Competition for Winter Territories in a Neotropical Migrant: The Role of Age, Sex and Color

Abstract

-Hooded Warblers (Wilsonia citrina) defend long-term feeding territories during the nonbreeding season. Experimental removals of territory owners early in the fall in Mexico resulted in rapid replacement by nonterritorial individuals (floaters). The sex ratio of replacement birds was similar to that of territory owners in the same habitat, but virtually all replacements were hatching-year (HY) birds. Females were able to acquire and defend territories in the presence of male floaters. Loss of wintering habitat would likely increase the proportion of nonterritorial birds and, therefore, increase overwinter mortality rates, especially of HY birds. The extent of malelike coloration of females was not correlated with habitat, indicating that dark females were not more likely to obtain territories in forested habitat where males predominate. Furthermore, the aggressive response of male territory owners toward models did not depend on intruder color. These results suggest that malelike coloration in females does not function in competitive interactions with males. Intense competition for winter territories coupled with reduced survivorship of winter floaters could constrain breeding strategies if young produced late in the summer have little chance of obtaining winter territories. Received 5 April 1993, accepted 30 September 1993. NEOTROPICAL MIGRANT songbird populations have declined in recent years (Askins et al. 1990). Habitat loss and fragmentation in the tropics is a major potential cause of this decline (Robbins et al. 1989). If habitat loss has a detrimental effect on wintering populations, then densities must be high relative to the carrying capacity of the habitat (Winker et al. 1990). Although many migrants are territorial on their wintering grounds (e.g. Greenberg 1986, Holmes et al. 1989), little is known about the intensity of competition for those territories (Morton et al. 1987, Winker et al. 1990) or the effects of such competition on population dynamics (Marra et al. 1993). When resources become limiting, are females at a disadvantage in competing for territories with males? Do yearling birds have difficulty obtaining territories? Age or sex biases in competitive ability for winter territories could have important implications for overwinter survival and productivity on the breeding grounds (Marra et al. 1993). The existence of nonterritorial populations on the wintering grounds has been well documented only for the Wood Thrush (Hylochichla mustelina; Rappole et al. 1989, Winker et al. 1990) and American Redstart (Setophaga ruticilla; Marra et al. 1993), and has been reported anecdotally for the Blackthroated Blue Warbler (Dendroica caerulescens; Holmes et al. 1989) and Hooded Warbler (Wilsonia citrina; Rappole and Warner 1980, Morton et al. 1987). Experimental removal of territory owners is an important tool for studying floating populations (e.g. Krebs 1971), but has been performed on the wintering grounds for only a few species (Rappole and Warner 1980, Morton et al. 1987, Winker et al. 1990, Marra et al. 1993). I conducted removal experiments with wintering Hooded Warblers to determine: (1) the size of the nonterritorial floater population; and (2) the relative abilities of birds in particular age/ sex classes to obtain territories. The winter ecology of the Hooded Warbler is among the best studied of any Neotropical migrant (Rappole and Warner 1980, Lynch et al. 1985, Morton et al. 1987, Morton 1990, Morton et al. 1993). There is habitat segregation by sex, such that most birds in the forest are males; however, both males and females are found in scrub habitat (Lynch et al. 1985). When territorial males are removed from forest habitat, females in adjacent scrub habitat do not move onto those territories even when such territories remain undefended, indicating that females are not being excluded from forest habitat (Morton et al. 1987). Experiments with hand-raised individuals and field data suggest that habitat segregation results from innate habitat preferences by each sex (Morton 1990, Morton et al. 1993). During winter females must compete directly