CALLS OF INDIVIDUAL TRANSIENT SWAINSON’S THRUSHES DURING FLIGHT AND STOPOVERS

The topic of flight calls is well summarized in Farnsworth’s review (Auk 122, 2005): Many passerine birds and their relatives utter flight calls, species-specific vocalizations given primarily during sustained flight, especially during migration. References to flight calls appeared in the ornithological literature as early as the 1890s, but some of the most basic features of these calls remain poorly known, including their functions, origins, ontogeny, distances over which they are used, and how much individual variation exists in the characters of the calls and their rates. With improved knowledge of these vocalizations, flight calls possibly will have a variety of applications. Identifying their function could illuminate how birds refine migration strategies during crucial decision-making periods. Because flight calls are relatively simple vocalizations, compared with many others that birds use, they provide useful characters for future evolutionary and comparative analyses. Monitoring flight calls can be a powerful method for studying nocturnal migration. One gram microphone transmitters attached to six Swainson’s thrushes provided vocalization data for comparison of the character of flight calls within and between individuals. We assign S4, S5, S6, S7, S8, and S9 designators to these birds following the presentation of their wing beat patterns and flight maps in Cochran et al. (Integrative and Comparative Biology 48, 2008). Data from these six birds were compared with a sample of calls of Swainson’s thrushes overflying ground based microphones (Evans and O’Brien 2002 Flight Calls of Migratory Birds, a CD). Variation in call durations and spectral characteristics were high and similar within and between individuals, between daytime and migratory flight, and between the individuals and the overflying birds. Calling rates among individuals varied from 3.8 per hour to 46.2 per hour during migratory flight and were appreciably lower but also highly variable during daytime. Females called more often than males in daytime and during nocturnal migration. The high variability in virtually all aspects of calling must tell us something about what the function of calling is or is not, but we haven’t figured that out yet.