Seasonal abundance codes are based primarily on data from point count surveys conducted by biologists from the USGS Patuxent Wildlife Research Center (PWRC) as part of the Bird Inventory Project for National Capital Parks--East (NACE). Surveys were conducted during April 30 - May 14, 1999; May 22 - June 27, 1999; August 24 - October 5, 1999; January 4 - February 26, 2000; and May 5 - May 18, 2000. For bird species observed on surveys conducted during the nesting season or winter, seasonal abundance codes were derived by dividing the number of points in the park at which a species was recorded in a particular season by the number of points at which the most common species was recorded in the same season. Bird species found at one third or more of the points were coded as "Common". Species found at fewer than one third but at least one tenth of the points were coded as "Uncommon". Species found at fewer than one tenth of the points were coded as "Occasional".

Seasonal abundance also was coded as "Occasional" for bird species that were not recorded on the point counts but whose occurrence in the park has been documented since 1980. Additional bird observations were documented by PWRC biologists on occasional visits to NACE parks for the Bird Inventory Project, on surveys of bird use of wetlands in or adjacent to several parks (Kenilworth, Anacostia, Oxon Cove, Harmony Hall, and Piscataway) conducted at roughly bi-weekly intervals in July through December 1999 and March through April 2000, and as participants in the Christmas Bird Counts (Washington, D.C.; Bowie, MD; Ft. Belvoir, VA/MD) and the May Counts. Also included were observations from bird surveys in NACE parks conducted by PWRC biologists for other projects (i.e., point count surveys in Greenbelt Park, 1997, and in Greenbelt and Piscataway Parks and along the Baltimore-Washington and Suitland Parkways, 1992 - 1994, as part of the Prince George's County Forest Bird Survey). Other sources of information include observations by NPS biologists and other birders, the Bio-Blitz at Kenilworth Park and Aquatic Gardens (May 31-June 1, 1996), and point count surveys conducted in NACE parks in June 1993 - 1995 for the DC Birdscape Project.