Is flight-calling behaviour influenced by age, sex and/or body condition?

Migratory organisms face many challenges as they travel to take advantage of changing resources, exhibiting a variety of strategies to successfully move between locations. Birds are a model taxa for understanding migratory systems, relying on a multitude of cues and showing diverse behaviours, one of which is their propensity to give calls during migratory flight. However, this behaviour is understudied and may have implications in orientation, navigation and migration monitoring. Because a variety of migratory behaviours differ among and within species as a function of age, sex and body condition, we evaluated whether flight-calling behaviour was related to any of these variables. We studied flight-calling behaviour in four species of passage migrant warblers in captivity during autumn migration at Powdermill Avian Research Center (Rector, PA, U.S.A.) and Braddock Bay Bird Observatory (Greece, NY, U.S.A.). Our results demonstrate extensive variation in flight-calling behaviour by migrant warblers. When all species were combined, young birds were more likely to call and call at a higher rate than adults. All species-specific models were consistent in suggesting a higher propensity and rate of calling by young birds, although the black-throated blue warbler, Setophaga caerulescens, was the only species-specific model with significant age-related differences. When all species were combined, sex did not have a significant effect on either propensity or rate of calling. The effect of sex was inconsistent across species, and only magnolia warbler, Setophaga magnolia, males were significantly more likely to respond to calls than females. Surprisingly, body condition did not significantly affect the propensity or rate of calling. While our results reflect the complexity of flight-calling behaviour, our finding that young migrants consistently gave more calls has broad utility, particularly in quantitative migration monitoring. These results demonstrate the need for additional study to determine the selective forces influencing flight-calling behaviour.