Habitat Segregation by Sex in the Hooded Warbler: Experiments on Proximate Causation and Discussion of Its Evolution

Although habitat choice has been shown to have an innate basis in some vertebrates, few experimental studies have been performed, and these most often relied on interspecific comparisons. Male and female hooded warblers (Wilsonia citrina) defend territories in markedly different habitats while wintering in the tropics. Males use forests; females use shrub or field habitats. I used naive handraised warblers in experiments designed to test for sex-related innate differences in habitat selection. Choosing between two three-dimensional artificial habitats, males chose the tall habitat with separated stems over the short habitat with dense stems, whereas females showed no preference. Field observations and a laboratory experiment with the naive birds showed no habitat preferences based on light intensity. Finally, naive warblers were exposed to a two-dimensional array of black stripes on opposite walls of an experimental room, one wall with the black stripes arranged vertically and the other with the stripes arranged at oblique angles to the floor. Males oriented toward the vertical stripes, whereas females oriented toward the oblique stripes. Since forests have more vertical structures than shrubby fields, an innate preference for cues based on vertical or oblique cues is a sufficient proximate mechanism to bring about the observed habitat segregation between the sexes in wild birds. I discuss possible sources of selection favoring the evolution of intraspecific habitat segregation by sex. A hypothesis is presented to explain the presence of a subadult female plumage and a highly variable adult female plumage as they relate to habitat segregation in the hooded warbler. I suggest that this unusual plumage variability in females resulted from changes in the habitat available to females after swidden agriculture began in their main wintering area.