. Proceedings of the Symposium on Management of Forest and Range Habitats for Nongame Birds, May 6-9, 1975, Tucson, Arizona. Birds Conservation Congresses; Birds Habitat Congresses; Range management Congresses; Wildlife habitat improvement Congresses. b. ^0 100 1000 Body Weight Figure between territory- size (acres) and body weight (grams) for birds of different feeding categories. Symbols designate varying percentages of animal food as follows: a = 90-100% (mainly arthropods), b = 90-100% (mainly vertebrates), c = 70-90%, d = 30-70%, e = 10-30%, and f = 0-10%. Adapted from S
. Proceedings of the Symposium on Management of Forest and Range Habitats for Nongame Birds, May 6-9, 1975, Tucson, Arizona. Birds Conservation Congresses; Birds Habitat Congresses; Range management Congresses; Wildlife habitat improvement Congresses. b. ^0 100 1000 Body Weight Figure between territory- size (acres) and body weight (grams) for birds of different feeding categories. Symbols designate varying percentages of animal food as follows: a = 90-100% (mainly arthropods), b = 90-100% (mainly vertebrates), c = 70-90%, d = 30-70%, e = 10-30%, and f = 0-10%. Adapted from Schoener (1968). territories for their body weights are usually those consuming the greatest proportion of animal matter (AO)." Other important conclu- sions reached by Schoener were (1) that 'home ranges of raptors in two areas were signifi- cantly correlated with prey density, (2) clutch size did not correlate with territory size in any group tested, (3) probably no correlation existed between territory size and number of birds defending the territory or feeding young, and (4) the habit of obtaining most or all food from the territory varied according to diet, being highest among predatory specialists (; of species in category A, in category AO, in 0, in HO, and in H). Body Size, Territory Size, and Local Extinction Careful studies of the reasons for local extinction of bird populations are practically non-existent. One exception is the recent report of Willis (1974) on declining numbers of bird species on Barro Colorado Island since its establishment in the Panama Canal Zone in 1910-14. The island, in Gatun Lake, is separated from the nearest mainland by about 500 m of open water. Between 1914 and 1923, when the island was established as a biological reserve, patches of forest were cut down. "Much of the eastern half of the islan>d is medium-height forest, probably under 100 yr old; much of the western half is tall fores't, probab
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