured) 

 V1 – Presence of low shrub openings SI=1.0 

 V2 - Shrub/seedling cover SI=1.0 

 HSI = (V1 x V22

)

1⁄3

 = 1.0 

TY 1 - V1 – no change from TY 0 

 V2 - no change from TY 0 

TY 60- V1 – no change from TY 1 

 V2 - no change from TY 1 

TY100 - no change from TY 60 

PA 4 - Future With Project (Inundation Area)

Assume: 1. All vegetation removed from temporary and permanent impact zones in year 1 

 2. Temporary easement areas will not be replanted with woody vegetation 

BOBCAT 

TY 0 – Baseline (measured) 

 V1 - % shrub cover SI=1.0 

 V2 - % herbaceous cover SI=0.98 

 V3 - degree of patchiness SI=0.6 

 V4 – rock outcroppings SI=1.0 

 HSI = V1 + V2 +V3 +2V4 = 0.72 

 5 

 

TY 1 V1 – no shrub cover SI = 0.2 

 V2 - no herbaceous cover SI = 0.2 

 V3 – patchiness (1) SI = 0.2 

 V4 – no rock outcroppings SI = 0.1 

 HSI = 0.2 + 0.2 + 0.2 + 0.2 = 0.16 

 5 

TY 60 V1 – no change from TY 1 

 V2 - no change from TY 1 

 V3 - no change from TY 1 

 V4 – no change from TY 1 

 HSI = 0.16 

TY100 - no change from TY 60 
Revised Draft- Subject to Change 82

WRENTIT 

TY 0 - V1 - % shrub cover 

 V2 - % shrub cover ≤ 5 feet 

 HSI = (V1 x V2)1⁄2 = 0.34 

TY 1 V1 – no shrub cover SI = 0 

 V2 - no shrubs SI = 0 

 HSI = (0 x 0)1⁄2 = 0 

TY 60 V1 – no change from TY 1 

 V2 - no change from TY 1 

 HSI = 0 

TY 100 - no change from TY 60 

CALIFORNIA THRASHER 

TY 0 – Baseline (measured) 

 V1 – Presence of low shrub openings 

 V2 - Shrub/seedling cover 

 HSI = (V1 x V22

)

1⁄3

 = 1.0 

TY 1 - V1 – no shrubs SI = 0 

 V2 - no shrubs/seedlings SI = 0 

 HSI = (0 x 02)

1⁄3

 = 0 

TY 60- V1 – no change from TY 1 

 V2 - no change from TY 1 

TY 100 - no change from TY 60 

MP 1 - Management Area - Future Without Project (Compensation Site)

Assume: 1. Annual grassland area selected for conversion to oak woodland. 

BOBCAT 

TY 0 – Baseline (estimated) 
Revised Draft- Subject to Change 83

 V1 - % shrub cover (no shrubs) SI = 0.2 

 V2 - % herbaceous cover (100%) SI = 0.8 

 V3 - degree of patchiness (1) SI = 0.2 

 V4 – rock outcroppings (no) SI = 0.1 

 HSI = V1 + V2 +V3 +2V4 = 0.8 + 0.8 + 0.2 = 0.2 = 0.28 

 5 5 

TY 1 V1 – no change from TY 0 

 V2 - no change from TY 0 

 V3 - no change from TY 0 

 V4 – no change from TY 0 

 HSI = 0.28 

TY 15 V1 – no change from TY 1 

 V2 - no change from TY 1 

 V3 - no change from TY 1 

 V4 – no change from TY 1 

 HSI = 0.28 

TY 30 V1 – no change from TY 15 

 V2 - no change from TY 15 

 V3 - no change from TY 15 

 V4 – no change from TY 15 

 HSI = 0.28 

TY 100 V1 – no change from TY 30 

 V2 - no change from TY 30 

 V3 - no change from TY 30 

 V4 – no change from TY 30 

 HSI = 0.28 

WRENTIT 

TY 0 – Baseline (estimated) 

 V1 - no shrub cover SI = 0 

 V2 – no shrubs SI = 0 

 HSI = (V1 x V2)1⁄2 = (0 x 0)1⁄2 = 0 

TY 1 V1 – no change from TY 0 

 V2 - no change from TY 0 

 HSI = 0 
Revised Draft- Subject to Change 84

TY 15 V1 – no change from TY 1 

 V2 - no change from TY 1 

 HSI = 0 

TY 30 V1 – no change from TY 15 

 V2 - no change from TY 15 

 HSI = 0 

TY 100 V1 – no change from TY 30 

 V2 - no change from TY 30 

 HSI = 0 

CALIFORNIA THRASHER 

TY 0 – Baseline (estimated) 

 V1 – no shrubs SI = 0 

 V2 – no shrubs/seedlings SI = 0 

 HSI = (V1 x V22)

1⁄3

 = (0 x 02)

1⁄3

 = 0 

TY 1 - V1 – no change from TY 0 

 V2 - no change from TY 0 

 HSI = 0 

TY 15 - V1 – no change from TY 1 

 V2 - no change from TY 1 

 HSI = 0 

TY 30 - V1 – no change from TY 15 

 V2 - no change from TY 15 

 HSI = 0 

TY 100- V1 – no change from TY 30 

 V2 - no change from TY 30 

 HSI = 0 

MP 2 - Management Area - Future With Project (Compensation Site)
Revised Draft- Subject to Change 85

Assume: 

1. Acquire lands (currently annual grasslands) 

2. Annual grassland area prepared for planting in TY 1 , provide access and maintenance roads 

3. Plant chaparral species at a density of 400 trees/acre and cover crop 

4. Watering, weed, pest control for minimum of 3 years and remedial actions as necessary to ensure plant 

establishment. 

5. Develop O&M manual 

BOBCAT 

TY 0 – Baseline (estimated) 

 V1 - % shrub cover (no shrubs) SI = 0.2 

 V2 - % herbaceous cover (100%) SI = 0.8 

 V3 - degree of patchiness (1) SI = 0.2 

 V4 – rock outcroppings (no) SI = 0.1 

 HSI = V1 + V2 +V3 +2V4 = 0.8 + 0.8 + 0.2 = 0.2 = 0.28 

 5 5 

TY 1 V1 – area cleared and planted (1%) SI = 0.2

 V2 – 100% SI = 0.8 

 V3 - no change from TY 0 SI = 0.2 

 V4 – no change from TY 0 SI = 0.1 

 HSI = 0.28 

TY 15 V1 – 30% SI = 1.0 

 V2 – 100% SI = 0.8 

 V3 – 2 SI = 0.6 

 V4 – no change from TY 1 SI = 0.1 

 HSI = 1.0 + 0.8 + 0.6 + 0.2 = 0.52 

 5 

TY 30 V1 – 50% SI = 1.0 

 V2 – 100% SI = 0.8 

 V3 – 2 SI = 0.6 

 V4 – no change from TY 1 SI = 0.1 

 HSI = 1.0 + 0.8 + 0.6 + 0.2 = 0.52 

 5 

TY 100 V1 – 50% SI = 1.0 

 V2 – 100% SI = 0.8 

 V3 – 2 SI = 0.6 

 V4 – no change from TY 1 SI = 0.1 

 HSI = 1.0 + 0.8 + 0.6 + 0.2 = 0.52 
Revised Draft- Subject to Change 86

 5 

WRENTIT 

TY 0 – Baseline (estimated) 

 V1 - no shrub cover SI = 0 

 V2 – no shrubs SI = 0 

 HSI = (V1 x V2)1⁄2 = (0 x 0)1⁄2 = 0 

TY 1 V1 – area cleared and planted (1%) SI = 0 

 V2 – area cleared and planted (100%) SI = 1.0 

 HSI = (V1 x V2)1⁄2 = (0 x1.0)1⁄2 = 0 

TY 15 V1 – 30% SI = 0.15 

 V2 – 80% SI = 0.8 

 HSI = (0.15 x 0.8)1⁄2 = 0.49 

TY 30 V1 – 50 % SI = 0.33 

 V2 – 80 % SI = 0.8 

 HSI = (0.33 x 0.8)1⁄2 = 0.64 

TY 100 V1 – 50 % SI = 0.33 

 V2 – 80 % SI = 0.8 

 HSI = 0.64 

CALIFORNIA THRASHER 

TY 0 – Baseline (estimated) 

 V1 – no shrubs SI = 0 

 V2 – no shrubs/seedlings SI = 0 

 HSI = (V1 x V22

)

1⁄3

 = (0 x 02)

1⁄3

 = 0 

TY 1 - V1 –no SI= 0 

 V2 - 1% SI= 0 

 HSI = 0 

TY 15 - V1 – yes SI = 1.0 

 V2 - 30% SI = 0.35 

 HSI = (1.0 x 0.352

)

1⁄3

 = 0.50 
Revised Draft- Subject to Change 87

TY 30 - V1 – yes SI = 1.0 

 V2 - 50% SI = 1.0 

 HSI = HSI = (1.0 x 1.0.2

)

1⁄3

 = 1.0 

TY 100- V1 – no change from TY 30 

 V2 - no change from TY 30 

 HSI = 1.0 
Revised Draft- Subject to Change 88

 

 APPENDIX A-2 

HSI MODELS 
Revised Draft- Subject to Change 89

 NORTHERN ORIOLE 

 HABITAT SUITABILITY INDEX MODEL
Revised Draft- Subject to Change 90

 HABITAT SUITABILITY INDEX MODEL 

 

 NORTHERN ORIOLE (Icterus spurius) 

 BREEDING HABITAT, CENTRAL VALLEY 

CALIFORNIA 

 U.S. Fish and Wildlife Service 

Ecological Services 

Sacramento, California 

January 1988 
Revised Draft- Subject to Change 91

COVER TYPE LIFE REQUISITE HABITAT 

VARIABLES

Average height of deciduous 

tree canopy 

 (V1) 

Valley Woodland (W) 

Reprod

uction/

Cover 

Percent 

deciduo

us tree 

Riparian (R) 

Crown cover (V2) 

 Stand width (V3) 

FOOD

The diet of the northern oriole is comprised mainly of insects. Fruits, berries, and nectar are also utilized 

(Bent 1958; Martin et al. 1961). For purposes of this model, it is assumed that if suitable habitat is 

available for nesting and cover, food resources are not limiting. 

Minimum habitat area

Minimum habitat area is defined as the minimum amount of contiguous habitat that is required before an

area will be occupied by a species. Based on reported pair densities (Walcheck 1970; Gaines 1974; 

Pleasant 1979), it is assumed that at least 0.25 acres of suitable habitat must be available for the northern 

oriole to occupy an area. If less than this amount is present, the HSI is assumed to be zero. 

VARIABLE HABITAT TYPE 

SUGGESTED TECHNIQUE

V1 Average height of R, W Range finder and 

clinometer 

deciduous tree canopy on belt transect 

V2 Percent deciduous R, W Line intercept 

 tree crown cover 

V3 Stand width R, W Visual observation, 

aerial interpretation 

HSI Determination

LIFE REQUISITE COVER TYPE 

EQUATION
Revised Draft- Subject to Change 92

Reproduction R, W 

 (V1 x V2 x 

V3)

1/3

The HSI value for the northern oriole is equal to the reproduction/cover value. 

Model Applicability

The model applies to breeding habitat of the northern oriole in the Central Valley of California up to 500 

feet in elevation. 

1. Average height of 

deciduous tree 

canopy. 

Assumption: 

Orioles nest 

almost exclusively 

in large, 

preferably 

deciduous, trees 

(derived from 

nesting data of 

Schaefer 

(1976A)). Tree 

height of 35 feet 

or greater is 

optimum the 

dominant canopy 

strata equals those 

trees comprising 50% of total canopy closure. 

Ave. height deciduous tree canopy

SI

0 5 101520253035404550556065707580859095100

0

0.2

0.4

0.6

0.8

1
Revised Draft- Subject to Change 93

2. Percent deciduous tree crown cover. 

Assumption: Orioles 

prefer open stands of 

deciduous trees for 

nesting (Grinnel and 

Miller 1944). Crown 

cover of 25-50% is 

assumed to be 

optimum. 

3. Stand width 

Assumption: Orioles prefer large blocks of riparian or oak woodland for nesting (USFWS 1981). 

A - Woodland a narrow band comprising the width of one tree. 

B - Woodland a strip less than 300 feet wide at its widest point. 

Percent deciduous tree crown cover

SI

0 5 101520253035404550556065707580859095100

0

0.2

0.4

0.6

0.8

1

Category

SI

A B C

0

0.2

0.4

0.6

0.8

1
Revised Draft- Subject to Change 94

C - Woodland greater than 300 feet wide at widest point. 
Revised Draft- Subject to Change 95

Literature Cited 

Bent, A.C. 1958. Life histories of North American blackbirds, orioles, tanagers, and their allies, U.S. 

Natl. Mus. Bull. 211. 549 pp. 

Gaines, D. 1974. A new look at the nesting riparian avifauna of the 

Sacramento Valley, California. West. Birds 5:61-80. 

Grinnell, J. and A. H. Miller, 1944. The distribution of the birds of 

California Pac. Coast Avifauna No. 27. 680 pp. 

Martin, A. C., H. S. Zim and A. L. Nelson. 1961. American wildlife and plants, guide to wildlife food 

habits. Dover Publ., Inc., New York 590 pp. 

Pleasants, B. Y. 1979. Adaptive significance of the variable dispersion pattern of breeding northern 

orioles. Condor 81:28-34. 

Schaefer, V. H. 1976. Geographic variation in the placement and structure of oriole nests. Condor 

78:443-448. 

United States Fish and Wildlife Service. 1981. Avian communities in riparian habitat of the upper 

Sacramento River: Predicting impacts of habitat alteration. Division of Ecological Services, Sacramento, 

CA. 13 pp. 

Walcheck, K. C. 1970. Nesting bird ecology of four plant communities in the Missouri River Breaks, 

Montana. Wilson Bull. 82:370-382. 
Revised Draft- Subject to Change 96

 WESTERN FENCE LIZARD 

 HABITAT SUITABILITY INDEX MODEL 

 HABITAT SUITABILITY INDEX MODEL 

 WESTERN FENCE LIZARD (Sceloporus occidentalis) 

by 

Daniel H. Strait 

 U.S. Fish and Wildlife Service 

 Division of Ecological Services 

Sacramento, California 
Revised Draft- Subject to Change 97

March 1989 

INTRODUCTION 

The western fence lizard (Sceloporus occidentalis) ranges from British Columbia southward through 

Washington, Oregon and throughout California and the Great Basin to northwestern Baja California 

(Smith, 1948; Stebbins, 1985). It occupies a wide variety of habitats, excluding extreme desert conditions, 

from sea level to over 9500 feet in the Sierra Nevada. In California, four subspecies are present 

(Jennings, 1987). Preferring wooded, rocky areas, it frequents talus and rocky outcrops of hillsides,

canyons and along streams. Western fence lizards are attracted to old buildings, woodpiles, fences, 

telephone poles, woodrat nests and banks with rodent burrows. It requires cover and, except for 

dispersing females (Jennings, personal communication) is seldom encountered in open fields or extremely 

barren areas (Stebbins, 1954). It is frequently a colonizer of disturbed habitats (Lillywhite, et. al., 1977). 

The western fence lizard can be semi-arboreal (Cunningham, 1955; Davis and Verbeek, 1972). Trees 

apparently do not constitute a life requisite as was shown by Sceloporus occidentalis populations in 

chaparral (Lillywhite, Friedman and Ford 1972) and at high elevations (Grinnell and Storer, 1924). Trees 

may simply act as another type of available cover. This indicates the microhabitat plasticity of this

species (Rose, 1978). 

 MODEL APPLICABILITY 

This model was designed for use in plant communities found in the Central Valley of California and 

surrounding foothills up to an elevation of approximately 1500 feet and applies to the subspecies S. o. 

occidentalis and S.o. biseriatus. The model is based on both empirical data provided by expert review 

and information obtained from current literature. 

Cover Type Life Requisite Habitat Variable

 Percent ground cover (V1) 

 Cover/Reproduction Average size of ground 

 cover objects (V2) 

Riparian (R) Structural diversity/ 

Oak savannah (O) Interspersion (V3) 

Oak woodland (W) 

Scrub (S) 

Annual Grassland (G) Percent ground cover (V1) 

 Thermoregulation 

 Percent canopy cover (V4) 

Habitat Variable Cover Type Suggested Techniques
Revised Draft- Subject to Change 98

V1 - Percent ground R.O.W.S,G Line intercept, measurement of cover 

 random points using a 3 feet 

 diameter loop. 

V2 - Average size of R.O.W.S,G Line intercept 

 ground cover objects 

V3 - Structural diversity/ R.O.W.S,G Ocular estimate 

 interspersion 

V4 - Percent canopy R.O.W.S,G Spherical densiometer, line 

 cover intercept, point intercept on 

 aerial photos. 

Variable 1. Percent ground cover 

Assumes: 

Only those objects less than 8 feet above the ground surface are considered. This includes rocks, logs, 

branches, tree trunks, fences, wood piles and live vegetation. Western fence lizards exhibit no welldefined habitat preference, but favor areas with logs, trees or other objects upon which they can climb, 

sun and display (Fitch, 1940). Brush piles and cavities under rocks and logs provide refuge (Marcellini 

and Mackey, 1979). An amount of ground cover beyond a particular density results in less than optimal

conditions as it conceals predators and interferes with movement and the ability to defend a territory

(Davis and Ford, 1983). Davis and Verbeek (1972) found that western fence lizards avoided dense 

grasslands. However, dispersing juveniles will cross dense grasslands and colonize any suitable isolated 

habitat found (Jennings, personal communication). 

In California, western fence lizards centered their territorial activities about logs, fence posts, stumps and 

exposed boulders from which males display (Carpenter, 1980) and to observe mates or rival males (Fitch, 

1940). 

Eggs are placed in damp, friable, well-aerated soil from mid-May to mid-July in pits dug by the female

and covered with loose soil (Stebbins, 1954) or under rocks and logs (Jennings, personal communication). 

In non-riparian conditions, nest sites are probably limited to areas within the shade of large cover objects. 

Ground cover ranging from 25 to 70 percent is considered optimum for western fence lizards as it 

provides sufficient cover for maximum use of an area while not being so abundant as to interfere with 

movement. Western fence lizards undergo hibernation from November to February (Smith, 1946) and 

require cover for winter survival (Jennings, personal communication). 
Revised Draft- Subject to Change 99

Variable 2. Average 

size of ground cover 

objects. 

Assumes: 

Ground cover objects include tree trunks but no other living material. The objects must be sufficiently 

large to provide escape cover. Western fence lizards have the habit of running to the opposite side of 

their perch (rock, log, etc.) when approached (Nussbaum et al., 1983). The objects must also be large

enough to provide cover for hibernation, nest building, shade for summer thermoregulation, and to offer 

vantage points for territorial defense and mating display. 

An average ground cover object size of 3.0 feet and larger is considered optimum as it is sufficiently large 

to provide for escape cover, thermoregulation and reproductive needs. 

The average size of ground cover objects greater than 4 inches is diameter are measured in the field using 

the line intercept method and is determined by the formula: 

Average size of ground Total feet of line intercepted 

 cover objects = Total number of ground cover objects intercepted 

Percent ground cover

SI

0 5 101520253035404550556065707580859095100

0

0.2

0.4

0.6

0.8

1

Ave size of ground cover objects (ft/object)

SI

0 1 2 3 4 5

0

0.2

0.4

0.6

0.8

1
Revised Draft- Subject to Change 100

Variable 3. Structural 

diversity/interspersion 

Assumes: 

This variable is related to the habitat heterogeneity. The western fence lizard areas have a mixture and 

sufficient quantity of cover types (rocks, logs, living vegetation, rodent burrows, cracks and crevices) in a 

semi-open environment with lots of habitat edge allowing for sufficient exposure to the sun (Ruth, 

personal communication), escape cover and a production base for food organisms (Jennings, personal 

communication). These areas usually have a significant vertical component in the form of large boulders, 

trees, fence rows, old buildings or log piles (Nussbaum et al, 1983). Davis and Ford (1983) found 

optimal habitat was provided by large fallen oaks in various stages of decay or by large, standing oaks 

from which limbs and branches had fallen to the ground creating massive tangles. Western fence lizards 

commonly show low distributions in climax communities due to the homogeneity of the habitat(Ruth, 

personal communication). 

Structural diversity/interspersion

SI

A B C

0

0.2

0.4

0.6

0.8

1
Revised Draft- Subject to Change 101

A - Low habitat diversity. Ground cover limited to 1 or 2 types (i.e., grassland and bare soil). Site 

mostly homogeneous with little edge. Cover component mostly one dimensional without a 

significant vertical element (average less than 1 foot above ground). An exception may be rock 

talus which can be good (Ruth, communication). 

B - Moderate habitat diversity. Two or more major ground cover types occur (i.e., large rocks, logs and 

woodpiles). A moderate amount of edge and interspersion is present between vegetation types 

and/or ground cover types. A significant vertical element to the cover component (average 1 -4 feet 

above ground) is present. 

C - High habitat diversity. Three or more major ground cover types are present (i.e., large rocks, logs 

and woodpiles). Heterogeneity is high with logs of edge between evenly dispersed vegetation and 

cover types. Overall, habitat has a significant vertical component (average greater than 4 feet above

ground). May include rock talus. 

Variable 4. Percent canopy cover 

Assumes: 

The canopy is defined as standing live vegetation greater than 6 feet above ground. This variable relates 

directly to the ability of the habitat to provide sufficient exposure so that western fence lizards can 

thermoregulate. 

The ability of a western fence lizard to thermoregulate in an area is a major determinant of its habitat 

occupancy. The ability of this species to absorb sunlight and warm quickly enables it to inhabit areas 

from sea level to over 9000 feet in elevation (Tanner and Hopkin, 1972). Western fence lizards typically 

move from areas of sunlight to shade to maintain their desired body temperature. Davis and Verbeek 

(1972) found this species shifted from rocks to trees and vice versa according to ambient temperature. 

Western fence lizards avoid dense, shaded woods (Stebbins, 1959). 

A canopy cover ranging from 0 - 45 percent is considered optimum as it provides sufficient sunlight on

the ground or ground cover surface for thermoregulation by western fence lizards. An area with a canopy 

cover greater than 90 percent is considered uninhabitable for western fence lizards due to a lack of 

sunlight on the ground surface for thermoregulation. 
Revised Draft- Subject to Change 102

CALCULATIONS 

Life Requisite Cover Type Index and Equation

Cover//Reproduction R.O.W.S,G CI = (2V1 x V2 x V3)

1/3

Thermoregulation R.O.W.S,G TI = (V1 x V4)

1⁄2

HSI Determination HSI = (CI x TI) 1⁄2

 Assumes percent ground cover is the major determining factor due to its 

importance in reproduction, predator avoidance and thermoregulation. 

An HSI value of 1.0 is considered optimum. An HSI value greater than 1.0 achieved 

through the use of this formula is to be considered 1.0. 

Percent canopy cover

SI

0 5 101520253035404550556065707580859095100

0

0.2

0.4

0.6

0.8

1
Revised Draft- Subject to Change 103

ASSUMPTIONS 

Feeding

It is assumed that where all necessary habitat components are present, food availability is not a factor 

limiting the use of an area by western fence lizards. Low availability of insects may be a limiting factor 

on winter recruitment of juveniles into the adult population (Jennings, personal communication). In arid 

areas, food can be limiting to adults in late summer (Ruth, personal communication). 

The western fence lizard is an opportunistic insectivore which feeds on a variety of insects and other

arthropods including leaf hoppers, aphids, beetles, wasps, termites, ants and spiders (Fitch, 1940; 

Johnson, 1965; Rose, 1976; Stebbins, 1954). 

Rose (1976) found the three primary groups in the fence lizard diet to be ants (Formicidae), beetles 

(Coleoptera) and termites (Isoptera). Johnson (1965) found flies (Diptera), beetles and ants to be 

important prey while Clark (1973) found grasshoppers (Acrididae) the most common prey item. Otvos 

(1977) found moths or butterflies (Lepidoptera) the most common prey item in stomachs analyzed. 

Western fence lizards commonly bask or loaf in the shade and eat whatever arthropod comes close 

enough to attract their attention (Tanner and Hopkin, 1972). It can therefore be assumed that food 

availability is not a limiting factor under normal lizard population levels and habitat conditions. 

Reproduction

It is assumed that, if ground cover of rocks, logs, trees, woodpiles, etc. of sufficient size and quantity are 

available for non-reproductive activities, then areas with moist, friable soil necessary for lizard nesting 

purposes would be present beneath the cover and should not be a limiting factor. Females may travel 

several hundred feed to find appropriate nesting conditions (Ruth, personal communication). 

Water requirements

Considering the wide distribution of this species in all but the most extreme desert regions, it is unlikely 

that water availability would be a limiting factor to the western fence lizard though densities are often 

highest where water (seeps, ponds, etc.)are nearby (Ruth, personal communication). This assumes that 

sufficient ground cover exists for thermoregulation and nesting. This species receives the bulk of its 

moisture through metabolic water from its prey (Ruth, personal communication). These lizards may 

lower metabolic rates to compensate for higher body temperatures and water stress during warm seasons 

(Tsuji, 1985). 

 ACKNOWLEDGMENTS 

We thank Mark R. Jennings, PhD., Department of Herpetology, California Academy of Sciences, Dixon, 

California and Stephen B. Ruth, Ph.D., Monterey Peninsula College, Monterey, California for reviewing 

the draft model and for providing field observations, data and suggestions which aided in establishing

field applicability for the model. Their contributions are greatly appreciated. 

REFERENCES 
Revised Draft- Subject to Change 104

Carpenter, C.C. 1980. In: Lizard ecology: A symposium, June 13-15, 1980. Univ. Of Missouri, Kansas 

City, p. 91. W.M. Milstead (ed.). Univ. of Missouri Press, Columbia. 300 pp. 

Clark, W.H. 1973. Autumnal diet of the San Joaquin fence lizard, Sceloporus occidentalis biseriatus

(Hallowell), in west-central Nevada. Herpetologica 29(1):73-75. 

Cunningham, J.D. 1955. Arboreal habits of certain reptiles and amphibians in southern California. 

Herpetologica 11:217-220. 

Davis, J. and R.J. Ford. 1983. Home range in the western fence lizard (Sceloporus occidentalis 

occidentalis). Copeia 1983(4):933-940. 

Davis, J. and N.A.M. Verbeek. 1972. Habitat preferences and the distribution of Uta stansburiana and 

Sceloporus occidentalis in coastal California. Copeia 1972:643-649. 

Fitch, H.S. 1940. A field study of the growth and behavior of the fence lizard. Univ. Calif. Publ. Zool. 

44(2):151-172. 

Grinnell, J. And T.I. Storer. 1924. Animal life in the Yosemite. Univ. Of Calif. Press, Berkeley. 

Jennings, M.R. 1987. Annotated check list of the amphibians and reptiles of California (second edition, 

revised). Southwestern Herp. Soc. Spec. Publ.(3):1-48. 

Jennings, M.R. 1989. Personal communication, Department of Herpetology, California Academy of 

Sciences, Dixon, California. 

Johnson, C.R. 1965. The diet of the Pacific fence lizard, Sceloporus occidentalis (Baird and Girard), 

from northern California. Herpetologica 21:114-117. 

Lillywhite, H.B., G. Friedman, and N.Ford. 1977. Color matching and perch selection by lizards in 

recently burned chaparral. Copeia 1977(1):115-121.

Marcellini, D. And J.P. Mackey. 1970. Habitat preferences of the lizards, Sceloporus occidentalis and S. 

graciosus (Lacertilia, Iguanidae). Herpetologica 26:51-56. 

Nussbaum, R.A., E.D. Brodie, Jr., and R.M. Storm. 1983. Amphibians and Reptiles of the Pacific 

Northwest, p. 226-237. Univ. Press of Idaho, Moscow, 332 pp. 

Otvos, I.S. 1977. Observations on the food of three forest-dwelling 

lizards in California. Herpetol. Rev. 8(1):6-7. 

Rose, B.R. 1976. Dietary overlap of Sceloporus occidentalis and Sceloporus graciosus. Copeia 

1976(4):818-820. 

Ruth, S.B. 1989. Personal communication. Monterey Peninsula College, Monterey, California. 

Smith, H.M. 1946. Handbook of lizards. Comstock Publ., Ithaca, NY. 557 pp. 
Revised Draft- Subject to Change 105

Stebbins, R.C. 1954. Amphibians and reptiles of western North America. Mcgraw-Hill Book Co., Inc. 

NY. 528 pp. 

Stebbins, R.C. 1959. Reptiles and amphibians of the San Francisco Bay region. Univ. of California 

Press, Berkeley, 72 pp. 

Stebbins, R.C. 1985. A field guide to western reptiles and amphibians (second edition, revised). 

Houghton Miflin Company, Boston. 336 pp. 

Tanner, W.W. and J.M. Hopkin. 1972. The ecology and life history of a population of Sceloporus 

occidentalis longipes (Baird) on Ranier Mesa. Nevada Test Site, Nye County, Nevada. Brigham Young 

Univ. Sci. Bull. Bio. Ser. 15:1-39. 

Tsuji, J.I. 1985. Seasonal changes in standard metabolism and habitat temperatures of Sceloporus 

occidentalis lizards. Am Zool. 25(40:136A. 
Revised Draft- Subject to Change 106

FWS/OBS-82/10.27 

 July 1982 

 HABITAT SUITABILITY INDEX MODELS: YELLOW WARBLER

by 

Richard L. Schroeder4

 Habitat Evaluation Procedures Group 

 Western Energy and Land Use Team 

 U.S. Fish and Wildlife Service 

 Drake Creekside Building One 

2625 Redwing Road 

 Fort Collins, CO 80526 

 Western Energy and Land Use Team 

 Office of Biological Services 

 Fish and Wildlife Service 

 U.S. Department of the Interior 

 Washington, DC 20240 

 

 

4

 Schroeder, R.L. 1982. Habitat suitability index models: yellow warbler. U.S. Dept. 

Int., Fish Wildl. Serv. FWS/OBS-82/10.27. 7 pp. 
Revised Draft- Subject to Change 107

PREFACE 

This document is part of the Habitat Suitability Index (HSI) Model Series (FWS/OBS-82/10), which 

provides habitat information useful for impact assessment and habitat management. Several types of 

habitat information are provided. The Habitat Use Information Section is largely constrained to those

data that can be used to derive quantitative relationships between key environmental variables and habitat 

suitability. The habitat use information provides the foundation for HSI models that follow. In addition, 

this same information may be useful in the development of other models more appropriate to specific 

assessment or evaluation needs. 

The HSI Model Section documents a habitat model and information pertinent to its application. The 

model synthesizes the habitat use information into a framework appropriate for field application and is 

scaled to produce an index value between 0.0 (unsuitable habitat) and 1.0 (optimum habitat). The 

application information includes descriptions of the geographic ranges and seasonal application of the

model, its current verification status, and a listing of model variables with recommended measurement 

techniques for each variable. 

In essence, the model presented herein is a hypothesis of species-habitat relationships and not a statement 

of proven cause and effect relationships. Results of model performance tests, when available, are 

referenced. However, models that have demonstrated reliability in specific situations may prove 

unreliable in others. For this reason, feedback is encouraged from users of this model concerning 

improvements and other suggestions that may increase the utility and effectiveness of this habitat-based 

approach to fish and wildlife planning. Please send suggestions to: 

Habitat Evaluation Procedures Group 

Western Energy and Land Use Team 

U.S. Fish and Wildlife Service 

2625 Redwing Road 

Ft. Collins, CO 80526 

 ACKNOWLEDGMENTS 

We gratefully acknowledge Douglas H. Morse for his review of this habitat model. The cover of this 

document was illustrated by Jennifer Shoemaker. Word processing was provided by Carolyn Gulzow and 

Dora Ibarra. 

YELLOW WARBLER (Dendroica petechia) 

HABITAT USE INFORMATION

General

The yellow warbler (Dendroica petechia) is a breeding bird throughout the entire United States, with the 

exception of parts of the Southeast (Robbins et al. 1966). Preferred habitats are wet areas with abundant 

shrubs or small trees (Bent 1953). Yellow warblers inhabit hedgerows, thickets, marshes, swamp edges 

(Starling 1978), aspen (Populus spp.) groves, and willow (Salix spp.) swamps (Salt 1957), as well as 

residential areas (Morse 1966). 
Revised Draft- Subject to Change 108

Food

More than 90% of the food of yellow warblers is insects (Bent 1953), taken in proportion to their 

availability (Busby and Sealy 1979). Foraging in Maine occurred primarily on small limbs in deciduous

foliage (Morse 1973). 

Water

Dietary water requirements were not mentioned in the literature. Yellow warblers prefer wet habitats 

(Bent 1953; Morse 1966; Stauffer and Best 1980). 

Cover

Cover needs of the yellow warbler are assumed to be the same as reproduction habitat needs are discussed 

in the following section. 

Reproduction

Preferred foraging and nesting habitats in the Northeast are wet areas, partially covered by willows and 

alders (Alnus spp.), ranging in height from 1.5 to 4 m (5 to 13.3 ft) (Morse 1966). It is unusual to find 

yellow warblers in extensive forests (Hebard 1961) with closed canopies (Morse 1966). Yellow warblers

in small islands of mixed coniferous-deciduous growth in Maine utilized deciduous foliage far more 

frequently than would be expected by chance alone (Morse 1973). Coniferous areas were mostly avoided 

and areas of low deciduous growth preferred. 

Nests are generally placed 0.9 to 2.4 m (3 to 8 ft) above the ground, and nest heights rarely exceed 9.1 to 

12.2 m (30 to 40 ft) (Bent 1953). Plants used for nesting include willows, alders, and other hydrophytic 

shrubs and trees (Bent 1953), including box-elders (Acer negundo) and cottonwoods (Populus spp.) 

(Schrantz 1943). In Iowa, dense thickets were frequently occupied by yellow warblers while open 

thickets with widely spaced shrubs rarely contained nests (Kendeigh 1941). 

Males frequently sing from exposed song perches (Kendeigh 1941; Ficken and Ficken 1965), although 

yellow warblers will nest in areas without elevated perches (Morse 1966). 

A number of Breeding Bird Census reports (Van Velzen 1981) were summarized to determine nesting 

habitat needs of the yellow warbler, and a clear pattern of habitat preferences emerged. Yellow warblers 

nested in less than 5% of census areas comprised of extensive upland forested cover types (deciduous or 

coniferous) across the entire country. Approximately two-thirds of all census areas with deciduous shrubdominated cover types were utilized, while shrub wetlands types received 100% use. Wetlands 

dominated by shrubs had the highest average breeding densities of all cover types [2.04 males per ha (2.5 

acre)]. Approximately two-thirds of the census areas comprised of forested draws and riparian forests of 

the western United States were used, but average densities were low [0.5 males per ha (2.5 acre)]. 

Interspersion

Yellow warblers in Iowa have been reported to prefer edge habitats (Kendeigh 1941); Stauffer and Best 

1980). Territory size has been reported as 0.16 ha (0.4 acre) (Kendeigh 1941) and 0.15 ha (0.37 acre)

(Kammeraad 1964). 
Revised Draft- Subject to Change 109

Special Considerations

The yellow warbler has been on the Audubon Society's Blue List of declining birds for 9 of the last 10

years (Tate 1981). 

HABITAT SUITABILITY INDEX (HSI) MODEL

Model Applicability

Geographic area. This model has been developed for application within the breeding range of the yellow 

warbler. 

Season. This model was developed to evaluate the breeding season habitat needs of the yellow warbler. 

Cover types. This model was developed to evaluate habitat in the dominant cover types used by the 

yellow warbler. Deciduous Shrubland (DS) and Deciduous Scrub/Shrub Wetland (DSW) (terminology 

follows that of U.S. Fish and Wildlife Service 1981). Yellow warblers only occasionally utilize forested 

habitats and reported populated densities in forests are low. The habitat requirements in forested habitats 

are not well documented in the literature. For these reasons, this model does not consider forested cover 

types. 

Minimum habitat area. Minimum habitat area is defined as the minimum amount of contiguous that is 

required before an area will be occupied by a species. Information on the minimum habitat area for the 

yellow warbler was not located in the literature. Based on reported territory sizes, it is assumed that at 

least 0.15 ha (0.37 acre) of suitable habitat must be available for the yellow warbler to occupy an area. If 

less than this amount is present, the HSI is assumed to be 0.0. 

Verification level. Previous drafts of the yellow warbler habitat model were reviewed by Douglass H. 

Morse and specific comments were incorporated into the current model (Morse, pers. comm.). 

Model Description 

Overview. This model considers the quality of the reproduction (nesting) habitat needs of the yellow 

warbler to determine overall habitat suitability. Food, cover, and water requirements are assumed to be 

met by nesting needs. 

The relationship between habitat variables, life requisites, cover types, and the HSI for the yellow warbler 

is illustrated in Figure 1. 

The following sections provide a written documentation of the logic and assumptions used to interpret the 

habitat information for the yellow warbler and to explain and justify and variable and equations that are 

used in the HSI model. Specifically, these sections cover the following: (1) identification of variables 

that will be used in the model; (2) definition and justification of the suitability levels of each variable; and 

(3) description of the assumed relationship between variables. 

Reproduction component. Optimal nesting habitat for the yellow warbler is provided in wet areas with 

dense, moderately tall stands of hydrophytic deciduous shrubs. Upland shrub habitats on dry sites will 

provide only marginal suitability. 
Revised Draft- Subject to Change 110

It is assumed that optimal habitats contain 100% hydrophytic deciduous shrubs and that habitats with no 

hydrophytic shrubs will provide marginal suitability. Shrub densities between 60 and 80% crown cover 

are assumed to be optimal. As shrub densities approach zero cover, suitability also approaches zero. 

Figure 1. Relationship between habitat variables, life requisites, cover types, and the HSI for the 

yellow warbler. 

______________________________________________________________________________ 

 Life 

Habitat variable requisite Cover types 

Percent deciduous shrub 

crown cover 

Average height of Deciduous Shrubland 

deciduous shrub canopy Reproduction Deciduous Scrub/ HSI 

 Shrub Wetland 

Percent of shrub canopy 

comprised of hydrophytic 

shrubs 

Totally closed shrub canopies are assumed to be of only moderate suitability, due to the probable 

restrictions on movement of the warblers in those conditions. Shrub heights of 2 m (6.6 ft) or greater are 

assumed to be optimal, and suitability will decrease as heights decrease to zero. 

Each of these habitat variables exert a major influence in determining overall habitat quality for the

yellow warbler. A habitat must contain optimal levels of all variables to have maximum suitability. Low 

values of any one variable may be partially offset by higher values of the remaining variables. Habitats 

with low values for two or more variables will provide low overall suitability levels. 

Model Relationships 

Suitability Index (SI) graphs for habitat variables. This section contains suitability index graphs that 

illustrate the habitat relationships described in the previous section. 

 

Cover-type Variable

DS,DSW V1 

Percent deciduous shrub 

crown cover. 

 

%

SI

0 10 20 30 40 50 60 70 80 90 100

0

0.2

0.4

0.6

0.8

1
Revised Draft- Subject to Change 111

DS,DSW V2 

Average height of 

deciduous shrub 

canopy. 

DS, DSW V3 

Percent of deciduous 

shrub canopy 

comprised of 

hydrophytic shrubs. 

Meters

SI

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

0

0.2

0.4

0.6

0.8

1

%

SI

0 10 20 30 40 50 60 70 80 90 100

0

0.2

0.4

0.6

0.8

1
Revised Draft- Subject to Change 112

Equations. In order to obtain life requisite values for the yellow warbler, the SI values for appropriate 

variables must be combined with the use of equations. A discussion and explanation of the assumed 

relationship between variables was included under Model Description, and the specific equation in this

model was chosen to mimic these perceived biological relationships as closely as possible. The suggested 

equation for obtaining a reproduction value is presented below. 

Life requisite Cover type Equation

Reproduction DS,DSW (V1 x V2 x V3)

1⁄2

HSI determination. The HSI value for the yellow warbler is equal to the reproduction value. 

Application of the Model

Definitions of variables and suggested field measurement techniques (Hays et al. 1981) are provided in

Figure 2. 

Figure 2. Definitions of variables and suggested measurement techniques. 

Variable (definition) Cover types Suggested techniques

V1 Percent deciduous shrub DS,DSW Line intercept 

 crown cover (the percent 

 of the ground that is 

 shaded by a vertical 

 projection of the 

 canopies of woody 

 deciduous vegetation 

 which are less than 5 m 

 (16.5 ft) in height). 

V2 Average height of DW,DSW Graduated rod 

 deciduous shrub canopy 

 (the average height from 

 the ground surface to the 

 top of those shrubs which 

 comprise the uppermost 
Revised Draft- Subject to Change 113

 shrub canopy). 

V3 Percent of deciduous DW.DSW Line Intercept 

 shrub canopy comprised 

 of hydrophytic shrubs 

 (the relative percent 

 of the amount of hydrophytic 

 shrubs compared to all shrubs, 

 based on canopy cover). 

SOURCES OF OTHER MODELS 

No other habitat models for the yellow warbler were located. 

REFERENCES 

Bent, A.C. 1953. Life histories of North American wood warblers. U.S. Natl. Mus. Bull. 203. 734 pp.

Busby, D.G., and S.G. Sealy. 1979. Feeding ecology of nesting yellow warblers. Can. J. Zool. 

57(8):1670-1681. 

Ficken, M.S., and R.W. Ficken. 1965. Territorial display as a population-regulating mechanism in a 

yellow warbler. Auk 82:274-275. 

Hays, R.L., C.S. Summers, and W. Seitz. 1981. Estimating wildlife habitat variables. U.S. Dept. Int., 

Fish Wildl. Serv. FWS/OBS-81/47. 173 pp. 

Hebard, F.V. 1961. Yellow warblers in conifers. Wilson Bull. 73(4):394-395. 

Kammeraad, J.W. 1964. Nesting habits and survival of yellow warblers. Jack-pine Warbler 42(2):243-

248. 

Kendeigh, S.C. 1941. Birds of a prairie community. Condor 43(4):165-174. 

Morse, D.H. 1966. The context of songs in the yellow warbler. Wilson Bull. 78(4):444-455. 

________________. 1973. The foraging of small populations of yellow warblers and American 

redstarts. Ecology 54(2):346-355. 

Morse, D.H. Personal communication (letter dated 4 March 19982). Brown University, Providence, RI. 

Robbins, C.S., B. Braun, and H.S. Zim. 1966. Birds of North America, Golden Press, N.Y. 340 pp. 

Salt, G.W. 1957. An analysis of avifaunas in the Teton Mountains and Jackson Hole, Wyoming. Condor 

59:373-393. 

Schrantz, F.G. 1943. Nest life of the eastern yellow warbler. Auk 60:367-387. 

Starling, A. 1978. Enjoying Indiana birds. Indiana Univ. Press, Bloomington. 214 pp. 
Revised Draft- Subject to Change 114

Stauffer, D.F., and L.B. Best. 1980. Habitat selection of birds of riparian communities: Evaluating

effects of habitat alternations. J. Wildl. Manage. 44(1):1-15. 

Tate, J., Jr. 1981. The Blue List for 1981. Am. Birds 35(1):3-10. 

U.S. Fish and Wildlife Service. 1981. Standards for the development of habitat suitability index models. 

103 ESM. U.S. Dept. Int. Fish Wildl. Serv., Div. Ecol. Serv. 

Van Velzen, W.T. 1981. Forty-fourth breeding bird census. Am. Birds 35(1):46-112. 
Revised Draft- Subject to Change 115

FWS/OBS-82/10.78 

September 1984 

 HABITAT SUITABILITY INDEX MODELS: RED-WINGED BLACKBIRD 

by 

Henry L. Short 

 Habitat Evaluation Procedures Group 

 Western Energy and Land Use Team 

 U.S. Fish and Wildlife Service 

2627 Redwing Road 

 Fort Collins, CO 80526-2899 

 Western Energy and Land Use Team 

 Division of Biological Services 

 Research and Development 

 Fish and Wildlife Service 

 U.S. Department of the Interior 

 Washington, DC 202240 

This report should be cited as: 

Short, H. L. 1985. Habitat suitability index models: Red-winged blackbird. U.S. Fish Wildl. Serv. 

Biol. Rep. 82(10.95). 20 pp. 
Revised Draft- Subject to Change 116

PREFACE 

This document is part of the Habitat Suitability Index (HSI) Model Series [Biological Report 82(10)] 

which provides habitat information useful for impact assessment and habitat management. Several types

of habitat information are data that can be used to derive quantification relationships between key 

environmental variables and habitat suitability. This information provides the foundation for the HSI

model and may be useful in the development of other models more appropriate to specific assessment or 

evaluation needs. 

The HSI Model Section documents the habitat and includes information pertinent to its application. The 

model synthesizes the habitat use information into a framework appropriate for field application and is 

scaled to produce an index value between 0.0 (unsuitable habitat) and 1.0 (optimum habitat). The HSI 

Model Section includes information about the geographic range and seasonal application of the model, its 

current verification status, and a list of the model variables with recommended measurement techniques

for each variable. 

The model is a formalized synthesis of biological and habitat information published in the scientific 

literature and may include unpublished information reflecting the opinions of identified experts. Habitat 

information about wildlife species frequently is represented by scattered data sets collected during 

different seasons and years and from different sites throughout the range of a species. The model presents 

this broad data base in a formal, logical, and simplified manner. The assumptions necessary for 

organizing and synthesizing the species-habitat information into the model are discussed. The model 

should be regarded as a hypothesis of species-habitat relationships and not as a statement of proven cause 

and effect relationships. The model may have merit in planning wildlife habitat research studies about 

species, as well as in providing an estimate of the relative quality of habitat for that species. 

 ACKNOWLEDGMENTS 

I gratefully acknowledge Dr. Gordon H. Orians, Department of Zoology, University of Washington, 

Seattle, for his review of this red-winged blackbird model . The cover of this document was illustrated by 

Jennifer Shoemaker. Word processing was provided by Carolyn Gulzow, Dora Ibarra, and Elizabeth 

Graf. 
Revised Draft- Subject to Change 117

RED-WINGED BLACKBIRD (Agelaius phoeniceus L.) 

HABITAT USE INFORMATION 

General

The red-winged blackbird (Agelaius phoeniceus L) nests in fresh-water and brackish herbaceous 

wetlands, bushes and small trees along watercourses, and certain upland cover types from (American 

Ornithologists’ Union 1983:723): 

... east-central, south-coastal and southern Alaska..., southern Yukon west-central and southern 

Mackenzie, northwestern and central Saskatchewan, central Manitoba, central Ontario, southern 

Quebec..., New Brunswick, Prince Edward Island, Nova Scotia and southwestern Newfoundland 

south to northern Baja California, through Mexico... and along both coasts of Central America to 

Nicaragua and northern Costa Rica .... and to southern Texas, the Gulf coast and southern Florida. 

[This blackbird winters] from southern British Columbia, Idaho, Colorado, Kansas, Iowa, the 

southern Great Lakes region, southern Ontario and New England... south throughout the remainder 

of the breeding range, with the southwestern and most of Middle American populations being 

sedentary. 

The red-winged blackbird traditionally was considered to be a wetland nesting bird. It has adapted, 

within the last century, to habitat changes brought about by man; it now commonly nests in hayfields, 

along roadsides and ditches, and in other upland sites (Dolbeer 1980). 

Food

Red-winged blackbirds vary their diet throughout the year, presumably in response to the nutritive 

demands of reproduction. The percent of waste grain and seeds in the diet of male blackbirds in one 

study in Ontario, Canada, was at least 80 to 87% in March and April, 46% in May, only 10% in July, and

85% in late July to October (McNicol et al. 1982). Insects amounted to 51 to 84% of the diet during May 

and July. The diet of female red-winged blackbirds varied between 67 and 79% insect parts in May and 

July but was only 15% insectivorous in late July-October, after fledging had occurred. 

Water

References describing the dependency of the red-winged blackbird on surface water for drinking and 

bathing were not found in the literature. Nesting occurs in herbaceous wetlands and upland habitat near 

surface water and in suitable vegetation distant from free water. Red-winged blackbirds seem to prefer 

habitats near wetlands for foraging. Communal roosting, which occurs after fledging is completed, is 

either in herbaceous wetlands or dense communities of young trees with thick canopies growing on moist

sites (Micacchion and Townsend 1983). 

Cover

The red-winged blackbird nests in a variety of habitats. Blackbirds in southern Michigan prefer old and 

new hay fields, pastures, old fields, and wetlands with robust vegetation capable of supporting nests and 

dense cover that provides protection for nests (Albers 1978). They avoid cut or fallow fields, woodlots, 

agricultural croplands, open water, and tilled soil. 

Areas with tall, dense, herbaceous vegetation seem to provide preferred nest sites. Blackbirds that nest 

early in the breeding season select tall, dense, old-growth herbaceous vegetation while blackbirds that 

nest late in the breeding season select tall, dense, new-growth herbaceous vegetation (Albers 1978). 

Upland nest sites of red-winged blackbirds in Ontario were in plant communities commonly dominated 

by goldenrod (Solidago spp.), alfalfa (Medicago sativa), fleabane (Erigeron spp.), clover (Trifolium spp.), 

various thistles (Cirsium spp.), and similar herbaceous weeds (Joyner 1978). Blackbirds in fresh water 
Revised Draft- Subject to Change 118

sites selected old- and new-growth of broad-leaved monocots, like cattails (Typha spp.) and broad-leaved 

sedges (Carex spp.), and commonly rejected old- and new-growth of narrow-leaved monocots and forbs 

(Albers 1978). Woody species, such as hightide bush (Iva frutescens) and groundselbush (Baccharis 

halimifolia), and robust herbaceous plants, like cattails, supported the most nests in tidal herbaceous 

wetlands (Meanley and Webb 1963). 

The density of preferred plant cover is not adequately described either in the literature or in this model. 

The height of preferred plant cover is inferred, below, from descriptions of nest sites. 

Red-winged blackbirds frequently use scattered trees and fence posts near their breeding territories as 

observation posts. Blackbirds use both herbaceous wetlands and trees for communal roosts after fledging 

is completed. Roost trees characteristically are young, occur at high densities, provide thick canopies, 

and are adapted to moist sites (Micacchion and Townsend 1983). 

Reproduction

Red-winged blackbirds are migratory in the northern portion of their range. Males migrate to or 

congregate at future nesting habitats in late winter, and females arrive at the territories in early spring 

(Case and Hewitt 1963). In areas with resident populations, individuals of both sexes may remain near 

breeding territories throughout the year, even though the areas are not actively defended or used in winter 

except, perhaps, as roosting sites (Orians pers. comm.). Males are polygynous, and up to six females 

commonly nest within a male's territory (Holm 1973). Harem size was larger in herbaceous wetlands 

with open stands of cattails than in herbaceous wetlands dominated by bulrushes (Scirpus spp.) or by 

closed stands of cattails (Holm 1973). Harem size has sometimes been observed to exceed 10 to 12 

females and, in one instance, numbered 32 females (Orians pers. comm.). 

Males do not participate in nest building, incubation, or feeding of the incubating female (Orians pers. 

comm.). Males may help feed nestlings and are likely to help feed fledglings. The timing of breeding 

varies throughout the range of the red-winged blackbird. Nesting frequently begins in March or April and 

is completed by mid-July in the more temperate habitats. Most young in North America are fledged by 

late July. 

Herbaceous wetlands dominated by cattails generally seem to be the most productive habitats for redwing blackbirds in terms of nests/ha or number of young fledged/ha (Robertson 1972). Favorable 

herbaceous wetland sites produce more suitable food per unit area and have higher nest densities, highly 

synchronous nesting, higher nest survival rates. and lower nest predation rates than do upland nest sites. 

Nests of red-winged blackbirds are placed on the edges of cattail clumps that border areas of open water 

(Wiens 1965). Herbaceous wetlands that are dominated by cattails and have open, permanent water have 

the optimum number of available nest sites. Early nests are placed in the old growth vegetation remaining 

from past growing seasons, while late nests may be built on new growth. Nest success in one herbaceous 

wetland habitat seemed related to: (1) increased depth of permanent water (up to 50 cm or more), which 

apparently reduced mammalian predation on nests; (2) nest placement close to water (greater nest success 

was observed for nests 20 cm above water than nests 100 cm above water), (3) nest placement in 

herbaceous wetland vegetation interspersed with open water, rather than in herbaceous wetland vegetation 

where no open water was present; and (4) nest placement in marsh grass and loosestrife (Decadon 

verticillatus), rather than in sweet gale (Myrica gale) and sedges (Weatherhead and Robertson 1977). 

Other studies have indicated that nests placed at 1.2 m heights were more successful than nests placed at 

0.6 m heights in tidal herbaceous wetlands on Chesapeake Bay (Meanley and Webb 1963) and that nest 

success was higher when permanent water levels were greater than 25 cm (Robertson 1972). 
Revised Draft- Subject to Change 119

Nests of red-winged blackbirds in upland sites typically are wound between and attached to stalks of 

herbaceous vegetation (Bent 1958). Early nests are entwined with old growth stems and late nests with

the sturdiest stems of the new growth. Activities, such as intensive livestock grazing, mowing, and 

burning of old growth stubble, make herbaceous uplands unavailable for early nest placement. Mowing 

hayfields during the nesting season disrupts nesting success on upland sites (Albers 1978). Red-winged 

blackbirds seem to prefer areas with the densest, tallest herbaceous vegetation for nest placement. 

Vegetation that restricted visibility was more important than the number of plant stems and leaves per unit 

area. Trees greater than 5.0 m in height were in most territories (Albers 1978). The mean height of nest 

placement was 15 cm in monotypic stands of reed canarygrass (Phalaris arundinacea) 58 cm high 

(Joyner 1978). Nest sites often are close to open water (Joyner 1978), although no specific descriptions 

of acceptable distances of upland nest sites from open water were found in the literature. 

Interspersion

The red-winged blackbird seems to be closely associated with the presence of standing water (Bent 1958) 

and certain types of dense herbaceous vegetation for nest placement. Herbaceous wetlands or sloughs I

with extensive cattails, bulrushes, sedges, reeds (Phragmites spp.), or tules (Scirpus spp.), historically 

have provided important nesting habitat for the blackbird (Bent 1958). However, blackbirds also nest in 

dense herbaceous cover in hayfields, along roadsides and ditches, and in other upland sites (Dolbeer 

1980). Red-winged blackbirds forage for insects in understory, midstory, and overstory canopies 

(Snelling 1968) during the nesting season. 

 

The blackbird is primarily a seed eater, except during fledging. The species sometimes forms large 

communal flocks in wetland herbaceous habitats or in trees and brushlands and these birds may forage on 

agricultural crops or understory seed sources (Mott et al. 1972; Johnson and Caslick 1982). After the

autumn migration from the northern portion of their range, red-winged blackbirds frequently roost in 

herbaceous wetland habitats, trees, or shrubs and feed on seeds within understory vegetation. 

Special Consideration

Red-winged blackbirds shift from a dispersed insectivorous feeding behavior during the nesting season to 

a communal granivorous feeding habit after fledging has occurred. They frequently move into 

agricultural areas at this time. Costs related to their consumption of grain can become high and may 

exceed the benefits of insect control related to their foraging habits during fledging (Bendell et al. 1981). 

Damage to ripening corn (Zea mays) occurs during August and September (Somers et al. 1981; Stehn and 

de Becker 1982), when blackbirds often congregate at night in herbaceous wetlands or in roosts in young 

deciduous trees in great concentrations (perhaps up to 1 million birds) (Stehn and de Becker 1982). The 

distance from these autumn roosts to corn fields and the proximity of corn fields to traditional flightlines 

strongly influences the amount of damage inflicted on individual corn fields. Bird damage to crops in

Ohio diminished consistently as distances from communal roosts increased from 3.2 to 8 km, and the 

level of damage remained constant and low at distances of 8 to 19.2 km (Dolbeer 1980). 

HABITAT SUITABILITY INDEX (HSI) MODEL 

Model Applicability

Geoqraphic area. This model will produce an HSI for nesting habitats of the red-winged blackbird. The 

breeding range and the year-round range of the blackbird occur throughout the contiguous 48 States. 

Season. The model will produce an HSI for nesting habitat throughout the nesting seasons, which 

generally occurs from March to late July. 
Revised Draft- Subject to Change 120

Cover types. This model was developed to evaluate habitat in herbaceous wetlands (HW) and upland 

herbaceous cover types, such as pasture and hayland (P/H), forbland (F), and grassland (G) (terminology 

follows that of U.S. Fish and Wildlife Service 1981). 

Minimum habitat area. Minimum habitat area is defined as the minimum amount of contiguous habitat 

that is required before a species will live and reproduce in an area. Specific information on minimum

areas required for red-winged blackbirds was not found in the literature. It is assumed, however, that a 

wetland area must contain at least 0.10 ha in emergent herbaceous vegetation, like cattails, to be 

considered nesting habitat for the blackbird. Several studies have described the minimum territory for 

male red-winged blackbirds as 0.02 ha (Weatherhead and Robertson 1977; Orians 1980). A 0.10 ha area 

of emergent herbaceous vegetation might, therefore, potentially provide territories for up to five male 

blackbirds. Territories in upland habitats are much larger than those in wetland habitats. It is assumed 

that a block of upland and habitat must be at least 1.0 ha in area to provide adequate breeding habitat for 

red-winged blackbirds. 

Verification level. This model was developed from descriptive information about nesting cover and 

species-habitat relationships identified in the literature. The HSI derived from the use of this model 

describes the potential of an area for providing nesting habitat for the red-winged blackbird. The model is 

designed to rank the suitability of nesting habitat as would a biologist with expert knowledge about the 

reproductive requirements of the blackbird. The model should not be expected to rank habitats in the 

same way as population data because many nonhabitat-related criteria can significantly impact 

populations of wildlife species. 

Model Description

Overview. The red-winged blackbird uses a variety of habitat layers throughout the year. Tall, dense, 

herbaceous vegetation seems to satisfy nesting, foraging, and cover requirements. The red-winged 

blackbird readily uses midstory and overstory layers of habitat at times but does not seem to be dependent 

on the presence of these layers. 

The red-winged blackbird typically nests in tall (over 0.5 m), dense (undefined) herbaceous vegetation, 

although it occasionally nests in shrubs and trees. This nest site requirement is best met in herbaceous 

wetland habitats where nest sites are available in sturdy cattails over open, permanent water. Nesting 

requirements also can be met by suitable herbaceous vegetation in upland sites. Tall, sturdy, herbaceous 

stems or midstory or overstory components are used as display perches or observation posts. Red-winged 

blackbirds nesting in herbaceous wetland habitats may feed on insects associated with shrub, tree canopy, 

or herbaceous vegetation within the wetland or on insects associated with midstory and overstory 

canopies or in the grass understory outside the wetland boundary (Snelling 1968). Birds nesting in upland 

sites typically forage for insects in understory vegetation near the nest site. 

This model attempts to evaluate the ability of a habitat to meet the food and reproductive needs of the redwinged blackbird during the nesting season. The logic used in this species-habitat model is described in 

Figure 1. The following sections document this logic and the assumptions used to translate habitat 

information for the red-winged blackbird into the variables selected for the HSI model. These sections

also describe the assumptions inherent in the model, identify the variables used in the model, define and 

justify the suitability level of each variable, and describe the assumed relationships between variables. 
Revised Draft- Subject to Change 121

FIGURE 1

Food and reproductive components (herbaceous wetland cover types). There are three conditions (A, B, 

and C) included in Figure 1. Condition A wetlands, with a minimum of 0.10 ha in emergent herbaceous 

vegetation, can be very productive nesting habitats for red-winged blackbirds if water is present 

throughout the year, water chemistry is favorable for photosynthesis, and abundant, persistent, emergent 

vegetation suitable for nest placement is present. The qua1ity of such a wetland as nesting habitat for 

red-winged blackbirds can be estimated with the following five habitat variables. 

Variable 1 (V1) refers to the type of emergent herbaceous vegetation available in the wetland. 

V1 = 1.0 if emergent herbaceous vegetation is predominantly old or new growth of broad-leaved 

monocots, like cattails. 

V1 = 0.1 if emergent herbaceous vegetation is predominantly narrow-leaved monocots or other 

herbaceous materials. 

 

Variable 2 (V2) considers the water regime of the wetlands. The suitability index of V2 is 1.0 if the

wetland is permanently flooded or intermittently exposed with water usually present throughout the year. 

This is a desirable condition because permanent water is necessary to support persistent populations of 

invertebrates that overwinter in various larval instars, maximizing the production of aquatic insects that 

emerge throughout the next spring and early summer. These insects seem to be the favored food source 

for blackbirds nesting in herbaceous wetlands (Orians 1980). The presence of permanent water within the 

wetland may reduce mammalian predation on nests of red-winged blackbirds (Robertson 1972). 

V2 = 1.0 if water usually is present in the wetland throughout the year. 

V2 = 0.1 if the wetland usually is dry during some portion of the year. 
Revised Draft- Subject to Change 122

Variable 3 (V3) pertains to the abundance of carp (Cyprinus carpio) within the wetlands. Carp disturb 

submergent vegetation within the wetlands, which may destroy habitat for emergent aquatic insects (like 

Odonates) and reduce wetland food sources for blackbirds. 

V3 = 1.0 if carp are absent from the wetland. 

V3 = 0.1 if carp are present within the wetland. 

Variable 4 (V4) in the model measures the abundance of larvae of emergent aquatic insects. The adult 

form of these species provides a potentially important food source for red-winged blackbirds nesting in 

wetland habitats. The biomass of these benthic invertebrates is variable within a herbaceous wetland at 

any one time, as well as between sampling periods (Hynes 1972). This biomass should not be regarded as 

a direct measure of productivity because production, in terms of both numbers and weight, is many times 

larger than that present at any one sample periods, and the assessment of numbers or biomass per unit of 

area presents formidable, perhaps insurmountable, difficulties (Hynes 1972). The presence or absence of 

suitable benthic invertebrates can be determined by sampling with a sieve net (Needham and Needham 

1970) along the edge of clumps of emergent vegetation. Sampling is more likely to be accurate than 

inferences about the presence of benthic invertebrates based on measures of water chemistry that may 

inadequately consider pollutants that impact aquatic food chains. Inferences about the presence of 

benthic invertebrates based on the appearance of aquatic vegetation also are less accurate than sampling 

(Orians pers. comm.). Therefore, sampling to determine the presence or absence of important benthic 

invertebrates is the preferred assessment technique. 

V4 = 1.0 if larvae of damselflies and dragonflies (Order Odonata) are present in the wetland. 

V4 = 0.1 if larvae of damselflies and dragonflies are not present in the wetland. 

Dense stands of emergent vegetation in wetlands prevent sunlight from penetrating to the water surface, 

which reduces aquatic productivity. A mat of vegetation can form a wetland "floor", which reduces the

availability of arthropods to red-winged blackbirds and may result in increased nest predation. Open 

water, interspersed throughout the emergent herbaceous vegetation, supports submergent vegetation 

within the wetland boundary that can be used by aquatic insects as food and cover. The openings also 

provide an interface between emergent vegetation and open water, which increases the vegetation surface 

area available to emerging insects and foraging red-winged blackbirds and may increase the presence of

potential nest sites. Blackbirds frequently nest on the edge of cattail clumps that border open water

(Wiens 1965). They are highly territorial, and the number of territories in a wetland is assumed to be 

dependent on the quantity of edge between emergent vegetation and open water that is available for nest 

sites. An exact measure of the amount of edge within a wetland can be difficult and unreliable because of 

the highly dynamic nature of the herbaceous vegetation, resulting from water level fluctuations, life 

cycles of the vegetation, and activities of animals like muskrats (Ondatra zibethica). Measures of the 

patchiness of emergent herbaceous vegetation and open water within a wetland is represented by variable 

5 (V5) in the model. 

Blackbirds prefer patchy stands of cattails interspersed with areas of open water over dense homogeneous 

stands of cattails (Robertson 1972). Variable 5 is assumed to have a suitability index of 1.0 when the

quantity of open water and emergent vegetation is about even (about 40% to 60%). Robertson (1972) 

found a nesting density of about 96 nests/ha in herbaceous wetland habitat when patchy vegetation was 
Revised Draft- Subject to Change 123

about 41% of the total wetland area. Wetlands with large areas of emergent vegetation and small areas of 

open water receive relatively low SIs because of the small quantity of suitable nest sites. Case and Hewitt 

(1963) described the Inlet Valley Marsh in New York as a small, closed herbaceous wetland with upland 

trees and shrubs immediately adjacent for nesting and foraging sites. The red-winged blackbird nesting 

density in this herbaceous wetland was about 33/ha. Variable 5 is assigned an SI of 0.3 when a wetland is 

completely covered with emergent herbaceous vegetation, as described above. 

Conditions where there are small areas of emergent vegetation and large areas of open water also receive 

a low SI because of the reduced availability of niche spaces. Moulton (1980) found red-winged 

blackbirds nesting in emergent vegetation along ditch banks that surrounded large areas of open water in 

rice (Oryza sativa) paddies in northern Minnesota. Nest densities averaged about 2.5 nests/ha of total 

wetland habitat, presumably because both nests and emergent vegetation were restricted to long, narrow

strips of edge. The territorial behavior of red-winged blackbirds may have restricted the nest density 

along the ditch banks. An SI of 0.1 is assigned to V5 for wetland habitats with a limited amount of 

emergent herbaceous cover. The SI's for wetlands with different amounts of emergent herbaceous 

vegetation are listed below. User's can interpolate between listed values as needed. 

V5 = 1.0 if the wetland area contains about an equal mix of emergent herbaceous vegetation and open 

water. 

V5 = 0.3 if the wetland area is covered by a dense stand of emergent herbaceous vegetation. 

V5 = 0.1 if the wetland area contains a few patches of emergent herbaceous vegetation and extensive 

areas of open water. 

Condition B wetlands are wetlands that are likely to be dry sometime during the year or that do not have 

an aquatic insect resource. These wetlands may still provide some habitat for nesting red-winged 

blackbirds. Blackbirds will tend to use the available emergent vegetation as nest sites and rely on 

vegetation surrounding the wetland as a foraging substrate. The distance that red-winged blackbirds will 

fly from wetlands to forage on insects in upland habitats is not known. In this model, only foraging sites 

within 200 m of wetlands that contain nest sites are assumed to be useful to blackbirds. The quality of a 

wetland without permanent water or an aquatic insect resource is assumed to be no better than the quality 

of available foraging sites outside the wetland (V6). Wetlands that only have upland habitats with 

understory vegetation (such as old fields, pastures, or hay fields) available as foraging substrates are given 

an SI of 0.1. Wetlands near uplands that have a deciduous midstory or tree canopy as a foraging substrate 

are assumed to have an SI of 0. 4. Red-winged blackbirds nesting in one herbaceous wetland will forage

on insects in other, close-by, herbaceous wetlands (Holm 1973). Condition B wetlands situated within 

200 m of a condition A herbaceous wetland that has an emergent aquatic insect fauna (Odonates) and 

undefended foraging areas are given an SI of 0.9. 

V6 = 0.1 if the only suitable foraging substrate is an understory layer. 

V6 = 0.4 if the suitable foraging substrates include a midstory and/or an overstory layer. 

V6 = 0.9 if the suitable foraging area is a condition A wetland. 

Food and reproductive components (upland cover types). Upland habitats (Fig. 1; condition C) frequently 

are less productive than are wetland habitats. The number of young red-winged blackbirds fledged per 

territory may be as large in upland sites as in some wetland habitats (Dolbeer 1976). The number of 

young fledged/ha in upland sites, however, frequently is less than 10% of the number fledged/ha in good 
Revised Draft- Subject to Change 124

quality wetland habitat. For example, Robertson (1972) reported 133 young fledged/ha in one wetland 

study area, while only 5 young fledged/ha in nearby upland sites. The nesting density in the wetland 

habitat, with patches of emergent, herbaceous vegetation interspersed with patches of open water, was 

about 10 times higher than in upland habitats. Robertson found about 100 red-winged blackbird nests/ha 

in suitable wetland habitat, 2 to 13 nests/ha in hay fields, and 0.1 nests/ha in a Christmas tree plantation. 

Robertson's (1972) data on the numbers of nests/ha and young fledged/ha suggest that, if the best wetland 

habitats have an HSI of 1.0, the best upland sites may have an HSI of about 0.1. Graber and Graber 

(1963) determined that summer populations of red-winged blackbirds (number/40 ha) in Illinois from 

1958 to 1959 were 301 birds in herbaceous wetlands (whether condition A or B is unknown), 342 birds in

edge shrubs, 204 birds in sweet clover, 158 birds along drainage ditches, 134 birds in mixed hay, 89 birds 

in red clover (Trifolium pratense), 65 birds in oat (Avena sativa) fields, 64 birds in ungrazed grasslands, 

58 birds in alfalfa, 30 birds in wheat (Triticum aestivum), 27 birds in fallow fields, 24 birds in 

pastureland, 23 birds in shrub-grown areas, 5 birds in corn fields, and 3 birds in soybeans (Glycine max). 

The observed nest densities would not exceed the values measured by Robertson (1972) for upland 

habitats even if all of the birds in each of these different habitat types were nesting females. 

The type of upland cover available as nest sites for the red-winged blackbird is represented by V7 in the 

model. Red-winged blackbirds nest in a wide variety of upland sites. For example, blackbirds nested in 

hay fields and old fields, but not in tilled and fallow fields, in southern Michigan (Albers 1978). 

Important characteristics of upland nest sites include the presence of dense, tall, herbaceous vegetation, 

the availability of fence posts and other structures that serve as display perches for males and as 

observation posts for both males and females, and a proximity to open water (Joyner 1978). Specific 

information on the preferred proximity of nest sites in upland habitats to open water were not found in the 

literature. 

Variable 7 (V7) describes the availability of dense, sturdy herbaceous vegetation in forbland, grassland, 

and pasture/hayland upland sites. Variable 7 has a habitat suitability index of 0.1 if the herbaceous

vegetation is dense and tall, like sweet clover (Melilotus spp.), mixed hay, alfalfa, and coarse weeds, 

which provide suitable nest sites and protective cover. Variable 7 has a suitability index of 0.0 if the 

habitat site has some other surface cover, such as cut or fallow fields, agricultural fields, woodlots, or 

tilled soils. 

V7 = 0.1 if upland habitat provides dense, tall, herbaceous vegetation. 

V7 = 0.0 if upland habitat has some other surface cover. 

Early nests of red-winged blackbirds in upland sites are more productive than are late nests (Dolbeer 

1976). Early nests are placed in robust, dense, old herbaceous growth. Activities that are destructive to 

this vegetation, such as mowing, heavy grazing pressure, or burning, reduce habitat suitability for redwinged blackbirds. The occurrence of disturbances that might impact nesting success in upland cover 

types is included as V8 in the model. 

V8 = 0.1 if disturbances, such as mowing, heavy grazing, or burning, do not occur to the potential 

habitat site in most years. 

V8 = 0.0 disturbances occur to the potential habitat site in most years. 

HSI determination. Three types of habitat conditions (A, B, and C) are described in Figure 1. Condition 

A represents a wetland that contains the preferred vegetative structure for nest placement, permanent 

water that supports a population of emergent aquatic insects that are available as food, the absence of 
Revised Draft- Subject to Change 125

carp, and the interspersion of open water within emergent herbaceous vegetation. The equation 

combining the SIs for VI to VS to estimate an HSI for condition A wetlands is: 

 HSI = (V1 x V2 x V3 x V4 x V5) 

Condition B habitats (Fig. 1) are wetlands where the emergent herbaceous vegetation does not have the 

preferred structure, there is no permanent water, carp are present, or benthic invertebrates are absent. 

Condition B habitats have a basic SI of 0.1, determined by the 0.1 SI for the unsuitable conditions of V1, 

V2, V3, or V4. The basic SI of 0.1 can be increased if suitable foraging substrate is available outside the 

boundary of the wetland. Food sources are considered more limiting if only an understory layer is 

available than if deciduous midstory and/or overstory layers also are available as foraging surfaces. A 

condition B habitat may be of highest value to red-winged blackbirds if the birds can readily feed on 

emergent aquatic insects in a nearby condition A herbaceous wetland habitat. The equation for estimating 

the HSI for condition B habitats is: 

HSI = (0.1 x V6) 1/2

Condition C habitats are upland sites, like grass, forb, and pasture/hayland cover types. Their HSI'S, 

which will be either 0.1 or 0, are described by the following equation: 

HSI = (V7 x V8) 1/2

The measure of habitat quality represented by the HSI actually reflects an estimate of the quantity of

niche space available to the blackbird. Habitats with higher HSIs are assumed to contain more niche 

space than habitats with lower HSI'S. More niche space in a habitat frequently means that more 

individuals will occur in that habitat. 

Application of the Model

Summary of model variables. This model can be applied by interpreting a recent, good quality, aerial 

photograph of the assessment area and making selected field measurements. The habitat to be evaluated

is outlined on the aerial photograph. Each wetland within the assessment area is identified and a 200 m 

zone drawn around its perimeter. The wetlands within the assessment area are evaluated, on a per ha 

basis, with field observations and measurements that determine: (1) the type of emergent vegetation 

present; (2) the probable permanency of the water; (3) the presence or absence of carp; (4) the presence or 

absence of larval stages of emergent aquatic insects; (5) the mix of open water and emergent herbaceous 

vegetation; and (6) the nature of vegetative cover within 200 m surrounding the wetland (Fig. 2). The 

proportion of open water and emergent herbaceous vegetation within the wetland is estimated from a map

made after boating or wading through the wetland. The presence of benthic invertebrates is determined

from field sampling. Upland habitats within the assessment area are evaluated by ground truthing to 

determine cover types and land-use practices. Habitat conditions, like the presence of dense, tall 

herbaceous cover and the probability that disturbances such as grazing, burning, mowing, and tilling will 

occur during the March to July nesting season, are noted. 

Definitions of variables and suggested field measurement techniques are provided in Figure 3. 

Model assumptions. I have assumed that it is possible to synthesize results from many studies conducted 

in different seasons of the year different locations in North America into a model years, and a wide 

variety of nest sites throughout North America into a model describing the relative quality of breeding 
Revised Draft- Subject to Change 126

habitat for the red-winged blackbird. My basic assumptions about habitat criteria important to redwinged blackbirds are based on descriptive and correlative relationships expressed in the literature. My 

descriptors of habitat quality will obviously be in error if authors made incorrect judgements or 

measurements or if I have emphasized the wrong data sets or misinterpreted the meaning of published 

data. 

I have assumed that the quality of some wetland habitats exceeds the quality of best upland habitats. This 

assumption was based largely on quality of the blackbirds fledged per hectare of wetland and upland 

habitats. I compiled and analyzed characteristics of wetland habitats that seemed to distinguish habitats 

where varying numbers of red-winged blackbirds were fledged. I assumed that I could meaningfully 

bound the size of study areas to be evaluated as nesting habitat as > 0.1 ha for wetland sites and ∃ 1.0 ha 

for suitable upland sites. I arbitrarily selected distances (200 m) that blackbirds might fly from their nests 

in wetlands to forage on insects and seeds in surrounding vegetative cover. I assumed that the presence of 

dense, tall, herbaceous cover reasonably close to water, coupled with a strong probability that the dense 

cover would remain relatively undisturbed during the breeding season, would adequately indicate the 

value of upland habitats as nest sites for the red-winged blackbird. 

The values for Variables 1 through 8 are estimates. The ecological information available does not seem 

sufficient to suggest: (1) other pertinent variables; (2) more appropriate values for the present variables; 

or (3) more definitive interrelationships between the variables. Finally, I have assumed that the 

multiplicative relationship described in the model is appropriate summary statement to provide a Habitat 

Suitability Index that reflects the relative importance of different habitats as nest sites for the red-winged 

blackbird. 

Figure 3. Definitions of variables and suggested measurement techniques. 

Variable (definition) Cover type Suggested technique

VI Type of emergent HW Identify the dominant species of 

emergent herbaceous vegetation in the 

wetland. Determine if the dominant 

species is a broad-leaved monocot. 

V2 Water regime HW Determine whether or not water will be 

retained in the wetland throughout the 

year in most years; use, if possible, 

indicators like muskrat houses and fish. 

Evaluate records describing permanence 

and level of water in wetland. 

Determine the classification type of 

wetland if the wetland has been 

classified. 

V3 Abundance of carp within HW Determine presence of carp by seining, 

the wetland. using local data about presence of carp 

within wetland or observations to see if 

water is clear or generally murky, as it is 

when carp are feeding. 
Revised Draft- Subject to Change 127

V4 Abundance of larval HW Collect insect larvae by dragging astages 

of emergent aquatic sieve net along 

water bottom near edge insects(Order 

Odonata) of clumps of emergent 

herbaceous within the wetland. 

vegetation. Sampling is done for some 

fixed time period. A second sampling 

procedure involves kicking up the 

substratum at the edge of clumps of 

emergent herbaceous vegetation in front 

of the mouth of a net in some 

standardized manner (Hynes 1972:240). 

The collected invertebrates are sorted 

and identified by comparison with 

illustrations in an appropriate manual 

(like Needham and Needham 1970) to 

determine the presence of damselfly and 

dragonfly larvae (Order Odonata). 

V5 Percent emergent HW Determine the mix of open 

water and herbaceous canopy emergent 

herbaceous vegetation within the 

wetland study area. Estimate the mix 

from a map prepared after wading, 

walking, or boating through the wetland 

or from a map made from a recent, high 

quality, aerial photograph 

V6 Types of foraging sites HW Use map measurer (Hays et al. 1981) available 

outside the wetland. to determine if another 

wetland with an emergent aquatic insect 

population occurs within 200 m of nest sites 

within the wetland being evaluated. Map 

vegetation within 200 m of the wetland and 

determine, using a dot grid (Hays et al. 1981) or 

a planimeter, if deciduous midstory and 

overstory layers comprise at least 10% cover 

when projected to the ground surface. If 

midstory and/or overstory do not provide at least 

10% cover, and a condition. A wetland does not 

occur within 200 m of the wetland being 

evaluated assume only the understory layer is 

available as a foraging substrate. 

V7 Presence of dense, sturdy F,G,P/H Interpret the aerial photograph or a herbaceous 

vegetation Vegetation on-site map prepared 

from the aerial photograph to determine areas of 

upland herbaceous vegetation. Ground truth to 

determine types of herbaceous vegetation 

occurring in the upland within the assessment 
Revised Draft- Subject to Change 128

area and determine if tall, dense, herbaceous 

cover covers at least 10% of the surface area. 

V8 Occurrence of disturbances F,G,P/H Ground truth to predict past and future like 

grazing, mowing, burning, land-use practices 

(types of and tilling on potential 

uplanddisturbances that may impact nesting nest 

sites. success). 

SOURCES OF OTHER MODELS 

Weatherhead and Robertson (1977) identified and quantified some parameters that affected the nesting 

success of red-winged blackbirds in wetland habitats in Ontario, Canada. They determined that nesting

success, as judged by numbers of young fledged per female, was positively correlated with territory 

quality scores based on nest placement. Nesting success seemed to be related to four parameters: (1)

water depth within the wetland; (2) height of nest above the herbaceous wetland floor; (3) relative 

openness of nesting cover within the wetland; and (4) the identity of the support vegetation holding the 

nest. Two of these variables are represented in the present model of habitat suitability for the red-winged 

blackbird: (1) presence or absence of permanent water; and (2) the relative openness of vegetation within 

flooded herbaceous wetlands. No other models for use in predicting the quality of nesting habitat for redwinged blackbirds were found in the literature. 

REFERENCES 

Albers, P. H. 1978. Habitat selection by breeding red-winged blackbirds. Wilson Bull. 90(4):619-634. 

American Ornithologists' Union. 1983. Checklist of North American birds. 6th edition. Am. Ornithol. 

Union. 877 pp. 

Bendell, B. E., P. J. Weatherhead, and R. K. Stewart. 1981 . The impact of predation by red-winged 

blackbirds on European corn borer populations. Can. J. Zool. 59(8):1535-1538. 

Bent, A. C. 1958. Life histories of North American blackbirds, orioles, tanagers, and allies. U.S. Natl. 

Mus. Bull. 211. 549 pp. 

Case, N. A. , and 0. H. Hewitt. 1963. Nesting and productivity of the redwinged blackbird in relation to 

habitat. Living Bird 2:7-20. 

Dolbeer, R . A . 1976. Reproductive rate and temporal spacing of nesting red-winged blackbirds in 

upland habitat. Auk 93(2):343-355. 

_____________ 1980. Blackbirds and corn in Ohio. U.S. Fish Wildl. Serv. Res. Publ . 136. 18 pp. 

Graber, R. R. , and J. W. Graber. 1963. A comparative study of bird populations in Illinois, 1906-1909 

and 1956-1958. Illinois Nat. Hist. Surv. Bull. 28(3):383-528. 

Hays, R.L., C. Summers, and W. Seitz. 1981. Estimating wildlife habitat variables. U.S. Fish Wi1dl. 

Serv. FWS/OBS-81/77. 111 pp. 
Revised Draft- Subject to Change 129

Holm, C. H. 1973. Breeding sex ratios, territoriality, and reproductive success in the red-winged 

blackbird (Agelaius phoeniceus). Ecol.54(2):356-365. 

Hynes, H. B. N. 1972. The ecology of running waters. Univ. Toronto Press, Toronto, Canada. 555 pp.

Johnson, R. J., and J. W. Caslick. 1982. Habitat relationships of roosting and flocking red-winged 

blackbirds. J. Wildl. Manage. 46(4):1071-1077. 

Joyner, D. E. 1978. Use of an old-field habitat by bobolinks and red-winged blackbirds. Can. FieldNat. 92(4):383-386. 

McNicol , D. K. , R. J. Robertson, and P. J. Weatherhead. 1982. Seasonal, habitat, and sex-specific food 

habits of red-winged blackbirds: Implications for agriculture. Can. J. Zool . 60( 12):3282-3289. 

Meanley, B. . and J. S. Webb. 1963. Nesting ecology and reproductive rate of the red-winged blackbird

in tidal marshes of the upper Chesapeake Bay region. Chesapeake Sci. 4(2):90-100. 

Micacchion, M. 9 and T. W. Townsend. 1983. Botanical characteristics of autumnal blackbird roosts

in central Ohio. Ohio Acad. Sci. 83(3):131-135. 

Mott, D. F. I R. R. West, J. W. DeGrazio, and J. L. Guarino. 1972. Foods of the red-winged blackbird in 

Brown County, South Dakota. J. Wildl . Manage. 36(3):983-987. 

Moulton, D. W. 1980. Nesting ecology of the red-winged blackbird in north central Minnesota. 

Minnesota Acad. Sci. 46(2):4-6. 

Needham, J. G., and P. R. Needham. 1970. A guide to the study of fresh-water biology. Holden-Day, 

Inc. San Francisco, CA. 108 pp. 

Orians, G. H. 1980. Some adaptations of marsh-nesting blackbirds. Princeton Univ. Press, Princeton, 

NJ. 295 pp. 

 Personal communication (letter dated August 14, 1984). Univ. Washington, Seattle, WA 

98195. 1 

Robertson, R. J. 1972. Optimal niche space of the red-winged blackbird (Aqelaius phoeniceus). 1. 

Nesting success in marsh and upland habitat. Can. J. Zoo 1. 50(2).247-263. 

Snelling, J. C. 1968. Overlap in feeding habits of red-winged blackbirds and common grackles nesting in 

a cattail marsh. Auk 85(4):560-585. 

Somers, J. D., R. G. Gartshore, F. F. Gilbert, and R. J. Brooks. 1981. Movements and habitat use by 

depreciating red-winged blackbirds in Simcoe County, Ontario. Can. J. Zool. 59(11):2206-2214. 

Stehn , R. A., and S. M. C. de Becker. 1982. Corn damage and breeding red-winged blackbird 

population density in western Ohio. Wildl. Soc. Bull. 10(3):217-223. 

U.S. Fish and Wildlife Service. 1981. Standards for the development of habitat suitability index models. 

103 ESM. U.S. Fish Wildl. Serv., Div. Ecol. Serv., Washington, DC. n.p. 
Revised Draft- Subject to Change 130

Weatherheadi P. J., and R. J. Robertson. 1977. Harem size, territory quality, and reproductive success in 

the red-winged blackbird (Agelaius phoeniceus). Can. J. Zoo l . 55(8) :1261-1267. 

Wiens, J. A. 1965. Behavioral interactions of red-winged blackbirds and common grackles on a 

common breeding ground. Auk 82(3):356-374.

FWS/OBS-82/10.78 

September 1984 
Revised Draft- Subject to Change 131

HABITAT SUITABILITY INDEX MODELS: GREAT EGRET

by 

Brian R. Chapman 

Department of Biology 

Corpus Christi State University 

Corpus Christi, TX 78412 

and 

Rebecca J. Howard 

 National Coastal Ecosystems Team 

U.S. Fish and Wildlife Service 

1010 Gause Boulevard 

Slidell, LA 70458 

Project Officer 

Paul L. Fore 

Regional Office, Region 2 

U.S. Fish and Wildlife Service 

Albuquerque, NM 87103 

Performed for 

 National Coastal Ecosystems Team 

Division of Biological Services 

Research and Development 

Fish and Wildlife Service 

U.S. Department of the Interior 

Washington, DC 20240 

This report should be cited as: 

Chapman, B.R., and R. J. Howard. 1984. Habitat suitability index models: 

great egret. U.S. Fish Wildl. Serv. FWS/OBS-82/10.78. 23 pp. 
Revised Draft- Subject to Change 132

PREFACE 

The habitat suitability index (HSI) model for the great egret presented in this report is intended for use in the habitat 

evaluation procedures (HEP) developed by the U.S. Fish and Wildlife Service (1980) for impact assessment and 

habitat management. The model was developed from a review and synthesis of existing information and is scaled to 

produce an index of habitat suitability between 0 (unsuitable habitat) and 1.0 (optimally suitable habitat). 

Assumptions used to develop the HSI model and guidelines for model applications, including methods for

measuring model variables, are described. 

This model is a hypothesis of species-habitat relations, not a statement of proven cause and effect. The model has 

not been field tested, but it has been applied to three hypothetical data sets that are presented and discussed. The 

U.S. Fish and Wildlife Service encourages model users to convey comments and suggestions that may help increase 

the utility and effectiveness of this habitat-based approach to fish and wildlife management. Please send any 

comments or suggestions you may have on the great egret HSI model to the following address. 

National Coastal Ecosystems Team 

U.S. Fish and Wildlife Service 

1010 Gause Boulevard 

Slidell, LA 70458 

ACKNOWLEDGMENTS 

Earlier versions of the habitat suitability index model and narrative for the great egret were reviewed by Dr. R. 

Douglas Slack and Jochen H. Wiese. The model's structure and functional relationships were thoroughly evaluated 

by personnel of the U.S. Fish and Wildlife Service's (FWS) National Coastal Ecosystems Team. Model and 

narrative reviews were also provided by FWS Regional personnel. 
Revised Draft- Subject to Change 133

 GREAT EGRET (Casmerodius albus) 

INTRODUCTION 

The great egret, also called common egret or American egret, is a large white heron in the order Ciconiiformes, 

family Ardeidae. Great egrets stand 37-41 inches tall and have a wing spread to 55 inches (Terres 1980). The 

species is associated with streams, ponds, lakes, mud flats, swamps, and freshwater and salt marshes. The birds feed 

in shallow water on fishes, amphibians, reptiles, crustaceans and insects (Terres 1980). 

Distribution

The great egret is a common breeding species in all coastal areas south from southern Oregon on the Pacific coast 

and from Maine on the Atlantic coast; in riverine, palustrine and estuarine habitats along the coast of the Gulf of 

Mexico; and in the Eastern-Central United States (Palmer 1962; Erwin and Korschgen 1979; American 

Ornithologists' Union 1983). The great egret undergoes an extensive postbreeding dispersal that extends the range 

of the species to most of the United States exclusive of the arid Southwest (Byrd1978). Young birds hatched in Gulf 

coast colonies tend to move northward for a short period (Byrd 1978; Ogden 1978). However, with the onset of 

colder weather most great egrets and other herons migrate south and many winter along the gulf coast in Texas, 

Louisiana, and Florida (Lowery 1974; Oberholser and Kincaid 1974; Byrd 1978). Analysis of banding data 

indicates that many birds winter in Cuba, the Bahamas, the Greater and Lesser Antilles, Mexico, and Central 

America (Coffey 1948). Lowery (1974) suggested that during severe winters, a higher proportion of the population 

winters farther south. 

Life History Overview

Great egrets nest in mixed-species colonies that number from a few pairs to thousands of individuals. A colony may 

include other species of herons, spoonbills, ibises, cormorants, anhingas, and pelicans. Colony and nest-site 

selections begin as early as December along the gulf coast, but most great egrets do not initiate nesting activities 

until mid-February or early March (Bent 1926; Oberholser and Kincaid 1974; Chaney et al. 1978; Morrison and 

Shanley 1978). Eggs have been recorded from March through early August, and young have been observed in nests 

from mid-May through late August (Oberholser and Kincaid 1974; Chaney et al. 1978). Clutch size varies from one 

to six eggs per nest, but three to four eggs is most common (Bent 1926). Incubation period in a Texas colony ranged 

from 23 to 27 days (Morrison and Shanley 1978). The first flights of young have been noted about 42 days after 

hatching (Terres 1980). 

SPECIFIC HABITAT REQUIREMENTS 

Food and Foraging Habitat

Fish constitute up to 83% of the great egret's diet (Hoffman 1978). Most fish taken by great egrets are minnowsized 3.9 inches, but fish up to 14 inches can be captured and swallowed (Willard 1977; Schlorff 1978). Other 

major food items include insects, crustaceans, frogs, and snakes, while small mammals, small birds, salamanders, 

turtles, snails, and plant seeds are occasionally taken (Baynard 1912; Bent 1926; Hunsaker 1959; Palmer 1962; 

Genelly 1964; Kushlan 1978b). 

Little specific information exists on the food habits of various age classes of great egrets. An adult great egret 

weighing 32.3 ounces (oz) (Palmer 1962) may require approximately 3.9 oz of food per day (estimated by using the 

wading bird weight-daily food requirement model proposed by Kushlan 1978b). Daily food requirements are 

undoubtedly higher during the nesting season when adults are feeding young (Kushlan 1978b). 

Great egrets usually forage in open, calm, shallow water areas near the margins of wetlands. They show no 

preference for fresh-, brackish, or saltwater habitat. Custer and Osborn (1978a,b) found that feeding habitat 

selection in coastal areas of North Carolina varied daily with the tidal cycle. During low tide, great egrets fed in 

estuarine seagrass beds. During high tide, freshwater ponds and the margins of Spartina marshes were used. Inland, 

great egrets feed near the banks of rivers or lakes, in drainage ditches, marshlands, rain pools (Bent 1926; Dusi et al. 

1971; Kushlan 1976b), and occasionally in grassy areas (Weise and Crawford 1974). Feeding sites are generally not 

turbid and are fairly open with no vegetative canopy and few emergent shoots (Thompson 1979b). 
Revised Draft- Subject to Change 134

Great egrets forage singly, in single-species groups, and in mixed-species associations (Kushlan 1978b). Great 

egrets generally fly alone to feeding sites (Custer and Osborn 1978a,b) and may use the same feeding site 

repeatedly. The density and abundance of fish at a given location in estuarine habitats may vary with season, time 

of day, tidal stage, turbidity, and other factors. If feeding success is low, great egrets may move to other areas 

(Cypert 1958; Schlorff 1978) and join other conspecifics in good feeding habitats (Custer and Osborn 1978a,b). 

Most instances of group feeding have been observed during specific environmental conditions, such as lowered 

water levels, that tend to concentrate prey (Kushlan 1976a,b; Schlorff 1978). 

Meyerriecks (1960, 1962) and Kushian (1976a, 1978a, b) provided detailed information on hunting techniques 

employed by great egrets. The "stand-and-wait" and "slow-wade" methods are used most frequently. Because of 

their long legs, great egrets can forage in somewhat deeper water than most other herons. In New Jersey, foraging 

depths ranged from 0 (standing on the bank while fishing) to 11 inches, but depths ranging from 4 to 9 inches were 

most commonly used (Willard 1977). In North Carolina, great egrets fed in water with a mean depth of 25.1 cm (9.8 

inches) in Spartina habitat and of 6.8 inches in non-Spartina habitat (Custer and Osborn 1978b). Mean water depth 

was 7.9 inches for foraging great egrets in California (Hom 1983). In addition to wading, great egrets can feed by 

alighting on the surface of deep waters to catch prey, a method rarely employed (Reese 1973; Rodgers 1974, 1975). 

Although recent declines of great egret populations in the central coastal region of Texas occurred simultaneously 

with declines in coastal marine and estuarine fish populations (Chapman 1980), no causal relationship has been 

proven. At present there are no known management practices that provide suitable food alternatives for piscivorous 

species, such as the great egret, during periods of fish population decline. Known fish nursery and feeding areas 

need protection from destruction or habitat alteration to ensure adequate prey populations for fish-eating birds. 

Water

The physiologic water requirement of great egrets is probably met during feeding activities in aquatic habitats (Dusi 

et al . 1971). Water depth affects the quantity, variety, and distribution of food and cover; great egret food and cover 

needs are generally met between the shoreline and water 1.6 feet deep (Willard 1977). 

Interspersion

Suitable habitat for the great egret must include (1) extensive shallow, open water habitat from 4 to 9 inches deep 

(Willard 1977); (2) food species present in sufficient quantity (Custer and Osborn 1977); and (3) adequate nesting or 

roosting habitat close to feeding habitat. Most great egrets at a colony in North Carolina flew less than 2.5 miles 

from nesting colonies (and presumably, from roosting sites) to feeding areas (Custer and Osborn 1978a), but flight 

distances of up to 22.4 miles have been recorded in the floodplain of the Upper Mississippi River (Thompson 

1979b). 

Several heronries may be close together. Great egrets from one colony may fly over or near an adjacent colony, but 

rarely feed in the same areas as conspecifics from the adjacent colony (Thompson 1979b). 

HABITAT SUITABILITY INDEX (HSI) MODELS 

Model Applicability

Geographic area. The habitat suitability index (HSI) models in this report were developed for application in coastal 

wetland habitats in Texas and Louisiana. Because there are few differences in habitat requirements along the 

Atlantic coast, the remainder of the gulf coast, and inland sites in the Southeastern United States, the HSI models 

may also be used to evaluate potential habitat in those areas. 

Season. This model will produce an HSI values based upon habitat requirements of great egrets during the breeding 

season (February to August). Because there is no apparent seasonal difference in feeding habitat preference and 

because winter nocturnal roosts are similar to nesting sites, the HSI models may also be used to evaluate winter 

habitat for the great egret. 
Revised Draft- Subject to Change 135

Cover types. Great egrets nest on upland islands and in the following cover types of Cowardin et al. (1979): 

Estuarine Intertidal Scrub-Shrub wetland (E2SS), Estuarine Intertidal Forested wetland (E2FO), Palustrine ScrubShrub wetland (PSS) (including deciduous and evergreen subclasses), and Palustrine Forested wetland (PFO) 

(including deciduous and evergreen subclasses). Great egrets may also feed in these wooded wetlands, but preferred 

feeding areas may be any one of a wide variety of wetland cover types. 

Minimum habitat area. Minimum habitat area is defined as the minimum amount of contiguous suitable habitat 

required before an area can be occupied by a particular species. Specific information on minimum areas required by 

great egrets was not found in the literature. If local information is available to define the minimum habitat area, and 

less than this amount of area is available, the HSI for the species will be zero. 

Verification level. The output of these HSI models is an index between 0 and 1.0 that is believed to reflect habitat 

potential for great egrets. Two biologists reviewed and evaluated the great egret HSI model throughout its 

development: Dr. R. Douglas -Slack, Texas A&M University, College Station, and Jochen H. Wiese, Environmental 

Science and Engineering Company, Gainesville, Florida. Their recommendations were incorporated into the modelbuilding effort. The authors, however, are responsible for the final version of the models. The models have not 

been field-tested. 

Model Descriptions

Feedinq HSI model. Great egret feeding habitat suitability is related to prey availability. Habitat suitability is 

optimal when two conditions are met: (1) the populations of minnow-sized fish are high; and (2) shallow open 

water (necessary for successful prey capture), aquatic vegetation (necessary for prey survival and reproduction), and 

deeper water are present in a ratio that maximizes prey density and minimizes hunting interference. Use of this 

model assumes that deep or permanent water environments are not limiting in coastal habitats and that fish 

populations are distributed uniformly. Because great egrets hunt a variety of species in many different habitat types, 

a general approach to modeling feeding habitat suitability is presented. Suitability of all wetland cover types for 

feeding is determined by integrating two factors: (1) the abundance of prey and (2) the accessibility of prey. 

The abundance of prey is determined by the ability of the habitat to support the major prey species, especially 

minnow-sized fish. It is assumed that the abundance of major prey species is related to the primary and secondary 

productivity of the aquatic habitat; however, few field studies have documented this relationship. The model 

assumes that prey abundance is not limiting in coastal habitats. Therefore, the accessibility of prey is used as the 

indicator of feeding habitat suitability. 

The accessibility of prey is determined by water depth and percentage cover of aquatic vegetation. A wetland with 

100% of its area covered by water 4-9 inches deep is assumed to be optimal for feeding by great egrets (V1). 

Although an absence of submerged or emergent vegetation would render fish species most vulnerable to capture, it 

is unlikely that many prey species would use such an area because it totally lacks cover. The model assumes, 

therefore, that optimal conditions for both the occurrence and susceptibility to capture of prey species exist when 

40%-60% of the wetland substrate is covered by submerged or emergent vegetation (V2). When such vegetation is 

lacking, the habitat has a low value for feeding great egrets because small fish may use unvegetated water that is too 

shallow for their larger aquatic predators. 

Habitat variable Component

V1 Percentage of area with water 

10-23 cm deep. 
Revised Draft- Subject to Change 136

 Food HSI 

 (Feeding) 

V2 Percentage of submerged or emergent 

vegetation cover in zone 10-23 cm deep. 

Suitability Index (SI) Graphs for Model Variables

This section provides graphic representation of the relationship between habitat variables and habitat suitability for 

the great egret in wetland (see Table 2 for abbreviations) and upland (U) cover types. The SI values are read 

directly from the graph (1.0 = optimal suitability, 0.0 = no suitability) for each variable. 

The SI graphs are based on the assumption that the suitability of a particular variable can be represented by a twodimensional linear response surface. Although there may be interdependencies and correlations between many 

habitat variables, the model assumes that each variable operates independently over the range of other variables 

under consideration. 

VI Percentage of study area with water 4-9 inches deep. In tidal areas, use depth at mean low tide. In nontidal 

areas, use average summer conditions. 

V2 Percentage of substrate in zone 4-9 inches deep covered by submerged or emergent vegetation. 
Revised Draft- Subject to Change 137

Feeding HSI. 

HSI = V1 + V2

 2 

Data representing three hypothetical study areas for great egret were used to calculate sample HSI values The HSI 

values obtained are believed to reflect the potential of the areas to support feeding or nesting great egrets. 

Field Use of Models

The level of detail needed for application of these models will depend on time, money, and accuracy constraints. 

Detailed field sampling of all variables will provide the most reliable and replicable HSI values. Any or all 

variables can be estimated to reduce the amount of time or money required to apply the models. Increased use of the 

subjective estimates decreases reliability and replicability, and these estimates should be accompanied by 

appropriate documentation to insure that decision makers understand both the method of HSI determination and 

quality of data used in the model. Techniques for measuring habitat variables included in the great egret HSI 

models are suggested in Table 5. 

A project area may contain both potential feeding and nesting habitat. To decrease the cost and time necessary to 

evaluate the area, assume that food is not limiting and apply only the nesting HSI model. This recommendation is 

based upon the following assumptions: (1) in most coastal areas of Texas and Louisiana, aquatic habitats suitable 

for feeding are abundant and are, therefore, less of a limiting factor to great egrets than are suitable nesting sites; and 

(2) nesting value is easier and more accurately estimated by using subjective methods than is food value. The 

variables used to measure food use of past colony sites, and (2) the enhancement of a site by the presence of other 

herons. These two factors are usually, but not always, interrelated. Great egrets tend to use the same colony site in 

successive years until the site is degraded, and the site may include great blue herons. When applying the HSI 

model , the user should be aware that an area known to be used by great egrets (or great blue herons) is more likely 

to be used in future years than an area with an equal HSI value not known to have a history as a colony site. 

Table 5. Suggested measurement techniques for habitat variables used in the great egret HSI 

models. 
Revised Draft- Subject to Change 138

 

Variable Suggested technique 

 

_____________________________________________________________________________________________

_

V1 The percentage of the area with water 4-9 inches deep can be determined by line 

transect sampling of water depth. 

 V2 The percentage of substrate in the 4-9 inches water depth zone covered by 

submerged or emergent vegetation can be determined from available cover 

maps, aerial photographs, or by line transect sampling. 

______________________________________________________________________________ 
Revised Draft- Subject to Change 139

 HABITAT SUITABILITY INDEX MODEL 

 CALIFORNIA VOLE (Microtus californicus) 

 U.S. Fish and Wildlife Service 

 Division of Ecological Services 

 Sacramento, California 
Revised Draft- Subject to Change 140

Cover-Type Life Requisite Habitat Variable

 

 Height of herbaceous vegetation (V1) 

Annual Grassland Food/Cover Percent cover of herbaceous vegetation (V2) 

Seasonal Wetland Reproduction Soil Type (V3) 

 Height of herbaceous vegetation (V1) 

 Percent cover herbaceous vegetation (V2) 

Riparian Woodland Reproduction 

Oak Woodland Food/Cover Soil Type (V3) 

 Presence of logs and other types of cover (V4) 

Variable Cover-Type Sampling Technique

V1 - Height of herbaceous Annual Grassland Average vegetation height in 1 m

2

 quadrat 

 Oak Woodland 

 Riparian Woodland 

 Seasonal Wetland 

 

 

V2 - Percent cover of Annual Grassland 1 m2

 quadrat 

 herbaceous vegetation Seasonal Wetland 

 Oak Woodland 

 Riparian Woodland 

V3 - Soil Type Annual Grassland Site inspection 

 Seasonal Wetland County Soil Survey 

 Oak Woodland 

 Riparian Woodland 

V4 - Presence of logs and Annual Grassland Visual inspections 

 other types of cover Seasonal Wetland Sample point 

 Oak Woodland 

 Riparian Woodland 
Revised Draft- Subject to Change 141

Variable 1: Height of herbaceous vegetation. 

Assumes: California voles require relatively tall herbaceous vegetation for both food (Gill 1977. Batzil 1986) and 

cover (Ingles 1965). Herbaceous vegetation > 6 in tall is considered optimum. 

Variable 2: Percent cover of herbaceous vegetation. 

 

Assumes: Relatively dense herbaceous vegetation is needed for cover percent cover > 100 percent is considered 

optimum (CDFG undated). 
Revised Draft- Subject to Change 142

Variable 3: Soil type 

Assumes: Friable soils such as silts and loams are optimum because voles can dig their burrows (Ingles 1965). 

Soils such as sands and clays are not optimum. 

Suitability Index (SI) 

SI = 1.0 if soil type is silty or loamy and friable. 

SI = 0.5 if soil type is not silty or loamy and is moderately friable 

SI = 0.2 if soil type is not silty or loamy and is not friable. 

Variable 4: Presence of logs and other cover types within the sample area. 

Assumes: California voles will use logs, brush piles, and rocks for cover in addition to their burrows (California 

Department of Fish and Game). These sources of cover are more important in woodland habitats than grassland and 

wetland habitats. 

SI = 1.0 logs, brush piles, and rocks are abundant and well distributed throughout the sample site (e.g., > 4 per 

sample site). 

SI = 0.7 if logs, brush piles, and rocks are moderate abundant and distributed throughout the sample site (e.g., 2-4 

per sample site). 

SI = 0.4 logs, brush piles, and rocks are absent or sparsely distributed throughout the sample site (< 1 per sample 

site). 

SI = 0.1 if logs, brush piles, matted vegetation, and/or rocks are absent From sample area. 

HSI Determination

For annual grasslands and seasonal wetlands. 

HSI = V1 + V2 + V3

 3 

For oak woodlands and riparian woodlands: 

HSI = V1 + V2 + V3 +V4

 4 

All variables are assumed to contribute equally to the availability of a given habitat type for the California vole. 

Water is assumed not be a limiting factor and is represented by the herbaceous vegetation variables. 

Model Applicability 
Revised Draft- Subject to Change 143

This model is a hypothesis of the relationships between various attributes of grassland, wetland, and oak riparian 

woodland habitats and the suitability of these habitats to California voles. The model is designed for use in the 

Central Valley of California up to 2,500 feet in elevation. California voles are permanent year-round residents, and 

this model can be applied to these habitats at all times of the year. 

Literature Cited

Batzil, G.O. 1986. Nutritional ecology of the California vole: effects of food quality on reproduction. Ecology 

67:406-412. 

California Department of Fish and Game. Undated. California wildlife and fish habitat relationships system species 

note: California vole (Microtus californicus). California Dept. of Fish and Game, Sacramento, CA. 4 pp. 

Gill, A.E. 1977. Food preference of the California vole, Microtus californicus. J. Mammal. 58:229-233. 

Ingles, L.G. 1965. Mammals of the Pacific States. Stanford University Press, Stanford, California. 506 pp.
Revised Draft- Subject to Change 144

HABITAT SUITABILITY INDEX MODEL 

 Plain Titmouse (Parus inornatus) 

by 

Michael Long and Daniel Strait 

U.S. Fish and Wildlife Service 

Division of Ecological Services 

Sacramento, California 

June 1989 
Revised Draft- Subject to Change 145

Habitat Use Information 

General

The plain titmouse inhabits oak and piñon-juniper woodlands from Oregon south and west to Texas. It is a yearround resident, and maintains a territory throughout the year. The species is generally a secondary cavity nester, 

although it may occasionally excavate its own hole.

Food

As a group, titmice take a wide variety of foods, but they are considered insectivorous during the summer, and 

consumers of fruit, seeds, and some insects in the winter (Ferrins 1979). Root (1967 - cited by Verner 1979), found 

that a large proportion of their food consisted of plant material and arthropods living on the bark of trees. Wagner 

(1981) found the plain titmouse took a great variety of arthropod taxa. 

The titmouse is primarily a bark forager, although it also forages on tree foliage and occasionally on the ground 

(Hertz et. al. 1976). Most foraging by this species is done between 0-30 feet (0-9 m) of the ground (Wagner 1981; 

Hertz et. al. 1976). Hertz et al. found that plain titmice showed a preference for foraging in blue oaks (Quercus 

douglasii) over coast live oaks (Q. agrifolia). Hertz et. al. (1976) attributed the avoidance of live oaks to their 

smooth bark which is poor habitat for arthropods. Block and Morrison (1986) also found the titmouse to use blue 

oaks more than valley oaks (Q. lobata), black oak (Q. kelloggii), and canyon live oak (Q. chrysolepis) for foraging 

at Tejon Ranch, California. The plain titmouse will forage extensively in live oaks however, especially when other 

oak species are not present (Dixon 1964). 

Reproduction

The plain titmouse is a secondary cavity nester, nesting in natural cavities, old woodpecker holes, or nest boxes. It 

prefers natural cavities over excavated cavities (Wilson, pers. comm.). Bent (1946) reported nests from 3-32 feet (1-

10 m) above the ground. Bent, citing Dawson (1923), reported the titmouse to occasionally excavate its own nest 

cavity in blue oaks. The plain titmouse prefers wooded areas with intermediate to high percentage canopy coverage 

dominated by blue, live and valley oaks (Verner and Boss 1980). 

Cover

Cover is provided by the oak woodlands and riparian areas in which the plain titmouse lives. Roost sites are 

provided by natural cavities, old woodpecker holes, or by dense foliage which simulates a cavity (Dixon 1949). 

Interspersion

Plain titmice maintain year-round territories. Three territories observed by Hertz et. al. (1976) averaged 2.0 acres 

(0.8 ha) in California oak woodland. Dixon (1949) found 12 territories ranged located primarily in live oak 

woodland. These territories ranged in size from 3.3-12.5 acres (1.3-5.1 ha) with an average size of 6.3 acres (2.6 

ha). According to Dixon (1956) 2.5 acres (1.0 ha) would probably be close to an absolute minimum size for a 

territory. 

Water Requirements

In a study by Williams and Koenig (1980), the plain titmouse was classified as an occasional drinker. 

Model Applicability

This model was developed for use in evaluating habitat suitability of oak savannah, oak woodland, and riparian 

woodland in Merced, Fresno, Stanislaus, and San Benito Counties in California from 500 - 2,500 ft in elevation. 

The basic assumptions for using the model are that meeting the reproductive needs of the plain titmouse will take 

care of its cover and food needs throughout the year. This assumption seems warranted. Verner (1979) believes that 

proper management for oaks for breeding birds should also provide the habitat needs for species that use oaks at 

other times of the year. In addition, it is assumed that water is not a limiting factor. It is assumed that the model is 

valid for use in riparian areas as well as the oak woodlands despite the fact that the model was initially developed for 

oak woodlands. 
Revised Draft- Subject to Change 146

Model Description

Little quantitative data were found on the habitat needs of the plain titmouse. The most useful information was the 

information on habitat factors related to breeding for the species presented by Ohmann and Mayer (1986). Using 

data from the California Wildlife Habitat Relationships data base and the Forest Inventory and Analysis Research 

Unit inventory, Ohmann and Mayer developed a habitat suitability index model for the plain titmouse from which 

Variable 1 was derived. 

Variable 1. Tree diameter. (A tree is defined as a woody plant species 16 feet high or greater) 

Ohmann and Mayer found tree size and percent canopy closure to be the major variables determining suitability of a 

habitat for the plain titmouse. Our model will assume that the diameter of a tree and the size of the canopy are 

correlated to the extent that they can be considered a single variable to be represented in this model by diameter at 

breast height (DBH). Presumably this variable best represents older trees with more cavities for nesting and greater 

bark surface which supports a greater prey base. 

Variable 2. Trees per acre. 

Plain titmouse abundance was found to increase as the number of trees increased (Wilson, pers. comm.). This may 

be particularly important in areas of low to moderate canopy cover. Studies at the Hopland, California field station 

found titmouse abundances to peak in areas with 60 trees/acre. 
Revised Draft- Subject to Change 147

Both Variables 1 and 2 relate directly to the extent of a stand's canopy closure such that the importance placed on 

canopy closure by Ohmann and Mayer is incorporated into this model through the use of Variables 1 and 2. 

Variable 3. Percent composition of tree species that are oaks (Quercus). 

Verner and Boss (1980) stated that the plain titmouse prefers stands dominated by blue, live and valley oaks. We 

have been unable to find and studies documenting the presence of the plain titmouse in an area without a major 

proportion of oaks. For the sake of this model then, we will consider the presence of oaks to be a life requisite such 

that the optimum titmouse habitat is one dominated by oaks. 

HSI Determination
Revised Draft- Subject to Change 148

In each sample area, tree diameter is measured along with the number of trees per acre and the percentage of those 

trees that are oaks. The Habitat Suitability Index for the sample site is then determined using the following formula: 

HSI = V1 + V2 + V3

 3 

Suggestions for Applying the Model

1. The tree diameter classes for calculating Variable 1 (DBH) were not specified by Ohmann and Mayer. 

Therefore, all trees within the sample plot should be included in the DBH determination. 

2. If no trees, 4-inch DBH or greater, are found in the sample plot, the HSI for the sample plot is 0.0. A 4-inch 

DBH tree is probably about the smallest tree that could have a cavity of sufficient size for the titmouse. 

3. Ideally, all tree species in the study area should be fully leafed out when applying the model. Therefore, the 

best time for sampling is spring and summer. 

Literature Cited 

Bent, A.C. 1946. Life histories of North American jays, crows and titmice. U.S. Natl. Mus. Bull, No. 191. 495 pp. 

Block, W.M. and M.L. Morrison. 1986. Conceptual framework and ecological considerations for the study of birds 

in oak woodlands. In: Proceedings of the Symposium on Multiple-use Management of California's Hardwood 

Resources, November 12-14, 1986, San Luis Obispo, California. Gen Tech. Rep. PSW-100, Berkeley, California. 

Pacific Southwest Forest and Range Experiment Station, For. Service., U.S. Dept. Agric.: 1987. 

Dixon, K.L. 1949. Behavior of the Plain Titmouse. Condor 51:110-136. 

Dixon, K.L. 1954. Some ecological relations of chickadees and titmice in Central California. Condor 56:113-124. 

Dixon, K.L. 1966. Territoriality and survival in the Plain Titmouse. Condor 58:169-182. 

Hertz, P.E., J.V. Remsen, and S.I. Zones. 1976. Ecological complimentary of three sympatric parids in California 

oak woodland. Condor 78:307-316. 

Ohmann, J.L. and K.E. Mayer. 1986. Wildlife habitat of California's hardwood forests - linking extensive inventory 

data with habitat models. In: Proceedings of the Symposium on Multiple-use Management of California's 

Hardwood Resources, November 12-14, 1986, San Luis Obispo, California. Gen. Tech. Rep. PSW-100, Berkeley, 

California. Pacific Southwest Forest and Range Experiment Station, For Serv., U.S. Dept. Agric.:1967.

Perrins, D.M. 1979. British Tits. William Calins and Sons and Co. LTD, Glasgow. 304 pp. 

Root, R.B. 1967, The niche exploitation pattern of the Blue-grey Gnatcatcher. Ecol. Monogr. 37:317-350. 

Verner, J. 1979. Birds of California's oak habitats - management implications. In: Plumb, Timothy R., tech. coord. 

Proceedings of the Symposium on the Ecology, Management, and Utilization of California Oaks, Claremont, 

California, Calif., June 26-28, 1979. Gen. Tech. Rep. PSW-44. Berkeley, Ca: Pacific Southwest Forest and Range 

Experiment Station. For. Serv., U.S. Dept. of Agri: 1980:246-264. 
Revised Draft- Subject to Change 149

Verner, J. and A.S. Boss, tech. coords. 1980. California Wildlife and Their Habitats: Western Sierra Nevada. Gen. 

Tech. Rep. PSW-37. Pacific Southwest Forest and Range Experiment Station, For. SHP LaserJet Series 

IIHPLASEII.PRSdland. Auk 97:339-350. 

Wagner, J.L. 1981. Seasonal change in guild structure: oak woodland insectivorous birds. Ecology 62:973-981. 

Wilson, R.A. Personal communication citing the California Dept. of Forestry publication. Silvicultural options in 

managed oak woodlands to benefit breeding birds. Humboldt State University, Arcata, CA.
Revised Draft- Subject to Change 150

 HABITAT SUITABILITY INDEX MODEL 

 BOBCAT (Felis rufus) 

 Pacific Gas and Electric Company 

1986 
Revised Draft- Subject to Change 151

Geographic Area: This HSI Model was developed for use on the west slope of the Sierra 

Nevada in Fresno County, California. 

Season: This model was developed to evaluate year-round habitat suitability for the bobcat 

(Felis rufus). 

Cover Types: This model was designed to evaluate habitat suitability for the bobcat in the 

Chaparral cover type (terminology follows that of Verner and Boss 1980). 

Guild: Feeding Breedinq

Surface Subsurface 

Equation: HSI = (V1 + V2 + V3 + V4)

 5 

V1 - Percent Shrub Cover 
Revised Draft- Subject to Change 152

V2 - Herbaceous Cover 

V3 - Degree of Patchiness 
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V4 - Rock Outcroppings 
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Appendix B 

Federal Endangered and Threatened Species that Occur in 

or may be Affected by Projects in Placer, Sacramento, and El Dorado Counties 

Document Number: 060915114416; Database Last Updated: September 15, 2006 

Red-Legged Frog Critical Habitat - The Service has designated final critical habitat for 

the California red-legged frog. The designation became final on May 15, 2006. 

County Lists 

Listed Species 

Invertebrates 

Branchinecta conservatio 

Conservancy fairy shrimp (E) 

Branchinecta lynchi 

Critical habitat, vernal pool fairy shrimp (X) 

vernal pool fairy shrimp (T) 

Desmocerus californicus dimorphus 

Critical habitat, valley elderberry longhorn beetle (X) 

valley elderberry longhorn beetle (T) 

Elaphrus viridis 

delta green ground beetle (T) 

Lepidurus packardi 

Critical habitat, vernal pool tadpole shrimp (X) 

vernal pool tadpole shrimp (E) 

Fish 

Hypomesus transpacificus 

Critical habitat, delta smelt (X) 

delta smelt (T) 

Oncorhynchus (=Salmo) clarki henshawi

Lahontan cutthroat trout (T) 

Oncorhynchus mykiss 

Central Valley steelhead (T) (NMFS) 

Critical habitat, Central Valley steelhead (X) (NMFS) 

Oncorhynchus tshawytscha 

Central Valley spring-run Chinook salmon (T) (NMFS)

Critical Habitat, Central Valley spring-run Chinook (X) (NMFS) 

Critical habitat, winter-run Chinook salmon (X) (NMFS) 

winter-run Chinook salmon, Sacramento River (E) (NMFS) 

Amphibians 

Ambystoma californiense 

California tiger salamander, central population (T)

Critical habitat, CA tiger salamander, central population (X) 
Rana aurora draytonii 

California red-legged frog (T) 

Critical habitat, California red-legged frog (X) 

Reptiles 

Thamnophis gigas 

giant garter snake (T) 

Birds 

Haliaeetus leucocephalus 

bald eagle (T) 

Plants 

Calystegia stebbinsii 

Stebbins's morning-glory (E) 

Castilleja campestris ssp. succulenta 

Critical habitat, succulent (=fleshy) owl's-clover (X) 

Ceanothus roderickii 

Pine Hill ceanothus (E) 

Fremontodendron californicum ssp. decumbens 

Pine Hill flannelbush (E) 

Galium californicum ssp. sierrae 

El Dorado bedstraw (E) 

Oenothera deltoides ssp. howellii 

Antioch Dunes evening-primrose (E) 

Orcuttia tenuis 

Critical habitat, slender Orcutt grass (X) 

slender Orcutt grass (T) 

Orcuttia viscida 

Critical habitat, Sacramento Orcutt grass (X) 

Sacramento Orcutt grass (E) 

Senecio layneae 

Layne's butterweed (=ragwort) (T) 

Candidate Species 

Fish 

Oncorhynchus tshawytscha 

Central Valley fall/late fall-run Chinook salmon (C) (NMFS) 

Critical habitat, Central Valley fall/late fall-run Chinook (C) (NMFS) 

Amphibians 

Bufo canorus 

Yosemite toad (C) 

Rana muscosa 

mountain yellow-legged frog (C) 
Birds 

Coccyzus americanus occidentalis 

Western yellow-billed cuckoo (C) 

Mammals 

Martes pennanti 

fisher (C) 

Plants 

Rorippa subumbellata 

Tahoe yellow-cress (C) 

Key: 

• (E) Endangered - Listed as being in danger of extinction. 

• (T) Threatened - Listed as likely to become endangered within the foreseeable future. 

• (P) Proposed - Officially proposed in the Federal Register for listing as endangered or threatened. 

• (NMFS) Species under the Jurisdiction of the National Oceanic & Atmospheric Administration 

Fisheries Service. Consult with them directly about these species. 

• Critical Habitat - Area essential to the conservation of a species. 

• (PX) Proposed Critical Habitat - The species is already listed. Critical habitat is being proposed 

for it. 

• (C) Candidate - Candidate to become a proposed species. 

• (V) Vacated by a court order. Not currently in effect. Being reviewed by the Service. 

• (X) Critical Habitat designated for this species 

Species of Concern - The Sacramento Fish & Wildlife Office no longer maintains a list of 

species of concern. However, various other agencies and organizations maintain lists of 

at-risk species. These lists provide essential information for land management planning 

and conservation efforts. See www.fws.gov/sacramento/es/spp_concern.htm for more 

information and links to these sensitive species lists.
Revised Draft- Subject to Change 181

Appendix C 

Summary Table of Impacted Acres by Cover-Type 
Auxiliary Spillway

6-gate Spillway 4-gate Spillway Tunnel Fuseplug

Chaparral 0.21 0.21 0.21 0.21Oak/Grey Pine Woodland 1.07 1.77 1.46 1.46Riparian Woodland 1.66 1.51 1.04 1.38

Flood Damage Reduction

Dikes 1-3 Raise 

(COE)

Inundation 3.5-foot 

Raise

Inundation 4-foot 

Raise

Inundation 7-foot 

Raise

Inundat

R

Chaparral 32.20 34.30 40.80 3

Oak/Grey Pine Woodland 8.46 781.50 820.20 935.10 13Riparian Woodland 0.02 45.47 48.68 56.50 4Seasonal Wetland 0.00 0.58 0.58 0.58 0

Dam Safety

Contractor and 

Construction 

Sites Haul Roads

Borrow and 

Stockpile 

Dike Construction 

Zones (BOR)

Chaparral 0.47 0.26Oak/Grey Pine Woodland 11.06 11.06 6.47 16.04Riparian Woodland 2.44 2.44 27.00 1.93

Seasonal Wetland 0.89 0.28

Impacted Acres by Cover-Type for the Various Components of the Folsom D

Project
Revised Draft- Subject to Change 183

Appendix D 

Fish and Wildlife Coordination Act Report for the American River Watershed 

Investigation Folsom Dam Outlet Modification Project