all photographs by Peter McGowan, USFWS

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Environmental Contaminants

In the spring of 1998, scientists from the U.S. Fish and Wildlife Service, U.S. Geological Survey and the New York State Department of Health biologists conducted a study to determine if organochlorine contaminants are affecting reproduction of black-crowned night-herons nesting in Baltimore Harbor.

Sources of organochlorine contaminants include chlorine-based pesticides such as DDT and its metabolites, DDE and DDD, chlordane, dieldrin, aldrin, and toxaphene. Other sources of organochlorines are PCB’s which were used in transformers, capacitors, and plasticizers where flame resistance was important.
In Baltimore Harbor area, chlordane and PCB’s are sources of organochlorines. However, PCBs are the main source. PCB’s are bound to the sediments and make their way into the food chain. Black-crowned night-herons are exposed to organochlorines when they eat contaminated prey.

PCB Levels in Eggs

Visits to the historic nesting site, the Baltimore Gas and Electric Riverside Synthetic Gas Plant, yielded neither herons nor nest building activities. On April 21st it was discovered that the heron colony had moved to Fort Carroll, about one mile south from their historic nesting site. Access to the site was graciously granted by Fort Carroll Associates, L.L.C.

Sample eggs were collected from each of the black-crowned night heron nests at the Baltimore Harbor colony. Hatching and nesting success of the remaining eggs in each nest, and the concentration of contaminants in the sample eggs, were evaluated to determine if, and at what level, contaminant exposure is affecting reproduction.

Sixty-five black-crowned night-heron nests were monitored between April 28 and June 22. A total of 77 eggs were collected for organochlorine contaminant analysis, one eggper nest plus eggs which failed to hatch. Seven regurgitated food samples were also collected. In addition, 12 black-crowned night-heron eggs and 2 regurgitated food samples were collected from a reference site at Holland Island, Maryland.

At the Fort Carroll site, 239 eggs were laid in the 65 nests. Sixty five eggs were collected from nests with 3 eggs or more. That left 174 eggs to be naturally incubated. Of these eggs, 2 cracked, 5 were infertile, 5 were fertile but failed to hatch, 3 were apparently predated, 7 were lost following storms or strong winds, and 18 were lost due to unknown causes. Of the eggs that were naturally incubated, hatching success was 91.7%. Success was defined as 1 egg successfully hatching per nest.

Of the 134 hatchlings, 28 were lost during the 15 day nestling period before young fledge (learn to fly). Four were found dead and 24 were absent from nest and possibly lost to predators (gulls). The remaining 106 nestlings survived to day 15.The daily survival rate was 99%, and fledging success for the entire 15 day nestling period was 87%. Mean number of nestlings per hen was 2.05, a value slightly above the recommended number necessary to maintain a stable population.
Eggshell thickness was measured for eggs collected. Individual shell thickness will be examined in relation to DDE concentrations, although a 1991 study in Baltimore Harbor suggested that DDE values rarely exceeded the level associated with eggshell thinning in black-crowned night-herons.

To date, a total of 48 egg samples have been evaluated for PCB concentrations. PCB concentrations in eggs range from 0.60 to 37 parts per million (ppm), one of the highest recorded for black-crowned night-heron eggs in the United States! Twenty four samples had levels greater than 5 ppm, 20 samples were between 5 and 10 ppm and 4 samples had levels greater than 10 ppm. Generally, concentrations greater than 10 ppm can affect reproduction of birds. The relationship between contaminant concentrations and reproductive success are currently being evaluated by statistical methods.

Heavy Metals in Nestlings

Feather and blood samples were collected also from 12-15 day old nestlings for metals analysis. Metal contamination in feathers and blood indicates exposure through the food supply. Black-crowned night-herons eat fish, frogs, snakes, crayfish, snails and other invertebrates. All samples were analyzed for 18 metals, metalloids and trace elements.The most significant finding was the greater concentration of lead in feathers of nestlings at the Fort Carroll site compared to nestlings at Holland Island. Studies are now being planned to investigate if and how exposure to lead and other heavy metals affects heron nestlings.

Unknown Lesions

During the course of this study, researchers discovered moderate to severe skin lesions on heron nestlings at the both the Fort Carroll and Holland Island colonies. With the assistance of staff from the Manomet Center for Conservation Sciences (MCCS), heron young from 19 nests at the Fort Carroll colony were carefully examined. Eleven nests (58%) contained one or more nestlings that had skin lesions. Of the 9 heron nests examined at Holland Island, 5 (56%) of the nests contained one or more nestlings with skin lesions. The size and nature of the lesions ranged from an abrasion a few millimeters in length to a large abscess approximately two centimeters in diameter that exposed internal organs.

This was unusual and unexpected. Previous findings by MCCS in east coast heronries suggested that lesions were restricted to birds nesting in heavily farmed estuaries. These birds however were nesting in a highly urbanized habitat influenced by heavy industry. Based upon these findings and those of MCCS, it appears that skin lesions in black-crowned night-herons, and possibly other colonial nesting waterbirds, is a regional, rather than a localized problem.

This suggests that, in addition to organochlorines and habitat loss, something else may threaten colonial waterbird populations nesting in coastal areas of the northeastern and mid-Atlantic states, including the Chesapeake Bay. Various species within this group of birds have declined in recent years. For example, between 1985 and 1995, black-crowned night-heron breeding populations within the Maryland portion of the Bay have declined by about 50%. The same is true for snowy egret and glossy ibis breeding populations.

More Information; More Questions

This study has raised some important questions regarding possible threats to colonial waterbirds such as:

• Are these lesions incidence a Bay-wide problem in nesting colonial waterbirds? Only two areas in the Chesapeake Bay have confirmed black-crowned night-heron populations with lesions.
  
• Why do some nests within the same colony have nestlings with lesions while others do not?
• Are birds feeding in different areas?
  
• Are lesion incidences higher in nesting populations that are closer to agricultural areas where contaminants such as organophosphate and carbamate pesticides are commonly used?
  
• Are other bird groups affected?
  
• What is the fate of nestlings with lesions?
• Are there any long-term lasting effects?
• Do they survive their first year?
• Do they enter the breeding population?
  

Environmental contaminant biologists will try to answer these questions as they continue to survey other colonial waterbird colonies in the Chesapeake Bay.