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BIOLOGICAL REPORT 82(10 .96)

MAY 1985

HABITAT SUITABILITY INDEX MODELS:

CACTUS WREN

- _.

SK

361

. U54

no . 82- 10 .96

and Wildlife Service

Department of the Interior
This is one of the first reports to be published in the new "Biological

Report" series. This technical report series. published by the Research

and Development branch of the U.S. Fish and Wildlffe Service. replaces

the "FWS/OBSII series published from 1976 to September 19.84. The Biological Report series is designed for the rapid publication of reports with

an application orientation. and it continues the focus of the FWS/OBS

series on resource management issues and fish and wildlife needs.
MODEL EVALUATION FORM

Habitat models are designed for a wide variety of planning applications where habitat information is an important consideration in the

decision process. However, it is impossible to develop a model that

performs equally well in all situations. Assistance from users and

researchers is an important part of the model improvement process. Each

model is published individually to facilitate updating and reprinting as

new information becomes available. User feedback on model performance

will assist in improving habitat models for future applications. Please

comp 1ete thi s form fo 11 owi ng app 1i cat i on or revi ew of the mode 1. Fee1

free to include additional information that may be of use to either a

model developer or model user. We also would appreciate information on

model testing, modification, and application, as well as copies of modified

models or test results. Please return this form to:

Habitat Evaluation Procedures Group

U.S. Fish and Wildlife Service

2627 Redwing Road, Creekside One

Fort Collins, CO 80526-2899

Thank you for your assi stance.

Species -------

Habitat or Cover Type(s)

Geographic

Location ----------------

Type of Application: Impact Analysis Management Action Analysis ___

Baseline Other

Variables Measured or Evaluated

Was the species information useful and accurate? Yes No

If not, what corrections or improvements are needed?------------
Were the variables and curves clearly defined and useful? Yes No

If not, how were or could they be improved?

Were the techniques suggested for collection of field data:

Appropriate? Yes No

Clearly defined? Yes No

Easily applied? Yes No

If not, what other data collection techniques are needed?

Were the model equations logical? Yes No

Appropriate? Yes No

How were or could they be improved?

Other suggestions for modification or improvement (attach curves,

equations, graphs, or other appropriate information)

Additional references or information that should be included in the model:

Model Evaluater or Reviewer Date ------------

Agency

Address

Telephone Number Comm:----------- FTS
Biological Report 82(10.96)

May 1985

HABITAT SUITABILITY INDEX MODELS: CACTUS WREN

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 20240
This report should be cited as:

Short, H. L. 1985. Habitat suitability index models: Cactus wren. U.S.

Fish Wildl. Servo Biol. Rep. 82(10.96). 15 pp.
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 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. 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 model 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 speci es. The mode 1 presents thi s broad data base ina 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 a species,

as well as in providing an estimate of the relative quality of habitat for

that speci es. User feedback concern i ng model improvements and other suggestions that may increase the utility and effectiveness of this habitat-based

approach to fish and wildlife planning are encouraged. Please send suggestions

to:

Habitat Evaluation Procedures Group

Western Energy and Land Use Team

U.S. Fish and Wildlife Service

2627 Redwing Road

Ft. Collins, CO 80526-2899

iii
iv
CONTENTS

Page

PREFACE iii

ACKNOWLEDGMENTS vi

HABITAT USE INFORMATION 1

Genera 1 1

Food 1

Water 2

Cover 2

Reproduct ion 3

Interspersion 4

HABITAT SUITABILITY INDEX (HSI) MODEL........... 4

Model Applicability............................................... 4

Mode 1 Descri pt ion 6

Appl ication of the Model 11

SOURCES OF OTHER MODELS 14

REFERENCES 14

v
ACKNOWLEDGMENTS

I gratefully acknowledge Dr. Ralph J. Raitt, Dept. of Biology, New Mexico

State University, Las Cruces, for his critical review of this cactus wren

model. The cover of this document was illustrated by Jennifer Shoemaker. Word

processing was provided by Carolyn Gulzow, Dora Ibarra, and Elizabeth Graf.

vi
CACTUS WREN (Campylorhynchus brunneicapillus)

HABITAT USE INFORMATION

General

The cactus wren (Campylorhynchus brunneicapillus) is resident (American

Ornithologists' Union 1983:524): "from southern California ... , southern

Nevada, southwestern Utah, centra 1 Ari zona, central New Mexi co, and centra 1

and southern Texas south to southern Baja California, the Pacific lowlands to

northwestern Sinaloa ... , and in the Mexican highlands to Michoacan, the State

of Mexico and Ht l da l qo".

The cactus wren is considered, in the guilding context of Short (1983),

to nest usually in shrub vegetation (the mi dstory 1ayer) and to be both a

primary and a secondary consumer within the midstory and terrestrial surface

layers.

Food

Bent (1948) summarized food habit data for cactus wrens sampled from

southern California in July through January. Animal matter constituted 83%

and vegetable matter constituted 17% of the stomach contents; no differentiation was made by month of collection. The animal matter in 41 stomachs

consisted of 27% beetles, 27% hymenoptera (wasps, bees, and ants), 15% grasshoppers, 5% hemiptera (bugs), 5% lepidoptera (caterpillars), and 3% spiders.

Seeds, such as those from sumac (Rhus spp.), filaree (Erodium spp.), and

fiddleneck (Amsinckia spp.), and fruit pulp from cactus (Opuntia spp.), elderberry (Sambucus spp.), and buckthorn (Rhamnus spp.) were important foodstuffs

during cooler months when many animal items were unavailable.

Anderson et al. (1982) determined that animal matter comprised 96.3% of

the contents of the gizzards of 12 wrens collected from March through October

and 90.1% of the contents of the gizzards of five wrens collected between

November and February. The birds all were collected in riparian vegetation

along the lower Colorado River.

Wrens glean among branches of midstory shrubs and al so forage on the

ground (Anderson and Anderson 1973). They frequently forage on paloverde

(Cercidium spp.), mesquite (Prosopis spp.), cholla (Opuntia spp.), and saguaros

(Carnegiea gigantea) by midsummer, after deciduous, perennial vegetation has

leafed out.

1
Cactus wrens in Chihuahuan desert habitats foraged principally on grasshoppers (Trimerotropis spp.) when they were feeding nestlings (Marr 1981).

They foraged on the ground in open areas with sparse, low vegetation, and in

and under shrubs (Raitt, pers. comm.).

Water

Pools of water rarely are available in the desert during the hottest

months of the year. Adult cactus wrens infrequently drink free water in July

and August (Anderson and Anderson 1973); they apparently obtain the necessary

water from their diet. Immature wrens, on the other hand, often have been

observed drinking water in August. Adult birds begin to drink free water in

September, and the rate of water consumption apparently increases to high

levels in December and January (Anderson and Anderson 1973). The insect food

eaten during the winter months presumably does not have as high a water content

as does the insect food consumed during the summer. Rainfall totals in the

Sonoran desert decrease during the spring. However, succulent plant growth

and associated insect populations increase in biomass, and foraging adult

wrens apparently can satisfy their water needs from their diet.

Ricklefs and Hainsworth (1968) stated that the cactus wren relies exclusively on water obtained from its food during the period of greatest heat

stress. The cactus wren also conserves water by behavioral mechanisms, such

as reduced activity and selection of cooler microhabitats.

Cover

The cactus wren is associated with the shrub life form within the warm

deserts of the southwestern United States and northern Mexico. The abundance

of appropriate shrub vegetation presumably affects the distribution and abundance of the wren within its range. Activities that impact suitable desert

scrub vegetation should be expected to directly impact the numbers and

distribution of any resident wrens.

Habitat for the cactus wren consists of thorny shrubs and trees or the

more arborescent species of cacti that occur on sunny hillsides and mesas next

to mountains and along gravelly watercourses, primarily in lowlands (Bent

1948). Selection of nest sites seems to favor placement among the protective

spi nes of desert vegetation. Cactus wrens use nests for both reproductive

purposes and roosting. A covered roosting nest is used throughout the year

(Anderson and Anderson 1973). The location of roosting nests in the Saguaro

National Monument near Tucson, Arizona, was associated with the presence of

cholla cactus. About 65% of 528 roosting nests were in jumping cholla

(Opuntia fulgida), 12% in Staghorn cholla (Q. versicolor), and 13% in saguaros.

The use of a variety of desert shrubs and cacti as reproductive nest sites is

described below.

Cover, such as shrubs and trees, also is important in providing shade and

cooler microhabitats, which cactus wrens use when temperatures within desert

habitats are high (Ricklefs and Hainsworth 1968). Wrens ignore open ground at

thi s time and hunt on shady ground or in the lower branches of mi dstory

vegetation.

2
Reproduction

The cactus wren apparently mates for life and, at least in part of the

Sonoran desert, remains in its established territory for life. Cactus wrens,

at least in the Chihuahuan desert, however, may not be resident on their

breeding territories throughout the year (Raitt, pers. comm.). The wrens may

remain in the general area, but overwinter in more mesic riparian habitats.

Reproductive nests characteristically are bulky, cylindrical to footballshaped, and comprised of plant stems and grasses, which rest horizontally on

branches of cacti or thorn trees. A passageway up to 15 cm long leads into

the closed-roofed, Il ret ort - shapedll nest (Bailey 1922). The nest usually is

placed in bushes, thickets, cacti (especially prickly pear and cholla, Opuntia

spp.), yucca (Yucca spp.), catclaw acacia (Acacia greggii), mesquite, and

various other thorny shrubs in Texas (Oberholser 1974); in yucca (Bent 1948),

little-leaf sumac (Rhus microphylla), whitethorn (Acacia constricta) (Marr

1981), mesquite, condalia (Condalia spp.), catclaw mimosa (Mimosa biuncifera),

desert willow (Chilopsis linearis), and netleaf hackberry (Celtis reticulata)

(Raitt, pers. comm.) in New Mexico; in the largest specimens of cholla and

prickly pear, large bushes, such as sugar sumac (R. ovata), and occasionally

in fruit trees in southern California (Bent 1948); and in a variety of desert

shrubs and cacti in southern and centra 1 Ari zona. Jumpi ng cho11 a was the

preferred nest site in desert shrublands near Tucson, Arizona; in the cholla

meadows of the Santa Rita Experimental Range south of Tucson; and in the

saguaro-paloverde (f. microphyllum) cacti association of the Saguaro National

Monument east of Tucson (Anderson and Anderson 1973). Nearly half of the

breeding nests were placed in jumping cholla; 11% in staghorn cholla; 3% in

paloverdes; and about 35% in the crotches, holes, or stumps of saguaro cacti

(Anderson and Anderson 1973:63). Nests in cacti were from 0.9 to 2.3 m above

ground, nests in paloverdes were 2 to 3.7 m above ground, and nests in saguaros

frequently were 3 to 4.3 m above ground (Anderson and Anderson 1973). Thorny

trees and bushes, especially catclaw (~. greggii) and jujube (Ziziphus spp.),

were used extensively, whereas mesquite and dense, shrubby hackberry (Celtis

spp.) were used only occasionally for nest sites at about the 1,210 m level at

the base of the Santa Rita mountains south of Tucson (Bailey 1922).

The wren begi ns to defend breedi ng territori es in January in southern

Arizona, and breeding territories frequently are established by mid-February.

The size of breeding territories averaged about 1.0 ha on the Saguaro National

Monument, about 1.9 ha on the cholla meadows of the Santa Rita Experimental

Range (Anderson and Anderson 1973), and about 4.7 ha in areas of the Chihuahuan

desert (Marr 1981).

Breedi ng nests usually are bui 1tin 1ate February and early March, and

egg laying commences by early to mid-March in the Tucson area (Anderson and

Anderson 1973). Egg laying may be initiated by an adequate rainfall which,

when coupled with rising temperatures, produces an abundant growth of ephemeral

plants and their associated insect fauna. The breeding season, in years of

normal rainfall, may last from March to July.

3
Cactus wrens in southcentral New Mexico seemed to initiate clutches

during a period of high temperatures during early spring. Marr and Raitt

(1983) hypothesized that this warm period would likely precede periods of

favorable temperatures that would occur during the period when the wren would

be feeding nestlings. Cactus wrens in this area fed heavily on band-winged

grasshoppers (Trimerotropis spp.), which emerged after soils warmed in the

spring. Cactus wrens started to develop eggs during the last week in April.

Each pair of wrens within the Tucson study area averaged two clutches/year

(Anderson and Anderson 1973). Incubation was 16 to 17 days, and nestlings

remained in the nest an additional 19 to 23 days after the eggs were hatched.

The second clutch frequently was initiated about 1 week after the first clutch

was fledged. Both parents fed insects to nestlings. About four young were

fledged/pair/year, with higher nesting success in chollas than in saguaros

(Anderson and Anderson 1973). Young birds, after fledging, use roosting nests

within the parental territory and initiate roosting nest construction at about

4 months of age (Anderson and Anderson 1973). Dispersal of immature birds may

be voluntary or initiated by the parents. Dispersal movement is only as far

as necessary to find an unoccupied territory (Anderson and Anderson 1973).

Interspersion

The cactus wren is attracted to the shrub life form (Dixon 1959) within

the hot deserts and arid lands of the southwestern United States and northern

Mexico. They apparently are most abundant where arborescent cacti and prickly

shrubs are most abundant. The life form of thorny bushes and trees and

arborescent cacti characteri sti ca lly produces an open midstory canopy and an

open understory. Thi s habitat structure is preferred by the wren, and no

interspersion of different vegetation structures or cover types is considered

necessary.

HABITAT SUITABILITY INDEX (HSI) MODEL

Model Applicability

Geographic area. This model for the cactus wren was developed from

descriptive data collected largely from Sonoran desert habitats. The model

shou1d have app1i cabil i ty throughout the Sonoran bi ogeographi c provi nee of

southcentral and southwestern Arizona and extreme southeastern California and

may also have some utility in the Mohavian province in southeastern California

and southern Nevada, the Chihuahuan province in southeastern Arizonia, southern

New Mexico and west Texas, and the Tamaulipan province of south Texas. The

approximate range of the Sonoran biogeographic province in the United States

as well as the range of the cactus wren in the southwestern United States is

indicated in Figure 1.

Season. This model will provide an estimate of the quality of habitat

for the cactus wren at least during the breeding season and perhaps throughout

the year. The bird builds conspicuous nests in preferred plants and frequently

4
General distribution of the cactus wren

- Sonoran desert

Figure 1. Distribution of the cactus wren in the southwestern United

States (adapted from a map prepared by D. B. Inkley and C. M. Raley,

Wyoming Cooperative Fishery and Wildlife Unit, Laramie, from information in American Ornithologists' Union 1983). The distribution of the

Sonoran biogeographic province within the United States also is

indicated on the map (after Brown 1982:13).

5
uses these nests as roosting sites throughout the year. Thus, the presence of

nests may be an indicator of habitation by cactus wrens, and the number of

useable nests in a given area of the Sonoran desert may be a rough indicator

of the number of cactus wrens present in that area (Anderson and Anderson

1973). This model estimates the suitability of habitat structure in terms of

providing nest sites for the wren.

Cover types. This model was developed for application in habitats

described as desertic shrublands (OS) and desertic woodlands (OW) (U.S. Fish

and Wildlife Service 1981).

Minimum habitat area. The rm mmum habitat area is the area required

before that habitat will be occupied by cactus wrens. The cactus wren is a

nonmigratory species that defends a territory. The minimum territory reported

in the 1iterature was about 0.4 ha (Anderson and Anderson 1973). A habitat

block of desert shrubland must, therefore, be at least 0.4 ha to be considered

as potential habitat for the cactus wren in this model.

Verification level. This model was developed from descriptive information

about nesting and foragi ng .habi tats pub1i shed in the 1iterature. The HSI

derived from this model describes the potential of an area as habitat for the

cactus wren. The model is des i gned to rank the sui tabi 1i ty of vari ous thorn

forest and semi-desert habitats as would a biologist with expert knowledge

about the wren. The model should not be expected to rank habitats in the same

way as popul ati on data because many nonhabitat-re 1ated criteri a can

significantly impact populations of wildlife species.

Model Description

Overview. This HSI model estimates the quality of arid shrubland habitats

in the southwestern United States for the cactus wren. The model assumes that

the wren prefers thorny shrubs and arborescent cacti for nest sites and

actively selects that vegetative structure.

The following sections provide documentation of the logic and assumptions

used to translate habitat information for the cactus wren into the variables

selected for the HSI model. Specifically, these sections describe: (1) the

variables used in the model; (2) the assumed suitability level of each

variable; and (3) the assumed relationships between variables.

The logic used to develop the HSI model is illustrated in Figure 2.

Roost and reproductive component. The nonmigratory cactus wren is assumed

to be restricted to arid savanna, open thorn forest, and semidesert cactus and

deciduous tree cover types (Selander 1964) in the southwestern United States.

These cover types provi de the combi nat i on of a scattered mi dstory 1ayer of

shrubs with dense foliage for nest placement and an insect population that is

available as food throughout the year.

6
Habitat is arid savanna,

open thorn forest, or

semi desert cactus and

deciduous tree cover

types in southwestern

United States.

y

Blocks of appropriate

habitat are at least

0.4 ha in area.

y

Habitat structure

provides potential

nest sites 0.9 to

4.3 m above ground.

y

Types of vegetation

vary in utility as

nest sites for the

cactus wren.

Density of midstory

vegetation may modify

the utility of habitats

for cactus wrens.

HSI for the cactus

wren.

N

N

N

Habitat is not

suitable nesting

habitat for the

cactus wren.

Figure 2. Logic used to develop the HSI model to estimate the quality

of habitat for the cactus wren.

7
The cactus wren is highly territorial. The mlnlmum territory size cited

in the 1i terature is about 0.4 ha (Anderson and Anderson 1973). A patch of

suitable habitat must, therefore, be 0.4 ha to be considered as habitat for

the cactus wren in this model.

The cactus wren makes and repairs nests that are used as roost sites

throughout the year and as reproductive sites in spring and early summer. The

useful ness of a habi tat for nest placement is dependent on the presence of

vegetation where the large, bulky, retort-shaped nest can be sturdily positioned horizontally on limbs. The cactus wren often selects vegetation with

spines that offer protection to the nest and its occupants. Suitable midstory

vegetation must provide nest sites within 0.9 to 4.3 m of the ground for the

area to be considered habitat for the cactus wren.

The abundance of cactus wrens in a habitat is assumed to be represented

by the abundance of preferred midstory vegetation that provi des nest sites.

The species of midstory vegetation are represented by Variable 1 (VI), and the

abundance of each species of suitable midstory vegetation is represented by

Variable 2 (V2) in the model. Suitability indices for Variables 1 and 2 may

be regionally biased because they were developed from descriptive data gathered

mostly in Sonoran desert habitats, which represent only a fraction of the

range of the cactus wren (Fig. 1). For example, high Sl t s are assigned to

jumping chollas and saguaros in VI, below, even though these species have a

restri cted range in the southwestern United States. The SI I S suggest that

these plant species are highly preferred as nest substrates of the cactus

wren. Other species of midstory vegetation are regionally important to the

wren. Their relative importance, compared to that of jumping chollas or

saguaros, is estimated by the SIrs listed in VI.

Cactus wrens, in Sonoran desertscrub habitats near Tucson, commonly build

their nests in the periphery of the crown of jumping cholla (Anderson and

Anderson 1973). Over 48% of 154 breeding nests and 65% of 528 roosting nests

were located in jumping cholla on the Saguaro National Monument. Hensley

(1959) found 21 of 22 active nests on the Organ Pipe National Monument in

jumping cholla. It appears that cactus wrens "select" suitable jumping cholla

as nest sites when this cactus is available in habitats. Therefore, jumping

cholla is assigned an SI of 1.0 in VI. The range of jumping cholla in the

United States is restricted to southwestern and southcentral Arizona (Lamb

1975).

Thi rteen percent of the roosting nests and about 36% of the breedi ng

nests of the cactus wren on the Saguaro National Monument were located in

saguaro cacti (Anderson and Anderson 1973:63). Saguaros were used about 74%

as frequently as jumping cholla as sites for breeding nests. Therefore,

saguaros are assigned an SI of 0.7 in VI. A variety of other chollas occur

throughout the Southwest and may be used as nest si tes by the cactus wren.

For example, chollas are uncommon in southcentral New Mexico but those of

large stature are often selected as nest sites by the cactus wren (Raitt,

pers. comm.). Staghorn cholla (0. versicolor) was used about 23% as frequently

as was jumping cholla as a site for breeding nests within the Saguaro National

Monument (Anderson and Anderson 1973:63); this cholla is assigned an SI of 0.2

8
in VI. Other arborescent forms of cacti (including chollas and prickly pears)

and yuccas (Yucca spp.) that occur throughout the Southwest and provide a

horizontal structure suitable for the placement of the bul ky nest of the

cactus wren at a height at least 1 m above the ground are assigned an S1 of

0.2 in VI.

The cactus wren also nests in a variety of thorn shrubs. Bailey (1922)

found that catclaw and jujube were used extensively, and mesquite and hackberry

were used occasionally as nest sites in southern Arizona. Thornbushes with

horizontal branches 1 m from the ground that can support the bulky nest of the

cactus wren are assigned an S1 of 0.2 in VI. Bailey (1922) found that 34 of

64 nests located in catclaw, jujube, and hackberry near the foothills of the

Santa Rita Mountains south of Tucson actually were placed in clusters of

mistletoe (Phoradendron californicum) in those bushes. The presence of

mi st1etoe ina thornbush habitat is assumed to increase the S1 of those

thornbushes to 0.4.

The S1 of VI differs depending on the type of midstory vegetation providing potential nest sites to the cactus wren. S1 1s for different types of

vegetation are estimated as follows:

VI = 1.0 if vegetation consists of jumping cholla at least 1 mtall.

VI = 0.7 if vegetation consists of saguaros with branches.

VI = 0.4 if vegetation consists of thorn shrubs with horizontal branches

1 m and more above ground and if the thorn shrubs a1so conta in

mistletoe.

VI = 0.2 if vegetation consists of thorn shrubs with horizontal branches

1 m and more above ground but wi thout mi st1etoe, or if vegetation

consists of arborescent forms of other cacti and yucca with a structure 1 m and more above ground capable of supporting wren nests.

VI = 0.0 if vegetation consists of open shrubs and other forms of cacti

and yucca « 1 m tall) or vegetation 1 m tall that does not have

the necessary structure to support wren nests.

The quantity of potential nest sites avail ab1e to the cactus wren is

assumed to increase (to some 1imit) as the density of arborescent cacti and

thorn shrubs increases per unit of habitat. A measure of cacti and shrub

density is Variable 2 (V2) in the model. The numbers of individuals in each

of the plant groups identified for VI are counted because of the assumption

that a mix of the plant groups provides a preferred habitat condition for the

cactus wren. Presumably, a mixture of suitable plant species should result in

a higher HS1 for a habitat because it provides a variety of nest sites and

foraging areas. Anderson and Anderson (1973:14) indicated that the number of

saguaros on a study area within the Saguaro National Monument varied from 0

and 98/ha, the number of pal overdes vari ed from 5 and 118/ha, and the total

number of jumping and staghorn cholla plants varied from 0 and 108/ha. These

densities of suitable midstory vegetation species are assumed to represent

optimum conditions for the cactus wren. Therefore, the suitability of a

habitat unit is assumed to increase as the number of large jumping chollas,

9
saguaros, and thorn shrubs per acre of habitat each increase from a to 125.

Densities greater than 125 plants/ha for any of these species presumably does

not provide any additional habitat benefits to the highly territorial wren.

V2 Density of each type

of midstory vegetation

providing potential

nest sites.

1.0

x 0.8

OJ

-0

....... 0.6

>,

+-'

.,...

.,... 0.4 ..c

ttl

+J

.,... 0.2 V1

0.0

25 50 75 100 125

Density ( number / ha )

Suitable nest sites are present in some xeric habitats only where shrubs

and cacti are linearly constrained along narrow arroyos (Raitt, pers. comm.).

The density of arborescent cacti or thorn shrubs is calculated per ha of

arroyo in this case. Cactus wrens occur in arroyos in southcentral New Mexico

if the arroyos contain shrubs along the arroyo without sizeable gaps between

clumps of shrubs (Raitt, pers. comm.).

HSI determination. The Habitat Suitability Index (HSI) for this cactus

wren model is developed from the identification of the types and density of

shrubs or cacti present on a study area. The model reflects the assumptions

that: (1) some shrubs or cacti are more useful to the cactus wren than are

other shrubs or cacti; (2) the suitability of a habitat is related to the

abundance of preferred shrubs or cacti; and (3) the quality of a habitat is

enhanced if a mixture of suitable shrubs and cacti are present because of an

increase in the number of potential nest sites and foraging areas available to

the cactus wren.

The equation for estimating the suitabil ity of habitats for the cactus

wren is listed below. HSI's greater than 1.0 are rounded to 1.0.

10
n

HSI = [l:

i=l

(SIV1. x SIV2.)]1/2 1 1

where i = 1, ... , n = the i ndi vi dua 1 types (or groups) of sui tab1e shrubs

or cacti present on an ha of habitat

VI = the SI for each type (group) of suitable shrubs or

cacti present on an ha of habitat

V2 = the SI that represents the density of each type

(group) of suitable shrubs or cacti

This model was developed from a few data sets that associated the density

of the cactus wren with quantitative or descriptive evaluations of habitat

structure. These data sets are listed below. Anderson and Anderson (1973)

determined that an average of 25 saguaros (range of 0 to 98), 55 paloverdes (0

to 118), and 38 chollas (0 to 108) occurred per ha on a study plot on the

Saguaro National Monument. A cactus wren population of 18 to 38 pairs/40 ha

occurred in this habitat. A second study area, on the Santa Rita Experimental

Range, contained jumping cholla (density unknown) on about one-half the plot

and mesquite, paloverde, and acacia (densities unknown) on the rema i nt nq

portion. A cactus wren population equal to 8 to 22 pairs/40 ha occurred on

the study area. A third plot occurred on residential acreages near Tucson

where jumping chollas averaged about 13 (0-50)/ha and cactus wrens occurred at

a maximum density of about 10 pairs/40 ha. A fourth study area occurred

between Las Cruces and the Organ Mountains in the Chihuahuan desert of southcentral New Mexico (Marr 1981). Nests of cactus wrens were placed in littleleaf sumac and whitethorn in the largest arroyos, which constituted a small

portion of the total test area. Cactus wren populations were about 0.3 pair/

40 ha. Densities were not calculated per ha of arroyo.

The selection of habitat variables, the determination of SIrs for the

habitat variables, and the development of the equation for combining the

habitat variables are best judgments, based on descriptive data published in

the 1iterature, and personal communications with species experts. The model,

if the assumptions are correct, should rank the relative importance of habitats

for the cactus wren, at 1east in the Sonoran desert. More cactus wrens are

expected to occur on habitats with a high HSI because more potential nest

sites are available to wrens in these habitats. It should be remembered that

the population level of the wren may be impacted by criteria other than the

structure of habitat.

Application of the Model

Summary of model variables. A biologist applying this model to estimate

the value of thorn desert habi tat shoul d conduct a terrestri a1 survey to map

midstory vegetation that could provide nest sites for the cactus wren. Aerial

photography is not recommended as a means of identifying potential habitat

11
because the structure and density of suitable desert scrub vegetation

frequently is not clearly discernible on aerial photographs. The following

questions should be considered for each land unit identified as potential

nesting habitat: (1) Is the land unit to be evaluated within the range of the

cactus wren (Fig. 1)? (2) Is the area to be evaluated arid savanna, open thorn

forest, semidesert cactus, or semidesert deciduous tree cover? The Southwestern deserts contain many isolated mountain ranges that receive more

precipitation than surrounding areas and support dense vegetation that is not

suitable habitat for the wren. Thus, the wren has a spotty distribution

pattern in lowlands and mountain foothills that support the preferred vegetation structure; (3) Is the block. of habitat to be evaluated 0.4 ha? and

(4) Is there a midstory vegetation structure that includes thorn shrubs or

arborescent cacti capable of supporting the bulk.y nest of the cactus wren at a

height of 0.9 to 4.3 mabove the ground?

If the answers to all four questions are positive, the biologist should

estimate the suitability of the land unit as habitat for the cactus wren. The

types of midstory vegetation present on the study area (VI) and the density

(number per ha) of each of the types of vegetation (V2), need to be determined.

The SI I S for VI and V2 are combi ned to estimate the HSI for each unit of

habitat (Fig. 3).

Habitat variable

VI Types of midstory

vegetation providing potential nest

site s , ---------,

Life requisite Cover types

V2 Density of each

type of midstory

vegetation providing potential

nest sites.-----~

Reproduct ion]-{Deserti c shrub 1and}-- HSI

Roosting cover Desertic woodland

Figure 3. The relationship of habitat variables, life requisites, and

cover types to an HSI for the cactus wren.

Definitions of variables and suggested field measurement techniques are

presented in Figure 4.

12
Variable (definition)

VI Types of midstory

vegetation providing

potential nest sites.

V2 Density of each type

of midstory vegetation

providing potential

nest sites.

Cover type

DS,DW

DS,DW

Suggested technique

Taxonomic identification of apparently

suitable midstory

species.

Count numbers per unit

area of each type of

plant apparently suitable as a nest substrate for the cactus

wren.

Figure 4. Definitions of variables and suggested measurement techniques.

Model assumptions. The cactus wren has been intensively studied in that

portion of the Sonoran desert near Tucson, Arizona, and little studied in

other portions of its range. My basic assumptions about habitat criteria

important to this wren probably are most pertinent to Sonoran desert habitats

conta in i ng thorny shrubs and arborescent cacti as the doml nant vegetat ion.

The model also may have relevance to appropriate habitats in the Mohavian,

Chi huahuan, and Tamaul i pan provi nces. My descri pti on of habi tat cri teri a

important to the wren are based on descriptive and correlative relationships

published in the literature. My description of habitat quality will be in

error if authors have 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: (1) the distribution of the cactus wren is limited

to desert habitats where arborescent cacti and prickly shrubs are dominant

vegetation and where insect populations are present during most of the year;

(2) species of arborescent cacti and thorn shrubs vary in their value to the

cactus wren and that the density of cactus wrens may increase as the density

of individual species of cacti or thorn shrubs increases; and (3) a land unit

must be at least 0.4 ha in area to be evaluated es habitat for the cactus

wren.

The values for VI and V2 are estimates. The ecological information

available does not seem sufficient to suggest other pertinent variables or

more appropriate values for the present variables. I also assumed that some

sort of arithmetric relationship, like that suggested, adequately combines the

estimates for VI and V2 to provide an HSI for the cactus wren.

13
SOURCES OF OTHER MODELS

Aspects of the biology of the cactus wren have been intensively studied,

especially in the Sonoran desert of southern Arizona. Anderson and Anderson

(1973), for example, reported results of a 30-year study of the general life

history of the wren around Tucson, whereas other authors have studied specific

adaptations of the wren to hot desert conditions (e.g., Ricklefs 1975; Ricklefs

and Hainsworth 1968,1969). Marr (1981) reported on aspects of the breeding

and foraging biology of the wren in the Chihuahuan desert of southcentral New

Mexico. Data about the wren from the Mohave Desert in southern California and

the Chihuahuan and Tamualipan thorn deserts of western and southern Texas are

limited.

No other models descri bi ng the habitat requi rements of the cactus wren

were found in the literature. The number of assumptions required to develop

the present model attest to the 1imitation in our understanding of habitat

requirements of the cactus wren.

REFERENCES

American Dr-n t tho l oqt st s ' Union. 1983. Check-list of North American birds.

6th edition. 877 pp.

Anderson, A. H., and A. Anderson. 1973. The cactus wren. Univ. Arizona

Press, Tucson. 226 pp.

Anderson, B. W., R. D. Ohmart, and S. D. Fretwell. 1982. Evidence for social

regulations in some riparian bird populations. Am. Nat. 120(3):340-352.

Bailey, F. M. 1922.

24:163-168.

Cactus wrens' nests in southern Arizona. Condor

Bent, A. C. 1948. Life histories of North American nuthatches, wrens,

thrashers, and their allies. U.S. Natl. Mus. Bull. 195. 475 pp. + 90

plates.

Brown, D. E. 1982. Biotic communities of the American Southwest - United

States and Mexico. Boyce Thompson Southwestern Arboretum, Superior, AZ.

342 pp.

Dixon, K. L. 1959. Ecological and distributional relations of desert scrub

birds of western Texas. Condor 61:397-409.

Hensley, M. M. 1959. Notes on the nesting of selected species of birds on

the Sonoran desert. Wilson Bull. 71(1):86-92.

Lamb, S. H. 1975. Woody plants of the Southwest. Sunstone Press, Santa Fe,

NM. 177 pp.

Marr, T. G. 1981. Breeding and foraging ecology of the cactus wren in a

variable environment. Ph.D. Diss., New Mexico State Univ., Las Cruces.

79 pp.

14
Marr, T. G., and R. J. Raitt. 1983. Annual variations in patterns of reproduction of the cactus wren (Campylorhynchus brunneicapillus). Southwest.

Nat. 28(2):149-156.

Oberholser, H. C. 1974. The bird life of Texas. Univ. Texas Press, Austin

and London. 1069 pp.

Raitt, R. J. Personal communication. New Mexico State Univ., Las Cruces,

N.M. 88003. Letter dated March 18, 1985.

Ricklefs, R. E. 1969. Temperature regulation in nestling cactus wrens: The

nest environment. Condor 71(1):32-37.

Ricklefs, R. E. 1975. Patterns of growth in birds. III. Growth and development of the cactus wren. Condor 77(1):34-45.

Ricklefs, R. E., and F. R. Hainsworth. 1968. Temperature dependent behavior

of the cactus wren. Ecology 49(2):227-233.

Selander, R. K. 1964. Speciation in wrens of the genus Campylorhynchus.

Univ. California Publ. Zool. 74. Univ. California Press, Berkeley and

Los Angeles. 305 pp.

Short, H. L. 1983. Wildlife guilds in Arizona desert habitats. U.S. Bur.

Land Manage. Tech. Note 362. 258 pp.

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. Servo n.p.

15
50272 -101

REPORT DOCUMENTATION 11. REPORT NO. 1 2•

PAGE Biological Report 82(10.96) 1---------

4. Title end Subtitle

Habitat Suitability Index models: Cactus wren

3. Reclplent·s Accession No.

50 Report Dete

r·1ay 1985

8.

B. Performlns Orsenlzetlon Rept. No.

.-- ..----~--~---------.--------------------I---------------1

7. Author(s)

Henry L. Short

1--------------_._-----------------.------------

9. Performlns O,.anizetiOl'l Neme and Address

Western Energy and Land Use Team

U.S. Fish and Wildlife Service

2627 Redwing Road

Fort Collins, CO 80526-2899

10. Protect/Tesk/Work Unit No.

11. Contrect(C) or Grent(G) No.

(C)

(G)

14.

13. Type 0' Report & Period Covered

-----------_.------

12. Sponsorlns Orsenlzatlon Neme end Address Vles tern Energy and Land Use Team

Division of Biological Services

Research and Development

Fish and Wildlife Service ..--__ _:c:---_=__=___=.--....LD)J;;:ep.a~fthe Interior,-We"h';:~+ -fl, DC 20240

IS.SupplementaryHotes The Biological Report series was initiated in October 1984 to replace the

FWS/OBS series published since 1976. The report number assigned to this model follows

consecutively from the Habitat Suitability Index models published as FWS/OBS-82/10. , now numbered as Bioloqical~ort82(lO, ). __ - __ 16. Abstrect (LImit: 200 words)

A review and synthesis of existing information were used to develop a Habitat

Suitability Index (HSI) model for the cactus wren (Campylorhynchus brunneicapillus).

The model consolidates habitat use information into a framework aporopriate for

field application, and is scaled to produce an index between 0.0 (~nsuitable

habitat) and 1.0 (optimum habitat). HSI models are designed to be used with

Habitat Evaluation Procedures previously developed by the U.S. Fish and Wildlife

Service.

17. Document Anelysls e. Descriptors

Birds

Wildlife

Habitabi 1ity

Mathematical models

b. Identifiers/Open-Ended Terms

Cactus wren

Campylorhynchus brunneicapillus

Habita t sui tab i l i ty

c. COSATI Field/Group

lL Avellablllty Stateme"~ 1'. SecurIty Cless (This Report)

Unclassified

Re1ease un 1imited 20. Security Cless (This Pase)

Unclassified

21. No. 0' Pelles

15

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'ANSI-Z39.18) See 'n.tructlon. on Reveree

-(, U.S. GOVERNMENT PRINTING OFFICE: 198&-579-391/25242 REGION NO.8

OPTIONAL FORM 272 (4-77.

(Formerly NTI5--3S)

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VirgIn ISlandS

Habitat suitability index roodels. 19544

Short, Henry L. SKJi1•U54 no.82

. I m1 WI

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ti l-leilQl)ua'ters D,vl Ion 0 BIologIcal

Sp· ces WastHnglon DC

)( Edster., E ergy and and Use Team

l..eetQ An WV

* ~~atlonal Coastal Ecosys ems Team

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