Wingbeat frequency of two Catharus thrushes during nocturnal migration

Radio transmitters emitting continuous signals were mounted on Swainson's Thrushes (Catharus ustulatus) and Veerys (C.fuscescens) during two spring migration seasons. Wingbeats modulated the continuous signals, and signal-processing techniques permitted examination of wingbeat frequency (WBF) during nocturnal migration. Distinct takeoff and cruising-flight phases were evident in both species. As birds climbed during the takeoff phase (which usually lasted about 15 min), WBF typically declined exponentially by at least 4 Hz. Median cruising WBF from seven flights varied from 9.2 to 10.9 Hz, values similar to those calculated from published equations predicting WBF based on flight mechanics. Cruising WBF typically varied slowly and irregularly throughout all flights by about 1 Hz. During takeoff and cruising flights, all birds flapped continuously, showing no evidence of the flap-coasting pattern common in passerine migrants previously studied with other techniques such as radar. All five birds exhibited departure times, course directions, and flight distances typical for migrating thrushes.