Energetics and Spread-winged Behavior in Anhingas and Double-crested Cormorants: The Risks of Generalization

SYNOPSIS. Spread-winged behavior is observed in a variety of avian species, and a number of different functions have been ascribed to it. Anhingas and Double-crested Cormorants, representing closely allied avian families, share a spread-winged behavior that is superficially identical. This paper examines the behavior and energetics of these two species to determine whether the primary function of this behavior is wing drying or the supplementation of metabolism. Double-crested Cormorants and Anhingas differ both in the frequency and the contexts in which spread-winged behavior is displayed: Double-crested Cormorants display the behavior sparingly and without relation to ambient temperature or solar radiant intensity, whereas Anhingas exhibit the behavior often, particularly when ambient temperatures are low and solar radiant intensity is high. Similarly Double-crested Cormorants show no significant reduction in metabolic rate at ambient temperatures below thermoneutrality when provided with incident simulated solar radiation, whereas Anhingas significantly reduce their rates of metabolism when exposed to ambient temperatures below thermoneutrality in the presence of incident simulated insolation. Despite the apparent similarities between these two species taxonomically, ecologically, and behaviorally, spread-winged behavior has quite different functions and energetic/ecological consequences for Anhingas as opposed to Double-crested Cormorants. This example illustrates the risks of generalizing from superficialities and of relying on inference and speculation as opposed to data.