201 Notes The Auk 119(1):201–209, 2002 Orientation of Nocturnally Migrating Swainson’s Thrush at Dawn and Dusk: Importance of Energetic Condition and Geomagnetic Cues ROLAND SANDBERG, 1,3 FRANK R. MOORE, 2 JOHAN BA ¨ CKMAN, 1 AND MARE LO ˜ HMUS 1,4 1 Department of Animal Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden; and 2 Department of Biological Sciences, University of Southern Mississippi, Hattiesburg, Mississippi 39406-5018, USA ABSTRACT.—We studied the early morning cage orientation of nocturnally migrating Swainson’s Thrushes (Catharus ustulatus) during three fall mi- gration seasons. The results were compared with earlier free-flight release tests under starry skies and were found to be consistent with continuation of mi- gratory flights in the expected seasonally appropri- ate direction. Energetic condition proved decisive: fat birds chose directions in accordance with migra- tion across the Gulf of Mexico, whereas lean birds oriented away from the coast, possibly in search of habitats suitable for refuelling. Whereas the orien- tation of fat Swainson’s Thrushes was affected by ex- perimental shifts of the magnetic field, the response during morning tests was larger than expected. A parallel series of orientation cage experiments per- formed during evening twilight showed a response to deflected magnetic fields that was close to the ex- pected shift, which suggests a difference in integra- tion of directional information between early morn- ing and evening twilight activity. However, within-individual response to deflected magnetic fields was of the same magnitude during both morn- ing and evening tests. RESUMEN.—Estudiamos la orientacio ´n al amane- cer de individuos de la especie migratoria nocturna Catharus ustulatus en jaulas durante tres estaciones migratorias de oton ˜o. Los resultados fueron com- parados con datos anteriores de ensayos de libera- cio ´n de vuelo libre bajo cielos estrellados y resulta- ron consistentes con la continuacio ´n de los vuelos migratorios en la direccio ´n esperada (adecuada para la estacio ´n). La condicio ´n energe ´tica resulto ´ ser de- cisiva: las aves con ma ´s grasa corporal eligieron di- recciones hacia el Golfo de Me ´xico, mientras que las aves con poca grasa corporal se orientaron en direc- cio ´n opuesta a la costa, posiblemente en busca de ha ´bitats apropiados para reabastecerse. Aunque la 3 E-mail: roland.sandberg@zooekol.lu.se 4 Present address: Department of Zoology, Go ¨te- borg University, Box 463, SE-405 30 Go ¨teborg, Sweden. orientacio ´n de los individuos de C. ustulatus con mayor contenido de grasa fue afectada por cambios experimentales en el campo magne ´tico, la respuesta durante las pruebas realizadas en la man ˜ana fue mayor que la esperada. Una serie paralela de expe- rimentos de orientacio ´n en jaulas realizados durante el ocaso mostro ´ un cambio cercano al esperado con los cambios del campos magne ´tico. Esto sugiere que existe una diferencia entre el amanecer y el ocaso en la manera en que la informacio ´n direccional es in- tegrada. Sin embargo, la respuesta de un individuo en particular a los cambios en el campo magne ´tico fue de la misma magnitud durante las pruebas rea- lizadas en la man ˜ana y en la tarde. Most neotropical landbird migrants fly at night. Nocturnal migration typically commences during the first hour after sunset, peaks around 2200 h, and wanes at midnight or shortly thereafter (Kerlinger and Moore 1989). Although most nocturnally mi- grating songbirds end their migratory flight well be- fore dawn, migrants have been observed to under- take ‘‘morning flights’’ (Rabøl 1967, Alerstam 1978, Gauthreaux 1978, Bingman 1980, Hall and Bell 1981, Wiedner et al. 1992). The function of morning flight is open to speculation (Moore et al. 1995), but is like- ly to depend on the bird’s physiological condition and the ecological context within which the migrant finds itself. When migrants end their flight at night, initial selection of habitat probably occurs early in the morning, and morning flights may reflect a pe- riod of exploration on the part of migrants seeking habitat in which to rest and forage (see Terrill and Ohmart 1984, Lindstro ¨m and Alerstam 1986, Wied- ner et al. 1992). Other observations suggest that birds may engage in morning flight to compensate for drift experienced during nocturnal migration (sensu Lack and Williamson 1959; see Gauthreaux 1978, Moore 1990). If nocturnal migrants select a direction at the time of takeoff and subsequently try to maintain that direction during a night’s flight, then the next morn- ing would be a convenient time to reorient if dis- placement has occurred (Vleugel 1954, Lowery and Newman 1955, Moore 1987). For other night mi- grants, movement in the morning hours may simply Downloaded from https://academic.oup.com/auk/article/119/1/201/5561870 by guest on 25 February 2023