Mute Swans in Maryland:
A Statewide Management Plan
Maryland Department of Natural Resources
Wildlife and Heritage Service
April 14, 2003
STATUS AND DISTRIBUTION
The mute swan population in Marylandís portion of the Chesapeake Bay originated when five birds escaped from an aviculture collection along the Miles River in Talbot County in March 1962.A pair of these birds bred successfully that summer, and the flock increased to more than 100 by 1974. Prior to 1986, the swan population grew slowly and remained fairly stable at <500 swans. However, periodic summer swan surveys conducted by the DNR revealed a rapid increase in swans after 1986 from 264 in 1986 to 3,955 in 1999 (Figure 1). During this period, the population grew at an annual rate of about 23%. A number of factors could have led to this increase, including milder winters and reduced mortality due to lead poisoning. Studies have shown that immigration of mute swans from other states contributed very little to the increase in Maryland, for mute swans rarely move more than 30 miles from their original banding site. Had this rate of growth continued, the population might have exceeded 30,000 by 2010.Between 1993 and 1999, the population grew more slowly, attributed, in part, to limited population control by the DNR and Federal National Wildlife refuges. The population decreased from 3,955 in 1999 to 3,624 in 2002 (Figure 1).This change was at least partly related to egg addling (about 70% of nests) by DNR, the removal of adult swans from Federal National Wildlife Refuges, and authorized scientific collecting.
Figure 1: Number of Mute Swans in Maryland 1962-2002
Mute swans have become common locally throughout Marylandís Eastern Shore and a few western shore tributaries and their range is expanding (Figure 2). Although most nest on the edges of tidal wetlands, the population has increased to the point where swans are now nesting on inland reservoirs, ponds, shallow impoundments, canals, and dredge spoil ponds. A small number of mute swans nest in the coastal bays of Worcester County.
The most recent Bay-wide survey of mute swans was conducted in summer 2002. During this survey, the largest numbers of mute swans were located in the mid-Bay, from Taylorís Island (Dorchester County) to Rock Hall (Kent County) on the Eastern Shore (Figure 2). Large concentrations also occur in the vicinity of Hoopers and Bloodsworth Islands. However, swan pairs have now established breeding territories in all Maryland tidal tributaries.
Figure 2: 2002 Mute Swan Distribution in Maryland (largest circle = 472 swans)
In the absence of population control, the only significant factors that currently limit population growth are flooding of nests, predation of eggs and young, and mortality from collisions with utility lines and other obstructions. Natural mortality of adult mute swans is quite low (less than 10% annually). Mute swans usually begin breeding at 3 years of age and can live up to 30 years. The number of breeding swan pairs in Maryland will increase rapidly as immature swans reach breeding age. In 2002, more than 83% of all the subadult and adult swans observed in Maryland were either nonbreeders or failed breeders. A recent example of how fast the number of nesting pairs of mute swans can increase was observed in the Patuxent River. In 2000, there were only 6 active nests located in the river. In 2001, the number of nests had increased to 40 (+660% increase in 1 year).
Considering the availability of unoccupied swan breeding habitat, the potential for the mute swan population to increase its numbers and expand its range is high. Territory size of mute swans has been reported to vary between less than 3 acres in high quality areas to about 15 acres on large bodies of water and open rivers. The upper Chesapeake Bay includes about 251,454 acres of coastal estuarine wetlands. Even assuming territories are at the upper limit of this range (15 acres) these wetlands could potentially provide nesting territories for about 16,960 mute swan pairs.
Maryland's coastal zone includes 4,358 miles of shoreline along the
Chesapeake Bay and Coastal Bays. During a 2001 survey of mute swans along the Talbot County shoreline, DNR observers
recorded 119 nesting mute swan pairs or about 0.27 nesting pairs per mile of shoreline. Assuming
coastal shorelines of all sixteen coastal counties could provide nesting
territory for an additional 1,180 pairs of mute swans.
Thus, considering the availability of unoccupied coastal wetlands and
shoreline, there is the potential in the state to provide nesting territories
for about 18,140 nesting mute swan pairs.
Including non-breeders, this could represent a population of about
100,000 mute swans. Furthermore,
this estimate does not account for mute swans that occupy inland freshwater
wetlands, ponds, impoundments, and reservoirs.
Therefore, unless there are
widespread disease outbreaks or serious degradation of the quality of
remaining wetlands, the size of the mute swan population will likely increase
dramatically, and impacts to native species will increase, unless growth is
limited by population control.
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contents (c) 2003
Maryland Department of Natural Resources.