Geographical Correlates of Abundance vs. Rarity in Some North American Winter Landbirds

Abstract

Using Christmas Bird Count (CBC) data, I compared range sizes and within-range abundances of 70 species of apodiform, piciform, and passerine landbirds whose ranges are 75% or more restricted in winter to the contiguous U.S. and southern Canada. Range size was computed as the number of occupied 5° latitude-longitude blocks. Three abundance measures were calculated: (1) mean birds counted/census hour across all occupied blocks, (2) maximum birds/hour in a single block, and (3) maximum birds/hour on a single CBC. Range size was positively but weakly correlated with each abundance measure, and the abundance measures were very strongly correlated with one another. Geography was a powerful predictor of the species' positions in a two-dimensional space difined by the axes of range size and average within-range abundance. Taxa that breed and winter at higher latitudes had larger total populations, and had significantly larger ranges and average local abundances. Species grouped by longitudinal areas of greatest local abundance had distinct range sizes but did not differ in average within-range abundance. Eastern species had larger ranges than comparably abundant western forms, probably because the eastern U.S. is characterized by relatively widespread habitat types. Results of this study suggest that a species' within-range abundance is influenced by the degree of its habitat generalization, whereas its range size will be larger if it is a habitat generalist or a specialist on widespread habitats. Because individual CBC's include many habitats, the same ecological attribute-habitat generalization-could cause species to be both widespread and abundant inside CBC circles. Carefully standardized within-habitat censuses will be required to determine whether or not these generalist species also dominate the individual habitats occupied by their more specialized and narrowly distributed relatives.