Wingbeat frequency and flap-pause ratio during natural migratory flight in thrushes

Powered flapping flight has evolved independently in many different taxa. For flapping fliers, wingbeat parameters such as frequency and amplitude are the primary determinants of these animals' energetic expenditure during flight. Here we present data on wingbeat frequency and amplitude for three New World thrush species during 15 entire nocturnal migratory flights over the Midwestern United States. Using continuous (non-pulsing) radio transmitters, we were able to measure wingbeat frequency and relative amplitude of wingbeats as well as the characteristics of flap-pauses. Contrary to previous telemetric findings, all of the individuals we followed used both flapping-only and flap-pause flight. During migratory flights, wingbeat frequency, effective wingbeat frequency, and amplitude were highest during initial ascent. Effective wingbeat frequency and amplitude were lowest during final descent. We show that identification of species based solely on characteristics of the wingbeat e.g., during radar studies, can be difficult because variables such as wingbeat frequency and amplitude, wingbeat pausing, and pattern of beats and pauses vary between individuals of the same species and even within individual flights. We also show that observed wingbeat frequencies were lower than those predicted by theoretical models. We speculate that this may be because theoretical predictions are generally based on (1) data from larger birds and (2) data from diurnal flights. We found that diurnal wingbeat frequencies of thrushes were generally higher than were those during nocturnal migratory flight. Finally, we suggest that rather than remaining at a single altitude during flight or climbing slightly as theoretical models predict, thrushes often moved up and down in the air column, perhaps searching for favorable atmospheric conditions.