WIKELSKI, Martin C; Princeton University: Migration ecology of New World Thrushes: energetics and orientation in the wild

George A. Bartholomew pioneered the investigation of physiological mechanisms in natural settings and sparked an amazing diversity of research directions. However, the migration physiology of free-flying songbirds is still largely unknown because adequate methodology to study it was lacking. I report data relating to two aspects of songbird migration in the wild: energy expenditure during nocturnal flight, and natural orientation. Catharus thrushes generally winter in Central/South America and migrate to Canadian breeding grounds. We caught birds in Illinois during spring migration, attached radio transmitters and followed birds during their natural migratory flights, up to 600 km. We measured the flight energetics (via doubly labeled water), heart rate, wing beat and respiration during migration. We show that although flight is expensive, the birds spent twice as much energy on stopover compared to migration. Cold nights in particular are energetically demanding. Our data have important implications for songbird conservation by highlighting the importance of small-scale stopover sites such as natural backyards in suburban areas. During migration, thrushes apparently use a magnetic compass that is calibrated daily from sunset cues. We tested their orientation capabilities experimentally by changing the birds� magnetic field prior to release and again following birds during natural flights. Birds for which the magnetic field was turned east migrated west, instead of north as expected. However, during their second migratory flight they re-oriented northward, after re-calibrating their magnetic compass with sunset cues. I propose a space-base tracking system that would allow us to follow small songbirds on a global scale. Such a system would finally solve the question of where and when songbirds migrate and disperse, where they die or what songbirds need to survive.