THE EXOTIC MUTE SWAN (Cygnus olor) IN CHESAPEAKE BAY, USA.
C. Perry, Peter C. Osenton, and
The exotic mute swan (Cygnus olor) has increased its population size in Chesapeake Bay (MD and VA) to approximately 4500 since 1962 when 5 swans were released in the Bay. The Bay population of mute swans now represents 30% of the total Atlantic Flyway population (12,600) and has had a phenomenal increase of 1200% from 1986 to 1999 (Figure 1). Unlike the tundra swan (Cygnus columbianus) that migrate to the Bay for the winter, the mute swan is a year-long resident, and, therefore, reports of conflicts with nesting native waterbirds and the consumption of submerged aquatic vegetation (SAV) have raised concerns among resource managers.Populations of black skimmers (Rynchops niger) and least terns (Sterna antillarum) nesting on beaches and oyster shell bars have been eliminated by molting mute swans. Mute swans are highly territorial during the breeding season which leads to localized depletions (eat-outs) of SAV during the growing period. The cumulative effect of their year-round foraging on aquatic plants and associated implications to SAV restoration efforts are unknown. Data on the reduction of SAV by nesting mute swans and their offspring during the spring and summer are limited, however.Mute swans are currently classified as a wetland game bird in Maryland and are protected under current state law. They are not protected in Virginia. Mute swans are also not protected by the federal government under the Migratory Bird Treaty Act of 1916, as they are considered non-migratory and exotic. Maryland had a program to control swan numbers by the addling of eggs and the killing of adult swans, but it was a contentious program with some residents of the Bay area.A management plan is being prepared by a diverse group of citizens appointed by the Governor to advise the Maryland Department of Natural Resources on viable and optimum options to manage mute swans in the Maryland portion of Chesapeake Bay. Hopefully, the implementation of the Plan will alleviate the existing conflicts, while not excessively diverting limited resources of the MD DNR.The expanding mute swan population should be continually monitored to determine its population status and seasonal distribution. More research is needed within the Chesapeake Bay to determine the impact that mute swans are having on other species indirectly by degrading the habitat or directly by aggressive territorial behavior that causes direct mortality. The current study is attempting to learn more about the food preferences of mute swans in all seasons of the year.
Mute swan populations are being surveyed by ground crews to learn more about the habitat they are using as feeding and resting areas during all seasons of the year. Exclosures are being established in several areas to measure the impact of mute swans on SAV and other vegetation. Findings from field exclosure studies will be compared with food habits analyses conducted on the gullet and gizzards of mute swans that were collected as part of another study. Any new carcasses that become available will also be analyzed for food habits. Attempts are being made to determine the amount of corn being fed to mute swans and the significance of corn to the status of mute swans in the Bay. The interaction of mute swans with other bird species, especially colonial nesting birds, during the nesting period will be evaluated on Barren Island and other areas of concern.
Figure 1. Mute swan mid-summer survey conducted by Atlantic Flyway Council (1986 to 1999).
Analyses of the gullet (esophagus and proventriculous) and gizzard of mute swans from Chesapeake Bay indicate that this species is primarily herbivorous during all seasons of the year and feed primarily on submerged aquatic vegetation (SAV). Widgeon grass (Ruppia maritima) constituted 66% and 78% of the food eaten at Eastern Bay and Smith Island respectively, whereas Eelgrass (Zostera marina) formed 2% and 32% respectively for these areas (Figure 2). Other SAV and invertebrates (including bryozoans, shrimp, and amphipods) formed a much smaller amount of the food percentage. Invertebrates are believed to have been selected accidentally within the vegetation eaten by the swans. Corn (Zea mays), fed by Bay residents during the winter, probably supplements limited vegetative food resources in late winter. Average food volume in the gullets of swans was 84 milliliters with maximum of 130 ml.
The food habits are different from those of tundra swans who feed commonly in agricultural fields in early winter and also feed heavily on clams in late winter. The major difference, however, between the food habits of tundra and mute swans is that the tundra does not occur in the Bay during the summer when SAV is growing.
Figure 2. The food habits of mute swans at Eastern Bay and Smith Island, MD, collected in both the winter and spring of 2000.
MANAGEMENT RECOMMENDATIONSThe continual increase of the mute swan population should be halted and the population stabilized. The population should eventually be reduced to lessen the impact of this non-migratory exotic species on habitat that is used by other species.
Specific methods to stabilize and eventually reduce the mute swan population in Chesapeake Bay are the following:
1.) Conduct an education program for citizens of the Chesapeake Bay area in regard to the status and impact of the mute swan on the populations and habitats of other wildlife in Chesapeake Bay.
2.) Require a permit for all persons who keep mute swans on their personal property and annual documentation of the number of swans maintained.
3.) Forbid the importation of new mute swans (adults, young, eggs) to the Bay and the sale of any mute swans that are currently in the Bay area.
4.) Issue permits to property owners and municipal authorities to addle eggs of nesting swans in problem areas.
5.) Manage mute swan populations in Chesapeake Bay at levels to have minimal impact on native wildlife and their habitats.
6.) Remove existing swans from state and federal problem areas by capture and humane euthanization.
It is recognized that there still is a vital need for more information about the mute swan population in Chesapeake Bay. It is important that while attempts to reduce the mute swan population are being conducted, that good records of all mute swan activities are obtained and maintained by the Maryland and Virginia wildlife agencies.
Research should continue to firmly establish the role of mute swan populations in regard to the growth and biomass of SAV available to other waterfowl migrating to Chesapeake Bay. Exclosure studies should be established in major mute swan concentration areas. Food habits analyses should continue to further document the use of SAV and other food items, including corn, by mute swans.
Numerous persons have assisted with this study including: Daniel Day, Michael Haramis, Michael Harrison, Larry Hindman, Janet Keough, and Daniel Stotts.
Last Updated 4/13/2001
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