Species richness among birds: body size, life history, sexual selection orecology?

Why do some avian families contain so many more species than other families ? We use comparisons between sister taxa to test predictions arising from s ix explanations to this puzzle: that differences between families are due t o chance, body size, life history, sexual selection, intrinsic ecological f actors or extrinsic abiotic factors, respectively. In agreement with previo us analyses, we find no support for the idea that differences in species ri chness are simply due to chance. However, contrary to most previous work, w e also find no support for the hypotheses that high species richness is cor related with small body size and fast life history. Rather, high species di versity is strongly associated with pronounced plumage dichromatism, genera list feeding habits and good dispersal capabilities as well as large and fr agmented geographical ranges. In addition, all of these relationships are r obust to the removal of the two most speciose avian lineages, the Ciconiifo rmes and the Passeriformes. The supposed relationships between species rich ness and both body size and life history are, however, due to phylogenetic non-independence. Together with previous work showing that differences betw een avian lineages in extinction risk are associated with variation in body size and life history, these results indicate that extinction rates and sp eciation rates are not necessarily determined by the same factors. Hence, h igh extinction rates are not inevitably associated with low speciation rate s. Extinction-prone. lineages may, in fact, have a high rate of speciation. In such lineages a high proportion of 'vulnerable' species would be a natu ral, ongoing phenomenon.