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.