Repeated evolution of unorthodox feeding styles drives a negative correlation between foot size and bill length in hummingbirds

We gathered records of hummingbirds clinging by their feet to feed legitimately as pollinators or illegitimately as nectar-robbers. We measured key features of bills and feet for 220 species of hummingbirds and compared the 66 known “clinger” species to the 144 presumed “non-clinger” species. Once the effects of phylogenetic signal, body size, and elevation above sea level are accounted for statistically, hummingbirds display a surprising, but functionally interpretable negative correlation. Clingers with short bills and long hallux (hind-toe) claws have evolved—independently—more than 20 times, and in every major clade.