Biotelemetry of New World thrushes during migration: Physiology, energetics and orientation in the wild

Billions of songbirds migrate between continents each year, but we have yet to obtain enough information on in-flight physiology and energetics to fully understand the migratory behavior of any one species. New World Catharus thrushes are common nocturnal migrants amenable to biotelemetry, allowing us to measure physiological parameters during migratory flight in the wild. Here, we review work by the authors on Catharus thrush in-flight physiology during spring migration in continental North America and present new data on individual variation in energy use during migratory flight. Previous work demonstrated that (1) a number of simple behavioral rules are sufficient to explain the initiation of individual migratory flights made by Catharus thrushes, (2) the thrushes used a magnetic compass to orient during the night rather than celestial cues and that they calibrated this magnetic compass each day using cues associated with the setting sun, (3) in total, Catharus thrushes used approximately twice as much energy during stopovers than they used during migratory flight, and (4) thrushes may use more energy when thermoregulating on cold days than on days when they make short migratory flights. Recently, we built upon this work and used newly-developed transmitters to measure heart rate, wingbeat frequency and respiration rate of free-flying Swainson's Thrushes (C. ustulatus). We found a large amount of between-individual variation in average heart rate after ascent (range 12.06-14.81 Hz, mean +/- SD, 13.48 +/- 0.75, n = 10), average wingbeat frequency after ascent (10.25-11.75 Hz, 10.82 +/- 0.49, n = 10), and the difference between the two variables (1.5-3.84 Hz, 2.53 +/- 0.76, n = 8). Both heart rate and wingbeat frequency were significantly higher during ascent than later in the flight. We propose biotelemetry as a means to understand energetic trade-offs and decisions during natural migratory flight in songbirds. To further our knowledge of intercontinental songbird migration and the connectivity between wintering and breeding sites, we outline plans for a satellite-based global tracking system for < 1 g transmitters.