SPATIAL PATTERNS IN THE GEOGRAPHIC RANGE SIZES OF BIRD SPECIES IN THENEW-WORLD

The attempt to identify and explain pattern in the extent of species' geographical distributions at regional scales has been central to macr oecology. However, with the exception of abundance, consistent relatio ns between other variables and species geographic extent have not been forthcoming. One reason may be that studies often encompass the entir e geographic ranges of only a fraction of the species in the taxon und er consideration, setting biologically artificial boundaries to the ar ea of study, and only revealing part of the pattern in question. Here, we examine patterns in the geographic range sizes of birds in the New World. By testing for patterns in the entire avifauna of a geographic ally isolated region (95% of species are endemic), we avoid many of th e problems of previous studies. Most New World bird species have small geographic ranges, although the frequency distribution of logarithmic ally transformed ranges is left-skewed. The geographic range size-body size relation is approximately triangular. Small-bodied species may h ave either large or small ranges, whereas large-bodied species have on ly large ranges. Species threatened with extinction more often fall ne arer to (or below) the lower edge defined by the majority of species i n his triangle than do non-threatened species, suggesting that this re presents the minimum area needed to sustain viable populations of spec ies of different sizes. The maximum range size attained by species is relatively constant across body sizes, but falls short of the maximum possible given the land area of the New World, and so cannot be limite d by this constraint. What does limit maximum range size is thus uncle ar. There is a latitudinal gradient in the size of species geographic ranges. Species which have the latitudinal mid-point of their geograph ic ranges at high latitudes either side of the equator tend have range sizes, whereas those with mid-points at lower latitudes tend to have small range sizes (as expected from Rapoport's rule). However, this pa ttern is not symmetrical about the equator, but rather, at about 17 de grees N. It appears to be a consequence of the biogeography of the New World, and implies that mechanisms suggested to explain Rapoport's ru le based on climatic variability are incorrect. Migrant birds have lar ger geographic ranges, on average, than do residents. They are also la rger-bodied, and tend to inhabit more northerly latitudes than residen ts. They are also larger-bodied, and tend to inhabit more northerly la titudes than residents, but their larger ranges are not the simple con sequence of these other patterns. The patterns we demonstrate, in part icular those relating to maximum range size across body sizes and to l atitudinal variation in range size, have significant consequences for the understanding of what determines species geographic range sizes.