Determining the Randomness of Western Sandpiper (Calidris mauri) Mist-Net Recaptures by Sex and Age Group in Ensenada de la Paz, Baja California Sur, Mexico

Background: Population sex ratios and age structure are fundamental demographic parameters in shorebird studies, and their accurate estimation depends on unbiased capture methods. Methods: To identify potential capture biases, we evaluated the randomness of mist-net recaptures between sex and age classes of the Western Sandpiper (Calidris mauri) in La Paz Lagoon, Baja California Sur, Mexico. Results: A total of 856 individuals were captured and banded, of which 89 (10.4%) were recaptured. Observed and expected recapture frequencies did not differ significantly between sexes (67 observed vs. 62 expected males; 22 observed vs. 26 expected females), indicating no sex-related bias in recapture probability. In contrast, significant differences were detected between age classes (20 observed vs. 42 expected adults; 69 observed vs. 46 expected juveniles), demonstrating a clear age-related bias. The elapsed time between initial capture and recapture was similar for males and females but differed significantly between age classes: adults were most frequently recaptured after 51–75 days, whereas juveniles were recaptured predominantly after 1–25 days. Conclusions: Juveniles were more likely than adults to be captured and recaptured. The higher capture probability of juveniles may be associated with their greater mobility, resulting from their limited experience and lower social status.