Sex differences in migratory restlessness behavior in a Nearctic–Neotropical songbird

Most seasonally migrating songbirds exhibit protandry, whereby males arrive to breeding sites in the spring before females. The proximate behavioral mechanisms of protandry are largely unknown for most species, but could include earlier migratory departure from wintering sites by males or overall faster migration by males. Using onset and intensity of migratory restlessness as proxies for departure timing and rate of migration, respectively, we evaluated these 2 hypothesized mechanisms in a Nearctic-Neotropical migrating songbird, the Black-throated Blue Warbler (Setophaga caerulescens). Birds were captured during fall migration, held in captivity over winter, and photostimulated in the spring to induce migratory behavior. Video analysis was used to separately quantify stereotypical nocturnal wing whirring and jumping migratory restlessness behaviors. The birds were then radio-tagged and released in mid-May to compare stopover duration between the sexes and validate migratory restlessness in captivity as a proxy for the motivation to migrate in the field. In captivity males initiated migratory restlessness earlier in the spring than females, demonstrating innate differences in the onset of spring migration in this species. Males also displayed higher-intensity wing whirring behavior, suggesting potential sex differences in flight behavior that could influence migration rate. We found no sex differences in stopover duration in the field following release. However, stopover duration was negatively correlated with total migratory restlessness intensity on the last night the birds were held in captivity, which supports migratory restlessness as a proxy for the motivation to migrate at the individual level.