Biometry, body condition, reproductive status and molt of Tyrannus s. savana in central Argentina

Abstract

Body condition may influence different phenological processes that, at the same time, may be performed differentially according to individual sex and age. We studied the biometry, body condition, breeding stage and feather molt of the Fork-tailed flycatcher (Tyrannus s. savana), a Neotropical austral migrant. Between October and February (2015-2018), we captured individual flycatchers in La Pampa, Argentina, which were banded and measured in a standardized fashion. We found larger wing chords and weights than previous studies, and tarsus and culmen lengths were reported for the first time. As is true for most passerines, females were smaller than males, both in nestling weight and in adult wing chord and tail length. During the breeding season, fat levels were mostly low, pectoral muscle scores were medium, brood patch development was high and cloacal protuberance development was low, as reported for most tyrant flycatchers. Additionally, active feather molt was mostly low, as has been previously documented, and wear of the remiges and rectrices was moderate. Also, these characteristics did not show temporal changes, except for the variation of fat levels and wear of the remiges and rectrices between breeding seasons, and a decrease in weight and greater brood patch development as the reproductive seasons progressed. Descriptions such as this are important because there exists a large gap in our understanding of the basic natural history of many Neotropical birds, which hinders our ability to address the alarming loss of biodiversity in the region.