Weather and Bird Populations in True Fir Forests of the Sierra Nevada, California

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Abstract
We monitored bird communities of true fir forests at 51 study sites in the western Sierra Nevada during the breeding seasons of 1983, 1984, and 1985. The summer of 1983 followed the El Nino winter of 1982-1983, with the greatest snowfall on record. The summers of 1984 and 1985 followed winters with moderate snowfalls. Bired species richness (BSR) and average total count (ATC) increased linearly from 1983 to 1985. The relative abundances of 12 common species increased from 1983 to 1985; abundances of two common species did not differ among years. Twenty-eight uncommon species increased and four decreased in numbers from 1983 to 1985. Bird response patterns differed between lower-elevation white fir (Abies concolor) forests and upper-elevation red fir (A. magnifica) forests. Bird numbers were similar in both habitats in 1983 but greater in white fir than in red fir in 1984 and 1985. Although abundances of the common species increased in both habitats in both years, those of uncommon species did not increase substantially in red fir until 1985. We suggest that bird numbers were depressed in 1983, but not atypically so for true fir forests. Numbers of permanent residents are often limited by frequent but unpredictable winters with excessive snowfall (Beedy 1982, Granholm 1982. Raphael and White 1984). Numbers of migrants, as in the case of this study, are sometimes affected similarly. White fir and lower-elevation forests may harbor "source" populations for red fir "sink" populations during periods of resoruce stress.