Mute Swan Population Survey Shows Decline

The Maryland Department of Natural Resources (DNR) - Wildlife and Heritage Service and U.S. Fish and Wildlife Service personnel conducted an aerial survey of mute swans in the Maryland portion of Chesapeake Bay between August 11 and September 9, 2002. The total number of mute swans counted was 3,624, a decline of about 8% from the 3,955 birds counted during the 1999 survey (Figure 1). The 2002 included 267 breeding pairs, 2,715 nonbreeders, and 374 cygnets (Table 1).The number of breeding pairs declined about 55% from the 594 pairs observed in 1999. Similarly, the number of cygnets declined 42% from the 646 cygnets observed in 1999. The smaller population estimate in 2002 can be attributed to an intensive egg addling effort in 2001 and 2002 combined with the removal of swans on Federal National Wildlife Refuges and elsewhere by entities authorized to collect waterfowl for scientific research.

The distribution of mute swans in 2002 (Figure 2) was very similar to the distribution in 1999. Swans are present in all major tributaries of the Bay. The largest circle in the attached map of the distribution of mute swans during August-September, 2002, represent 472 swans (Figure 2). 

In spring 2001, the DNR initiated a concerted effort to addle mute swan eggs to slow the growth rate of Maryland's mute swan population. Population modeling and experience in other states has demonstrated that egg addling can reduce the population growth rate. However, reducing the size of the population through egg addling alone is unlikely (about 80% of nests must be addled just to stabilize numbers). 

Based upon past experience, the DNR established a target of treating more than 60% of all swan nests mapped. DNR personnel and volunteers visited nests located on public lands and waters and on private lands where property owner permission was obtained. All swan eggs encountered in nests were covered with corn oil to terminate embryo development. Most nests were visited by boat, however, nests located in inland ponds and impoundments were accessed by land. Nests in remote salt marsh habitats and coastal islands on the lower Eastern Shore were treated by helicopter. 

Between April 8 and May 17, 2002, a total of 232 mute swan nests containing 1,243 eggs or about 68% (224/328) of nests mapped were treated. High spring tides on April 27 further reduced productivity, as evidenced by several destroyed nests and with cold eggs found outside the nests. 

Doug Forsell, U.S. Fish and Wildlife Service (FWS) – Chesapeake Bay Field Office, and Mark Koneff, FWS - Division of Bird Habitat Conservation, performed the surveys on the Western Shore. Brent Evans, Bill Harvey, Don Webster, and Larry Hindman, Maryland DNR - Wildlife and Heritage Service, and Maryland Civil Air Patrol pilots performed the aerial survey of the Eastern Shore. Brent Evans and Doug Forsell did the data entry and mapping. 

Table 1. 2002 Aerial Mute Swan Survey

*Noted one adult with 1 cygnet

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