Radar Tracking of Experimentally Released Migrant Birds

Demong, N.J. & Emlen, S.T. (1978). Bird Banding, 49:342-359.

We describe a new technique that makes it possible for a researcher to exercise manipulative control over the orientation systems of free-flying migrant birds. As such, it bridges the gap between descriptive (but uncontrolled) field observations of migration and controlled (but unnatural) laboratory investigations of orientation. The technique involves sending a migrant aloft in an especially designed release box, suspended beneath a helium-filled weather balloon. At a predetermined altitude, the box opens and the bird is released, free to continue its migratory journey. The migrant can be followed by visual observation, by radio-tagging, or by radar tracking.

Potential orientation cues can be controlled either (1) by choosing to release birds under different meteorological conditions (when high over-cast obscures celestial information; when low fog blocks visual landmarks on the ground; when winds are from inappropriate directions for migration, etc.), or (2) by subjecting birds to various experimental manipulations prior to release (phase-shifting the bird's time sense; attaching miniature body magnets, etc.).l

The flight behavior of artificially released birds was compared to, and found consistent with, the known behavior of naturally occurring passerine migrants. Specifically: (1) more than 70 percent of all birds released maintained altitude and initiated long flights; (2) the tendency to migrate correlated with internal motivational state, with fatter birds being more prone to initiate flights; (3) the proportion of birds initiating flights was greatest when released under stimulatory synoptic weather conditions; (4) all birds that initiated migratory flights exhibited the "flap-pause" wing beat pattern typical of nocturnally migrating songbirds; and (5) the departure bearings selected by the artificially released birds were consistent with expected migratory directions in each of three spring migration seasons.

We present initial results gathered from radio-tracking of Swainson's and Gray-cheeked thrushes in Ithaca, NY, and from radar tracking of White-throated Sparrows at Wallops Island, VA.