The Plum Island Sound ecosystem is a valuable habitat for migratory birds. During the 1940's, the United States Fish and Wildlife Service (USFWS) began acquiring land on Plum Island to preserve wildlife habitat. Today, the Parker River National Wildlife Refuge (PRNWR) includes over 4,600 acres of protected salt marsh, beach, and upland areas on Plum Island. Over 300 species of birds have been recorded on the Island where large numbers of migratory birds converge to feed and rest each spring and fall (USFWS, 1990). Some of the salt marshes and uplands along the western shore of Plum Island Sound and the Parker River have been designated as state wildlife management areas because of the many shallow pannes within them that provide resting and feeding areas for thousand of migratory shorebirds, herons, egrets and waterfowl.
Despite the recognized importance of the Plum Island Sound ecosystem to wildlife and the large numbers of people who visit the area to observe birds, little data exists on the historical trends in bird numbers for this areas. National Audubon Christmas Bird Counts and winter waterfowl surveys conducted by the Massachusetts Division of Fisheries and Wildlife provide some long term information. During the past few years, members of the Brookline Bird Club, in cooperation with staff biologist at the Refuge, have carried out weekly surveys of waterfowl, raptors, and shorebirds during migration, and biweekly surveys at other times of the year. PRNWR personnel have recently been monitoring brood success of waterfowl, and carrying out yearly monitoring of tern populations as part of a statewide survey of coastal nesting birds. The Eastern Massachusetts Hawk Watch Association carries out weekend monitoring of raptors flying over Plum Island during spring hawk migration season.
As part of the Plum Island Sound Minibays project, we evaluate historical and current information on the use of Plum Island by water birds, waterfowl, shorebirds, gulls and terns. Our analysis examines long term trends and synthesizes baseline data about birds currently using Plum Island Sound for breeding, feeding, and resting.
We used two major sources of data for our evaluation of birds on Plum Island from the 1930s through the 1990s. For the 1990s we analyzed the results of bird surveys conducted by the Brookline Bird Club in the refuge during 1990, 1991, and 1993. These surveys were conducted weekly during migration periods (March to May and mid-July to October), and biweekly during the remainder of the year. We would like to thank the Parker River National Wildlife Refuge and the Brookline Bird Club for making the results of their bird surveys available to us for this project. To provide a historical comparison, we analyzed the journals of ornithologist Ludlow Griscom who kept notes on the birds he observed on field trips throughout the state during the 1930s, the 1940s, and the 1950s. Many of Griscom's weekly trips were to Essex County and Plum Island. Griscom's journals are currently housed in their original forms at the Peabody/Essex Museum in Salem, Massachusetts.
Based on our evaluation of the adequacy of data, we included four shorebirds (black-bellied plover, greater yellowlegs, semipalmated plover, and semipalmated sandpiper), six waterfowl (American black duck, common loon, green-winged teal, mallard, red-breasted merganser, and white-winged scoter), one gull (Bonaparte's gull), and one tern (common tern) in our analysis. We evaluated the highest number of birds observed each year at Plum Island during any one survey, and developed averages for the maximum number of birds observed during the 1930s, 1940s, 1950s, and 1990s. Comparisons were then based on the three highest peak migration numbers during each decade for each species.
Despite the recognized data limitations, the comparison between bird numbers recorded in Griscom's journals and the more recent data from the PRNWR was sufficient to identify historical changes for certain species of birds on Plum Island.
During the period between the 1930s and the 1950s, the average peak population for three out of four of the shorebirds evaluated (greater yellowlegs, semipalmated sandpipers, and black-bellied plovers) decreased, while semipalmated plovers remained relatively stable. In contrast, four out of six waterfowl species showed trends of increasing between the 1930s and the 1990s. Mallard, common loons, green-winged teal, and red-breasted mergansers observed in the Plum Island Sound ecosystem showed an overall increase, while white-winged scoters and American black ducks declined. The average peak number of Bonaparte's gulls using the Sound has remained relatively stable with large fluctuations on a year to year basis. The peak number of common terns observed during this period has declined.
There has been a recognizable decline in waterfowl populations throughout the northeast region. "Rafts" of winter ducks are no longer seen along the North Shore. However, it is difficult to attribute population trends for the birds measure in this report to specific local changes since most of these birds are migratory. In general, there is little evidence that Plum Island Sound as a habitat for birds has changed significantly between the 1930s and today. We do know that ditches, which have been dug throughout the marshes to reduce mosquito breeding habitat, have reduced the umber of salt pannes available to birds, and that humans have affected mallard populations by feeding them. However, most of the changes in the average peak numbers of birds in Plum Island may be related to regional and global factors such as the following:
- Changes in the adequacy of breeding habitat in other regions may impact the
bird species that come to Plum Island Sound during the non-breeding season.
- Shifts in the number and type of fish found in Plum Island Sound caused by
overfishing in the Gulf of Maine and other factors may have increased some of
the food species available to birds in the Sound.
- Migratory birds often shift their migration patterns n response to weather
conditions and the availability of food.
In addition to identifying historical trends for birds recorded at Plum Island, our analysis provides valuable baseline data about the birds found in the Sound today. This information can be used in the future to evaluate changes in the use of this important habitat by birds.