Additional Evidence of Nocturnal Migration by Yellow-Rumped Warblers in Winter

Abstract

Migration is a response to changing environmental conditions (Gauthreaux 1982). Migratory behavior spans a continuum from highly fixed behavior (obligate migration) to very flexible behavior (facultative migration). Whereas facultative migration occurs in response to deteriorating environmental conditions, obligate migration appears to be fundamentally a result of an endogenous, circannual program (see Gwinner 1986 for a recent review). Even obligate migrations, however, are not performed entirely without integration of environmental information. For example, migrants make daily decisions to migrate or not to migrate based upon environmental factors such as weather conditions, food and water availability at stopover sites, and competition (e.g., Rappole and Warner 1976, Richardson 1978, Mehlum 1983, Biebach 1985, Gwinner 1986, Biebach et al. 1986). The degree to which migratory behavior is subject to environmental influence appears to be quite variable (Terrill and Ohmart 1984). There is evidence that at least some species of annual, migrant passerines facultatively extend migration after the fall migratory period if environmental conditions become unfavorable for overwinter survival at a particular locality (Perdeck 1964, Niles et al. 1969, Moreau 1972, Lack 1983; Terrill and Ohmart 1984; Terrill 1987, in press). Presumably, the ability to perform facultative migrations enables these birds to respond to relatively unpredictable winter environments by migrating in an appropriate direction if conditions necessitate it. Patterns of migration and distribution of Yellowrumped Warblers (Dendroica coronata) indicate that they switch from an obligate phase to a facultative phase over the course of fall migration (Terrill and Ohmart 1984). Winter populations shift southward if conditions become unusually severe. When placed in Emlen funnels (Emlen and Emlen 1966), Yellowrumped Warblers showed southerly nocturnal orientation into early January (after which no birds were tested). While the orientation data support the hypothesis that winter movements like fall movements are nocturnal, the evidence is indirect. Here we present direct evidence that winter movements by Yellow-rumped Warblers are in fact noctur' Received 15 April 1987. Final acceptance 27 August 1987. 2 Present address: 208 Junius St., Thomasville, GA 31792. SHORT COMMUNICATIONS 261