Birds, body size and the threat of extinction

A relation between body size and threat of extinction for animal species has often been hypothesized. However, evidence for the form of the relation is equivocal, and studies can be found reporting positive, negative, or no relation between body size and extinction risk. One way to assess this relation is to compare the body sizes of species considered to be globally threatened with those of species considered to be less at risk. We adopt this approach for birds, considering a bird to be in danger of global extinction if it was listed by Collar & Andrew (ICBP technical publication no. 8 (1988)). Threatened species of bird are, on average, larger-bodied than non-threatened species. This difference is not due to size differences between island endemic species and species with continental distributions. Island endemic and continental species show no consistent body size differences. The relation between body mass and threat of extinction is not due to differences between higher taxa: within taxa, there is still a relation between body size and extinction threat. We present evidence that the degree of threat faced by endangered species may also be related to body mass. We discuss possible explanations for the observed patterns, and conclude that a genuine tendency for large-bodied birds to be more at risk from extinction than small-bodied species is the most likely.

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