The Orientation of Passerine Nocturnal Migrants Following Offshore Drift

Abstract

Abstract Nocturnal autumn passerine migrants are frequently drifted by wind or carried by downwind flight from New England to offshore islands. Most individuals in these flights are immatures. Many recover the mainland via reoriented northward flights. During fall 1972, I made field observations and performed orientation cage tests on nocturnal migrants on Block Island, Rhode Island. Migrants reached the island during four southward movements in following winds. Migration was very light or absent on most other nights. Reoriented diurnal flights left from the island immediately after the arrival of a large nocturnal migration. Northward movements occurred on two nights following daytime reoriented flights. These nocturnal movements are tentatively interpreted as constituting reoriented flights. About 27% of the 79 individuals tested in orientation cages showed significantly directional nocturnal activity. About two-thirds of these oriented basically northwestward, corresponding to the reoriented flight of free-flying birds. These and similar reorientations of immature birds can be explained by a simple compass reorientation in response to wind drift over the ocean without invoking complicated navigation mechanisms. The remaining individuals oriented toward the southeast. Eleven birds exhibited a significant eastward orientation during the first few hours after dawn; this is interpreted as a positive phototaxis. Three of four Blackpoll Warblers oriented southward. No correlation existed between the quantity of Zugunruhe and the amount of subcutaneous fat, but fatter birds were significantly more likely to show oriented nocturnal activity.