Females are larger and less detected than males but experience similar annual survival in the lek-displaying Eleothreptus anomalus (Sickle-winged Nightjar)

Abstract Intraspecific variation in body size and demographic rates is central to understanding life histories and to conserving many animal populations, but remains poorly known for nocturnal aerial insectivorous birds. We show that for a globally threatened lek-breeding nightjar species with extreme male ornamentation (sickle-shaped wing), females have larger body size than males throughout their lives, and, as adults, females have much lower capture rates but similar annual survival compared to males. We examined age- and sex-specific variation in body size, apparent annual survival, and recapture probability using morphometric measurements, molecular sexing, logistic regression, and Cormack-Jolly-Seber models on 1,006 captures of 370 Eleothreptus anomalus (Sickle-winged Nightjars) in northeastern Argentina (2012–2025). Adult females were larger than males in all measurements, with a dimorphism of 45% in length of secondary feathers, 30% in weight, 16% in tail length, and 10% in total length. Sexual size dimorphism (females larger than males) also occurred in juveniles, such that we were able to accurately determine the sex of juveniles by their measurements, especially the length of their secondaries. The best Cormack-Jolly-Seber models included age-specific survival (adults: 0.67 annually, CI: 0.6–0.73; juveniles: 0.58, 0.48–0.68) and a sex-specific capture probability for adults (females: 0.19, 0.11–0.31; males: 0.64, 0.53–0.74), highlighting the need to account for intraspecific variation in avian demographic parameters. We propose that the larger size of females, not previously recorded in any nightjar species, is related to sexual or social selection for the males’ extraordinary “sickle-shaped” wings.