Time of Departure by Summer Tanagers (Piranga rubra) from a Stopover Site Following Spring Trans-Gulf Migration

Abstract

Nocturnal passerinc migration typically commences within an hour of sunset (Drury and Nisbet 1964, Casement 1966, Cochran et al. 1967, Parslow 1968, Gauthreaux 1971, Hebrard 1971, •kesson et al. 1996), and the bulk of migrants depart before the end of nautical twilight (which ends when the sun is 12 o below the horizon or almost an hour after sunset). This window of nightly departure is probably tied to the availability of directional information from a wide array of environmental cues (Emlen 1980, Moore 1987, Piersma et al. 1990) as well as to the onset of atmospheric onditions favorable for migratory flight (Kerlinger and Moore 1989). Although several orientation studies have quantified the cage activity of migrants during this window of time (see Moore 1987), species-specific departure of free-ranging passetines has been reported in only a few species (Cochran et al. 1967, •kesson et al. 1996). Discounting birds artificially stimulated to initiate flight, 7 of 12 radio-tagged Catharus thrushes that departed under clear skies during spring migration through Illinois did so within an hour of sunset, while greater variation occurred when birds (n = 6) .departed on overcast evenings (Cochran etal. 1967). Akesson et al. (1996) found that half of the 10 radio-tagged Song Thrushes (Turdus philomelos) that stopped over on the island of 61and, Sweden, initiated migration before the end of nautical twilight or within about 90 rain of sunset, whereas the others departed much later in the night and possibly the next morning. We report the time of departures of Summer Tanagers (Piranga rubra) from a stopover site along the northern coast of the Gulf of Mexico following trans-Gulf migration and consider several factors (e.g. fat loads) that might explain observed variation.