The relative sizes and asymmetry of kidneys in passerine birds from Australia and North America.

Despite their close taxonomic affinities, nectar-feeding passerine birds from Australia had smaller kidneys, on average, than sympatric passerines of equivalent weight that fed entirely upon insects. Insectivorous passerines from North America had larger kidneys, on average, than comparable insect-feeding passerines from the separate endemic radiation in Australia. Dietary and other environmental differences, rather than phylogenetic origins, may account for these differences. The left kidney of Australian passerines was significantly longer, on average, than the right. Kidney widths showed no lateral asymmetry.