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ESTIMATE OF TOTAL ACUTE MORTALITY TO BIRDS RESULTING FROM THE 

CHALK POINT OIL SPILL, SWANSON CREEK, MARYLAND, APRIL 7, 2000

INTRODUCTION

On 7 April 2000, approximately 126,000 gallons of a mixture of No. 2 and No. 6 fuel oils 

were released from a ruptured underground pipeline that transported oil to the Potomac Electric 

Power Company Chalk Point Facility, near the town of Aquasco, Maryland (Fig. 1). The spill 

spread into Swanson Creek, a tidal tributary of the Patuxent River. The main stem of the 

Patuxent River, associated shoreline habitats, and other tributaries were also impacted by oil 

after heavy winds on the evening of April 9 blew oil over containment booms placed at the 

confluence of Swanson Creek and Patuxent River. More details about the incident can be found 

at the following web site: www.darp.noaa.gov/neregion/chalkpt.htm

Wildlife rescue efforts began immediately, and resulted in the collection of 109 live birds 

and 55 dead birds. Most of the oiled birds were treated and released, with only 12 birds dying 

during rehabilitation (USFWS, 2000).

The number of birds retrieved after an oil spill represents only a fraction of the actual 

number of birds affected by the spill. Oiled and dead birds are not recovered because they sink, 

drift out to sea, are scavenged, are overlooked by search teams, or they hide. Several approaches 

can be used to estimate the actual mortality. Their applicability to the Chalk Point spill is 

discussed below:

1. Selection of a Multiplier. In this approach, the actual number of carcasses recovered is 

multiplied by a factor to arrive at an estimate of the total bird mortality. Burger (1993) 

summarized data for 21 spills where the actual and estimated bird mortalities were reported. 

On average, the estimates were 4.4 higher than the actual counts. For the North Cape oil spill 

off Rhode Island, the natural resource Trustees (NOAA, state of Rhode Island, and US Fish 

and Wildlife Service) used a multiplier of 6, after evaluating the spill conditions (Sperduto et 

al., 1998). However, this case and many others where seabird mortality has been well studied 

occurred in open coastal settings where dead seabirds drifted out to open seas. It is not 

appropriate to use multipliers developed on such different physical settings, compared to the 

restricted upper estuarine setting of the Chalk Point spill site. Furthermore, most of the birds 

affected were overwintering ducks at the end of their migration to breeding grounds. These 

oiled birds could have left the spill area and expired from the stress of migration. Thus, their 

carcasses would not be found in the surveys.

2. Computer Modeling. In this approach, computer models are used to analyze the trajectory of 

the oil, the spatial distribution of birds, and probability functions to predict the number of 

oiled birds. These models have been used for spills where large numbers of seabirds were 

affected or potentially at risk, such as the Nestucca spill off Washington where an estimated 

56,000 birds were killed (Ford et al., 1991) and the Apex Houston spill in central California 

where over 10,000 birds were estimated to have died (Page et al., 1990). This approach is not 

cost-effective for the Chalk Point oil spill, considering the number of birds likely to have
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Figure 1. Map showing the spill site and the locations of place names cited in the text.
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been killed and the costs of developing a model for the spill site and conditions.

3. Risk Assessment Approach. In this approach, data collected during the spill are used to 

estimate the population at risk and the percent of the population oiled, and data from the 

literature are used to estimate total mortality. It considers the life history and behavior of 

different groups of birds. This approach is appropriate where field teams can make good field 

observations during the spill. We selected this approach for the Chalk Point oil spill site. The 

specific methodology and assumptions for this assessment were developed cooperatively by 

the wildlife injury workgroup of the Chalk Point Oil Spill Natural Resource Damage 

Assessment (NRDA) Council. It uses a combination of field data and literature reviews, 

which are two of the assessment methods listed in the NRDA regulations (15 CRF Part 990).

BIRD RESCUE AND REHABILITATION

Wildlife rescue and rehabilitation efforts at the spill lasted from 7 April to 20 June. 

Detailed logs were kept of the animals that were rescued alive and turned into the rehabilitation 

center (Table 1). Only three birds turned into the rehabilitation center did not have visible oil on 

them: one common grackle, one house finch fledgling, and one mallard hen. Dead animals that 

were delivered directly to the morgue or died during rehabilitation are listed in Table 2. One 

dead osprey and one ring-billed gull were definitely not attributable to the spill (USFWS, 2000). 

As can be seen in Table 2, ruddy ducks were impacted the most, comprising nearly 60 percent of 

all known dead birds.

Table 1. List of birds recovered alive and the results of rehabilitation efforts during the Chalk 

Point oil spill during the period April 7 to June 20, 2000. 

BIRD SPECIES

NO. RECOVERED FOR 

REHABILITATION NO. DIED NO. RELEASED

BIRDS

Canada Goose 8 0 8

Double-crested Cormorant 1 1 0

Coot 3 1 2

Domestic & Hybrid Ducks 9 0 9

Grackle (not oiled) 1 0 1

House Finch (not oiled) 1 1 0

Mallard Duck (1 was not oiled) 37 1 36

Mute Swan 7 0 7

Osprey 4 0 4

Peking Duck 9 0 9

Ring-Billed Gull 1 1 0

Ruddy Duck 24 4 20

Savannah Sparrow 1 1 0

Barn Swallow 1 0 1

Virginia Rail 1 1 0

Warbler 1 1 0

BIRD TOTALS 109 12 97
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Table 2. Summary of dead wildlife, including dead wildlife collected by field teams, and deaths 

incurred during rehabilitation at the Chalk Point oil spill from April 7 to June 20, 2000. 

BIRD SPECIES COLLECTED 

DEAD 

REHABILITATION

DEATHS

TOTAL

DEAD

BIRDS

Double-crested Cormorant 3 1 4

Coot 1 1 2

Great Blue Heron 2 0 2

Herring Gull 1 0 1

House Finch 0 1 1

Kingfisher 1 0 1

Loon 1 0 1

Mallard Duck 1 1 2

Osprey 3 0 3

Ring-Billed Gull 2 1 3

Ruddy Duck 35 4 39

Savannah Sparrow 0 1 1

Unidentified Tern 2 0 2

Unidentified Bird 1 0 1

Virginia Rail 0 1 1

Unidentified Warbler 2 1 3

TOTALS 55 12 67

WILDLIFE SURVEY TEAM OBSERVATIONS

The USFWS coordinated Wildlife Survey Teams to document wildlife impacts and use 

within the spill zone. The teams were made up of 2-4 individuals from the USFWS, Maryland 

Department of the Environment, Maryland Department of Natural Resources (MDNR), and later, 

by trained volunteers. Each team was equipped with a radio and assigned a specific shoreline 

zone to survey each day. All surveys were conducted from shore; sometimes the teams were 

transported from one area to another by boat. They recorded the following information on field 

data sheets:

- number and species of wildlife observed

- degree of oiling (none, light, medium, heavy) on wildlife

- extent of oiling (none, light, medium, heavy) on the shoreline

They reported by radio the location of oiled and live wildlife to the Wildlife Trailer, and a 

rehabilitation response team would be dispatched to attempt the capture of the impacted wildlife. 

The Wildlife Survey Teams were instructed not to attempt capture.

Wildlife surveys were conducted 9-17 April. The surveys were terminated when the 

numbers of oiled and dead wildlife dropped to minimum levels. The data sheets for each day 

were compiled onto base maps showing the shoreline sections surveyed and the field 

observations by species and degree of oiling, if reported. MDNR staff also conducted an aerial 
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survey of mostly the eastern shore of the Patuxent River on 13 April. Their observations were 

recorded onto base maps for analysis.

IMPACTS TO BIRDS

Estimates of the number of birds oiled and killed have been made for selected species or 

species groups, as discussed below. The spill impact area extends from Eagle Harbor in the north 

to Broomes Island in the south, based on field observations and the extent of shoreline oiling 

documented by the Shoreline Cleanup Assessment Teams (SCAT). Figure 1 shows the extent of 

shoreline oiling compiled by Entrix (2000) from the SCAT data.

Ruddy Ducks. For ruddy ducks, 59 birds are known to have been oiled; of these, 39 died. 

Table 3 lists the observations for ruddy ducks from the Wildlife Survey Teams for the period 10-

15 April 2000, the period for which the most extensive surveys were conducted. The data sheets 

for 9 and 16 April were reviewed, and there were no observations of ruddy ducks in the area of 

concern on those days. 

Based on the observations summarized in Table 3, it is difficult to determine the actual 

number of ruddy ducks present in the area affected by the oil spill. The number of ruddy ducks 

present is expected to change from day-to-day, because the spill occurred during the end of the 

migratory period. Ruddy ducks are highly mobile, and they could have moved to cleaner and less 

disturbed areas. According to Chris Swarth of the Jug Bay Wetlands Sanctuary, the Swanson 

Creek area has the highest density of wintering ruddy ducks in the Patuxent River; with 3,500 

and 7,500 ruddy ducks there in February of 1999 and 2001, respectively. In the February 2000 

count, only 30 ruddy ducks were observed, because of very cold weather before the count. 

The wildlife surveys on 11 April were the most extensive, covering the western shoreline 

from Eagle Harbor south to Horse Landing and the eastern shoreline from the south shore of 

Hunting Creek to Buzzard Island. USFWS personnel used a spotting scope to count ruddy ducks 

on the eastern shoreline from the Highway 231 bridge north to Hunting Creek and on the western 

shoreline from Swanson Creek south to Indian Creek.

In Table 3, note that there are no counts of ruddy ducks in Trent Hall Creek on 11 April; 

the field form only notes that there were "ruddies" present. Of the 35 dead ruddy ducks collected, 

27 came from Trent Hall Creek, including 14 on 13 April. Also, of the 24 live ruddy ducks 

rescued, 17 came from Trent Hall Creek between 12 and 16 April. Thus, a total of 41 ruddy 

ducks came from this area, although the maximum reported on any day was 13. The videotapes 

from the aerial overflights of Trent Hall Creek on 11 April were viewed, but the flight was too 

high to observe the number of ducks present. Therefore, the number of oiled ruddy ducks in 

Trent Hall Creek on 11 April is assumed to be at least 41 (the number of oiled ruddy ducks 

collected from this area between 12-16 April. The number of ruddy ducks in the spill area is 

estimated by the Trustees to be 851 birds on 11 April. 
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Table 3. Observations of ruddy ducks in the Chalk Point area of the Patuxent River, April 2000, 

as reported by Wildlife Survey Teams and a MDNR aerial survey over mostly the eastern shore 

on 13 April 2000. A (-) indicates that the zone was not surveyed on that date. For each date, the 

number of ruddy ducks and the degree of oiling (heavy, medium) are listed. The entry “NR” is 

used when information on the degree of oiling was not reported.

Date/

Zone

4/10 4/11 4/12 4/13

4/13 

MDNR 

Aerial

4/14 4/15

West Shore

Eagle Harbor 

to Swanson 

Ck

- 0 0 - - 2, no oil -

Swanson 

Creek

70, 10% 

oiled

2, heavy oil 

110, oil NR

80, medium 

oil

6 ducks, 

oiling NR 0 - 0 -

Swanson to 

Indian Ck - 120, oiled

30, light 

3, no oil

1, heavy oil

1, no oil 2, oil NR 0 -

Indian Creek 150, no oil

151, heavy 

oil

150,

oil NR

12, medium 

oil 1, no oil

15, oil NR

0 0

Trent Hall 

Creek 1, heavy oil "ruddies"

3, heavy oil; 

8-10, oil NR

18 ducks, 

oil NR

- 0

1, oil NR

2, dead 1, dead

East Shore

Hunting 

Creek -

180, oil NR 15-20, no 

oil - 295, oil NR - -

Gods Grace to 

Hallowing Pt. -

90, some 

oiled

80, oil NR

2, heavy oil

20 ducks, 

oil NR

41, heavy 

oil

1, dead

0 0 0

Hallowing to 

Sheridan Pt - - 6, heavy

4, no oil

3, no oil 0 0 23 ducks, 

oil NR

TOTAL 374 810+ 134 48 335 5 24

The Responsible Party (RP) agreed with using the overall approach, but disagreed with 

the inclusion of the ruddy ducks in the Hunting Creek area in the population at risk. They 

believed that this area fell outside of the area impacted by the spill, hence it was unlikely birds in 

this are would become oiled. The Trustees believed that the proximity of these birds to the spill 

zone and observations indicating birds were moving around in the area put them at a high risk of 

exposure to oil. The Trustees and RP could not agree on this issue, therefore the bird injury was 

estimated with and without the Hunting Creek bird count.

Table 4 shows the calculations for determining the estimated number of birds that were 

oiled and died in the field, by species. Where the field teams made estimates of the number of 

oiled ruddy ducks, the estimates were usually about half of the total number of ducks that were 

observed in the spill area. Therefore, it is estimated that 50 percent of the ruddy duck population
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Table 4. Calculations to estimate the number of birds oiled and dying in the field.

Species

Number

Observed 

in Field

Number 

Observed/

Estimated to 

be Oiled

Total Number 

Collected 

(birds 

collected live 

and dead)

Estimated

Number 

Not 

Collected

Estimated

Mortality

Rate in 

the Field

Estimated 

Number 

Dying 

in the Field

Ruddy Duck 

(Trustees)

851 426 (est. 50% of 

number in field)

59 367 0.85 312

Ruddy Duck 

(RP1)

671 336 (est. 50% of 

number in field)

59 277 0.85 236

Greater Scaup 41 10 (est. 25% of 

number in field)

0 10 0.85 9

Green-winged 

Teal

50 12 (est. 25% of 

number in field)

0 12 0.85 10

Canada Goose 13 12 (observed) 7 5 0.23 1

Mallard 29 53 (est. 50% of 

number in field 

+ all 38 

recovered)

38 15 0.5 8

Double-crested 

Cormorant

200 50 (25% of 

number in field)

4 46 0.85 39

American Coot 40 40 (observed) 4 36 0.85 31

Virginia Rail 0 1 (observed) 1 0 - 0

Osprey 15 6 (observed) 6 0 - 0

Great Blue 

Heron

7 7 (observed) 2 5 0.23 1

1

 Does not include the 180 ruddy ducks in Hunting Creek

in the spill impact zone was oiled, for an estimated total of 426 oiled ruddy ducks, or 336 ruddy 

ducks using the RP numbers. A total of 59 oiled ruddy ducks were collected (both alive and 

dead), so the number of oiled birds not collected is 367 birds, or 277 birds using the RP numbers 

(Table 4). 

Other bird species were expected to have a lower probability of exposure to oil either 

because of their behavior or because they were observed in areas that were not heavily oiled. 

Consequently, the Trustees assumed that 25% of the number of scaup, teal and cormorants 

present in the area affected by the spill were oiled. This estimate is half of the field observations 

of the percent of ruddy ducks that had visible oiling. The Trustees used this estimate because the 

other waterbirds were mostly in areas with lower amounts of surface slicks, and it is based on the 

experience of the Trustees from other oil spills. The Trustees used the actual observed percent of 

the population that was oiled for the remaining species (coot, rail, osprey, heron).

Some of the oiled birds that were not collected for rehabilitation are expected to have 

died in the field. The only data from the literature is on the survival of oiled, rehabilitated, and 

released birds, and these data were extrapolated as follows. For large birds such as geese and 

herons, the 23 percent mortality rate of Anderson et al. (1996) for pelicans was used directly. For 

migratory ducks, coots, and cormorants, there were no data, but it was the judgment of the 
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workgroup that mortality would be very high. A mortality rate of 85 percent was considered 

reasonable, since some of the birds were very lightly oiled and likely to survive. For mallards, a 

mortality rate of 50 percent was used, based on the work of Anderson et al. (2000), discussed 

below.

Birds that were oiled, rehabilitated, and released are also estimated to have been affected 

(Table 5). Anderson et al. (2000) studied the survival, condition, and behavior of 

oiled/rehabilitated American coots, compared to unoiled, wild-caught coots over a four-month 

period. Both groups were randomly mixed and released into fenced marshes. They reported 51 

percent mortality in the oiled/rehabilitated birds, compared to 24 percent for unoiled birds. For 

oiled migratory ducks, it is estimated that 50 percent of the oiled/rehabilitated birds eventually 

died as a result of the spill. The mortality rate for mallards was estimated to be half that for 

migratory ducks, so 25 percent of the oiled/rehabilitated mallards were assumed to have died as a 

result of the spill. For larger aquatic birds, the 23 percent mortality rate for pelicans from the 

Anderson et al. (1996) study was used. Table 5 shows the estimated mortality for oiled birds 

dying after release alive.

Table 5. Estimated number of birds dying after release alive.

Species

Number Oiled

Released Alive

Mortality Rate

After Release

Estimated

Number Dying

After Release

Ruddy Duck 20 0.5 10

Greater Scaup 0 - 0

Green-winged Teal 0 - 0

Canada Goose 7 0.23 2

Mallard 36 0.25 9

Double-crested 

Cormorant

0 - 0

American Coot 2 0.5 1

Virginia Rail 0 - 0

Osprey 4 0.23 1 (was collected)

Great Blue Heron 0 0.23 0

Other 20 - -

Total 96 22

The total estimated number of ruddy ducks that died as a result of the spill is 361 (or 285 

using the RP numbers), as shown in Table 6. For comparison, Perry et al. (1978) reported that an 

estimated 12,500 ruddy ducks were killed during five winter spills in the upper Delaware River 

over the period 1973-1976. They also reported on two spills in the Chesapeake Bay where an 

estimated 350 and 90 ruddy ducks were killed, respectively (Perry et al., 1978).

Other Migratory Ducks. Wildlife Survey Teams reported other migrating ducks in the 

oiled area as follows: 12 greater scaup, three oiled on 11 April; 41 greater scaup, none visibly 

oiled on 12 April, and 50 green-winged teal with no information on oiling degree on 13 April. 
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Table 6. Summed columns to calculate total number of dead birds.

Species

Number 

Collected 

Dead

Number Oiled

Dying In 

Rehab

Number 

Dying 

After Release 

Number 

Dying 

in Field

Total Number 

of Dead Birds

Ruddy Duck 

(Trustees)

35 4 10 312 361

Ruddy Duck 

(RP1)

35 4 10 236 285

Greater Scaup 0 - 0 9 9

Green-winged 

Teal

0 - 0 11 11

Canada Goose 0 - 2 1 3

Mallard 1 1 9 8 19

Double-crested 

Cormorant

3 1 0 39 43

American Coot 1 1 1 31 34

Virginia Rail 0 1 0 0 1

Osprey 2 0 0 0 2

Great Blue Heron 2 0 0 1 3

Other 7 2 9

Total (Trustees) 51 11 22 412 495

Total (RP1) 51 11 22 336 419

1 Does not include the 180 ruddy ducks in Hunting Creek

Most of the teal and scaup were located in the southern half of the oiled area and were likely at a 

lower risk of oiling. The only field count of the percent of birds oiled was 25 percent on 11 

April. Therefore, it was estimated that 25 percent of the scaup and teal in the area affected by the 

oil spill were oiled (Table 4). 

Three American coots were collected and rehabilitated from Benedict pier, Golden 

Beach, and Washington Creek. One dead coot was recovered from Indian Creek. For the area 

between Benedict and Indian Creek, Wildlife Survey Teams reported 40 oiled coots on 11 April, 

8 coots 12 April, and one on 13 April. 

Calculations for estimating the number of migratory waterfowl that died in the field are 

shown in Table 4. An estimated nine scaup, ten teal, and 31 coots died in the field as a result of 

the spill. Two oiled/rehabilitated coots were released, and it is estimated that one of them died 

(Table 5). Total mortality for migratory ducks is estimated to be nine scaup, eleven teal, and 34 

coots (Table 6).

Mute Swans. Seven oiled mute swans were collected and rehabilitated. No mortalities 

were observed or are estimated to have occurred. Mute swans are large, white birds that are 

readily observed as to their oiling and survival post-oiling. It is believed that all of the oiled birds 

were collected and survived rehabilitation efforts, thus they are not listed in Tables 4-6.
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Canada Geese. One unoiled and seven oiled birds were collected and rehabilitated. Six 

came from the Golden Beach/Trent Hall Creek area from 12-15 April, one from Benedict marina 

on 11 April, and one with no oiling from Route 4 highway on 17 April. Eight oiled Canada geese 

and three without oiling descriptors were observed on 12 April, including the three heavily oiled 

birds in the Trent Hall Creek area that were collected that day. Thus, twelve Canada geese were 

observed oiled, and five oiled Canada geese remained in the field. Of these five, one is estimated 

to have died in the field (Table 4). 

It is likely that most of these large birds were able to survive oiling and rehabilitation 

efforts. The survival rate for oiled, rehabilitated, and released brown pelicans following the 

American Trader oil spill in southern California in the first 60 days after release was 77 percent 

(Anderson et al., 1996). It is assumed that oiled Canada geese had a survival rate of 77 percent, 

similar to that reported for pelicans. For the seven rehabilitated birds that were released alive, it 

is estimated that 23 percent of them (2 birds) eventually died (Table 5). Thus, a total of three 

Canada geese are estimated to have died as a result of the spill (Table 6).

Mallards. Most of the 38 live mallards collected came from two areas: Benedict and 

Golden Beach from 11-18 April. Only one dead mallard was recovered, and one mallard died in 

rehab. The numbers of mallards observed by the Wildlife Survey Teams in other areas were 29 

on 11 April, 22 on 12 April, 24 on 13 April, nine on 14 April, and 27 on 15 April. When 

provided, the number of oiled mallards was about half of the total number observed. The total 

number of oiled birds is estimated to be 53, half of the birds observed in the field plus the 38 

recovered. Half of those not recovered are estimated to have died, for a total of eight mallards 

dying in the field (Table 4).

According to the wildlife specialists at Tri-State Bird Rescue and Research Center, 

mallards have a high survival rate after being oiled and rehabilitated (Eileen Gilbert, pers. comm, 

2000). Therefore, the mortality rate after release is estimated to be 25 percent, half that of other 

ducks, resulting in nine mallards dying after release (Table 5). Total mallard mortality is 

estimated to be 19 birds (Table 6).

Double-crested Cormorants. One oiled cormorant was collected near Chalk Point, and it 

died in rehab. Three dead birds were collected: one at Captain Point on 14 April (no visible oil), 

one at Benedict on 20 April, and one at Cremona Creek on 1 May (no visible oil). Wildlife 

Survey Teams reported decreasing numbers of cormorants over time: 200 on 11 April (all within 

the area of surface slicks), 30 on 12 April, 15 on 13 April, four on 14 April, and 0 on 15 and 16 

April. They were usually not able to determine if the birds were oiled. Recovery rates for 

cormorants are low because they seldom come ashore. This behavior is even more prevalent 

where there are numerous isolated areas available for roosting, such as the many structures over 

water on the Patuxent River. It is assumed that 25 percent of the cormorants in the area were 

oiled, for a total of 50 oiled cormorants. Once oiled, cormorants are expected to have low 

survival in the wild, so the 85 percent mortality rate was used. Therefore, 39 cormorants are 

estimated to have died in the field (Table 4). No cormorants were released alive, so the total 

estimated mortality is 43 (Table 6)
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Virginia Rails. Only one Virginia rail was collected, and it died in rehab. Rails are very 

secretive and are not likely to be found. To determine the potential population of rails at risk, 

contact was made with Gregg Kearns, of the Maryland National Capital Park and Planning 

Commission, a local naturalist who has extensive experience in conducting bird surveys on the 

Patuxent River. He advised that few rails overwinter in the Swanson Creek area. Rails usually 

start arriving mid-April and the migration peaks in May. He did a survey of the break site a few 

days after the spill and did not see or hear rails in the area. He also noted that the Swanson Creek 

area is not good rail habitat. Based on this information, it is estimated that one Virginia rail died 

as a result of the Chalk Point oil spill (Table 6).

Osprey. Four live, oiled osprey were collected, rehabilitated, and released. Three dead 

osprey were turned into the morgue, including one of the birds that had been treated and 

released, and one whose death was not related to the spill. Thus, two osprey were directly killed 

as a result of the spill. Fifteen osprey were observed on 11 April, three were described as oiled. 

Six were observed on 12 April, of which three were oiled. Ten were observed on both 14 and 15 

April, of which one was oiled. The total number of oiled osprey is estimated to be six. Because 

of the successful efforts to capture oiled birds and rehabilitate them, it is assumed that the two 

dead oiled osprey in the morgue represents those killed as a result of the spill (Table 6). 

Impacts to osprey nesting success were assessed in a separate study (Cardano, 2001). 

Results of the reproductive success study showed that the number of young osprey produced per 

nest met or exceeded the values determined to maintain a stable population (0.95-1.30). 

Statistical analysis of the data showed that there were no differences between upstream and 

middle river nests. Overall, the Chalk Point spill did not have an effect on osprey brood 

production for the Patuxent River. 

Bald Eagles. A separate study was conducted to monitor potential impacts on bald eagle 

reproductive success (Wearmouth, 2000). Of the three bald eagle (two in Swanson Creek, and 

one at Cremona Creek) nests selected for monitoring, only the Cremona Creek nest was 

monitored for the entire nesting season. The other two nests were destroyed early in the 

monitoring period by high winds (Stanley Run), and failure of a young eagle pair to produce 

eggs (mouth of Swanson Creek). The Cremona Creek eagles successfully fledged two young, 

exceeding the 0.7 per nest required to maintain a stable Chesapeake Bay population, and the 1.1 

per nest value established by the Chesapeake Bay bald eagle recovery team.

Great Blue Herons. Two dead great blue herons were turned into the morgue, collected 

on 17 April from Chalk Point and on 19 April from the Benedict boat ramp. Five oiled great blue 

herons were observed on 12 April. It is assumed that five oiled birds were not recovered and one 

of these birds eventually died (mortality rate in the field of 23 percent, Table 4). Thus, three great 

blue herons are estimated to have died as a result of the spill (Table 6). 

Impacts to great blue heron nesting colonies were assessed in a separate study (McGowan 

et al., 2001). Overall survival rates of great blue heron nestlings in the Swanson Creek colony 

were not statistically different from survival rates at an upstream reference site. In addition, both 

populations exhibited survival rates that were similar to or higher than values reported in the 

literature. There were also no statistical differences in nest productivity (defined as the number 
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12

of young per successful nest); however, productivity values for both colonies appear to be on the 

low end of the range of literature values from other areas along the east coast. Productivity of 

both great blue heron populations may have been affected by the weather, because April 2000 

had the highest precipitation recorded for the month in 30 years. There is some uncertainty 

associated with the study results due to the delay in initiating monitoring and questions 

concerning the appropriateness of the reference site. As a follow-up to this study, the Trustees 

will be conducting a nesting pair survey at both colonies in the Spring 2001..

Other Birds. Three dead gulls were turned into the morgue, including one that was 

reported as unoiled and collected from tangled wires on an unfinished pier. Other oiled dead 

birds included a belted kingfisher, two terns, one loon, and one unidentifiable bird. Dead birds 

with no visible oil on them (and so are not attributed to the spill) that were turned into the 

morgue included two warblers, a house finch fledgling, and a common grackle. Therefore, seven 

other species of birds that were collected dead are attributed to the spill. One heavily oiled 

savannah sparrow and one moderately oiled common yellowthroat (warbler) died in rehab. Two 

lightly oiled gulls were reported by Wildlife Survey Teams for the period 9-16 April. Thus, nine 

other dead birds can be attributed to the Chalk Point oil spill (Table 6).

In summary, the total number of adult birds estimated to have died is 495 using the 

Trustee numbers and 419 using the RP numbers.

ESTIMATING PRODUCTION FOREGONE 

Based on the number of adult birds killed, the Trustees decided that the ruddy duck was 

the only species for which the adult injury was large enough to potentially affect productivity. 

Ruddy ducks that overwinter on the Atlantic coast have a steady population and are expected to 

recover within one breeding cycle (Mike Haramis, Patuxent Wildlife Research Center, USFWS, 

Laurel Maryland). In 1999, 64,000 ruddy ducks were killed by hunters, 26,000 of

which were in the Atlantic Flyway (John Debney, Delta Waterfowl Research Station, Manitoba, 

Canada, pers. comm.). Thus, estimates were made for the loss associated with the first generation 

of fledgling ruddy ducks that would have been produced by the birds affected by the spill.

Production foregone calculations for ruddy ducks were based on life history information 

taken from Johnsgard and Carbonell (1996) and Bellrose (1978). The calculations are based on 

the following information:

- Ratio of males to females = 1.63:1 (38% females in the spring population)

- Percent of females that are active nesters = 79%

- Broods per season = 1 (double brooding occurs only occasionally)

- Mean clutch size = 8 eggs

- Mean hatching success = 50%

- Mean brood reduction = 25% (fully feathered young)

- Total number of birds unable to reproduce in 2000 (Trustees) = 361 dead birds + 55 

birds estimated to be oiled that survived (15 percent of 367, the total oiled and not 

collected in the field in Table 4) + 10 oiled birds that were treated, released and survived 

(Table 5) = 426 birds
Final Report 

May 7. 2001

13

- Total number of birds unable to reproduce in 2000 (RP) = 285 dead birds + 42 birds 

estimated to be oiled that survived (15 percent of 277, the total oiled and not collected in 

the field in Table 4) + 10 oiled birds that were treated, released and survived (Table 5) = 

337 birds

It is assumed that birds that were oiled but survived did not attempt breeding. This 

assumption is based on studies by Anderson et al. (1996) of oiled, rehabilitated, and released 

brown pelicans that showed no breeding activity in the two years after oiling, compared to 

unoiled control birds that were active at breeding colonies over the same period. Ruddy ducks, 

being smaller and having to undertake a significant migration, are expected to be significantly 

stressed and not attempt nesting after being oiled. 

The sex ratio for ruddy ducks varies depending on which population is surveyed. Bellrose 

(1978) reported that a compilation of sex ratios from several areas indicated that males 

comprised 62 percent of spring populations. Johnsgard and Carbonell (1996) reported a 3:1 male 

to female ratio on the breeding grounds. Because the birds were oiled in their overwintering 

areas, the sex ratio for the spring populations is the appropriate one to use.

The calculations using the Trustee numbers are shown below.

Number of females that were oiled: 426 X 0.38 = 162 females

Number of females that were active nesters: 162 X 0.79 = 128 nesting females

Number of eggs laid: 128 X 8 (mean clutch size) = 1024 eggs 

Number of hatched young: 1024 eggs X 0.5 (mean hatching success) = 512 young

Number of fully fledged young: 512 young X 0.75 (25% brood reduction for fully 

feathered young) = 384 fully feathered young

384/128 nesting females = 3 fledged young produced per nesting female

Johnsgard and Carbonell (p. 109, 1996) state "that each nesting female produces an 

average of about 3 fledged young." 

The calculations using the RP numbers are shown below.

Number of females that were oiled: 337 X 0.38 = 128 females

Number of females that were active nesters: 128 X 0.79 = 101 nesting females

Number of eggs laid: 101 X 8 (mean clutch size) = 808 eggs 

Number of hatched young: 808 eggs X 0.5 (mean hatching success) = 404 young
Final Report 

May 7. 2001

14

Number of fully fledged young: 404 young X 0.75 (25% brood reduction for fully 

feathered young) = 303 fully feathered young

Production foregone is estimated to be 384 fledged young ruddy ducks using the Trustee 

numbers, or 303 fledged young ruddy ducks using the RP numbers.

ACKNOWLEDGEMENTS

This report was prepared by the Wildlife Injury Workgroup, for the Chalk Point NRDA 

Council, consisting of representatives from the USF&WS, NOAA, Maryland Department of the 

Environment, Maryland Department of Natural Resources, Pepco, and ST Services. The Wildlife 

Injury Workgroup members are: Beth McGee and Peter McGowan, USFWS Chesapeake Bay 

Field Office; Mark Hoffman, Maryland Department of Natural Resources; Jacqueline Michel, 

Research Planning, Inc.; and Richard Greer, Entrix, Inc., representing PEPCO. Ann Wearmouth 

(formerly of PEPCO) assisted with assembling the observational data. James Hoff of NOAA, 

Heidi Stout of the Tri-State Bird Rescue and Research Center, and Wayne Kicklighter of Entrix, 

Inc. are acknowledged for their review and comments on the draft report. 
Final Report 

May 7. 2001

15

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