Feeder counts as indicators of spatial and temporal variation in winter abundance of resident birds

Project FeederWatch (PFW) is a North American-wide volunteer program that h as compiled weekly observations of birds at feeders throughout each winter since 1987-1988. Three population indices are calculated for each bird spec ies: percent of feeders visited, group size (number observed per visit), an d mean abundance per feeder over the entire season. For nine resident speci es in the northeastern United States, we tested whether PFW indices reflect spatial and temporal patterns of abundance similar to those derived from t he Breeding Bird Survey (BBS). For most species, PFW indices were strongly correlated with BBS relative abundances from the same states (averaged acro ss years). Percent of feeders visited was significantly correlated with BBS in all nine species, and mean abundance was significantly correlated in ei ght species, whereas average group size was correlated in only four species . However, there was poor correlation between PFW and BBS annual abundance indices (averaged across states). The range of temporal variation in PFW an d BBS indices is more limited than the range of spatial variation across th e northeast region, and PFW annual indices evidently do not track subtle po pulation changes in the same manner as BBS. For two species (Carolina Wren, House Sparrow) that exhibited marked among-year changes in abundance, howe ver, PFW tracked the same pattern of change as BBS, and annual indices from both programs were significantly correlated. We conclude from our analyses to date that winter feeder counts, if interpreted with care, accurately re flect spatial, and in some cases temporal, variation in abundance of common resident birds.