Geographic range structure in North American landbirds: variation with migratory strategy, trophic level, and breeding habitat

We investigated the relationship between abundance and geographic range structure of 258 North American landbirds. For this purpose we used six measures of range structure based upon fractal geometry and geostatistics, and three ecological characteristics that can influence avian distribution. Permanent residents (PRs) that were abundant showed little fragmentation of their abundance surface at the periphery of their breeding range. Conversely, common Neotropical migrants (NTMs) exhibited low fragmentation of their central populations. The abundance surface was smoother for PRs than NTMs or short-distance migrants (SDMs), indicating that changes in abundance occurred more gradually across space for this group. The areas of high abundance for grassland species were had little demographic fragmentation, but other populations showed little spatial autocorrelation in abundance. Species breeding in late-successional forests were relatively rare compared tospecies breeding in other habitat types. Among carnivores, PRs had ahigher average abundance than either NTMs or SDMs. Although carnivores had more distributional gaps within their ranges than other trophic groups, the number of gaps did not differ between rare and abundantspecies, indicating that increased abundance did not change significantly influence their presence/absence distribution maps. Knowledge of patterns and variations of geographic range structure among speciesmay provide insights into processes that shape and maintain the biodiversity of a continent. [ABSTRACT FROM AUTHOR]