DICKISON, M.R.; Duke University: Are birds small? A macroecological perspective.

Are birds small? Birds seem smaller than mammals, because almost all birds fly while few mammals do, and flight imposes severe constraints on body size. The most commonly cited study of avian and mammalian body size found modal sizes of about 10 g and 100 g respectively. This study, however, compared North American birds (which all fly) with terrestrial North American mammals (which do not). Better comparisons are between flightless birds and flightless mammals, and between flighted birds and bats. Moreover, almost all previous studies neglect recently extinct taxa. Since most large or flightless birds, as well as many large mammals, have been exterminated by humans in the last few thousand years, such exclusions necessarily bias any comparisons. Without this bias, birds and mammals have approximately the same mean and modal size. Nevertheless, the largest mammals are far bigger than the largest birds, especially given new, more conservative, estimates of the largest known bird’s weight. Biomechanical, phylogenetic, and biogeographical explanations for the lack of truly giant birds have been proposed; each will be examined.