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

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 1006 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. The data and code are in two parts: (1) morphometric measurements and logistic regression in a Monte Carlo Cross Validation framework; (2) Cormack-Jolly-Seber models of sex-specific adult and juvenile survival. Methods Field, lab, and statistical analysis methods are described in Cockle et al. Females are larger and less detected than males but experience similar annual survival in the lek-displaying Eleothreptus anomalus (Sickle-winged Nightjar). Ornithology. Additional details are provided in the supplementary material to that article.