Nests, Threats, and Leks: Nonrandom Distributions of Nests in Ruffs (Calidris pugnax)

Habitat selection determines an animal's spatial distribution at various scales. In ground-breeding birds, selecting the right nesting location can be decisive for the survival of parents and offspring. However, it remains often unclear what cues birds use to settle in their breeding habitat. Ruffs (Calidris pugnax) are waders with highly divergent sex roles: males aggregate for competitive display to attract females (reeves), who then care for the nest and offspring alone. Ruffs frequently breed in coastal wetlands of higher latitudes where they often face the threat of nest loss because of flooding or predation. We investigated which environmental and social cues determine Ruff nest distributions in a coastal meadow habitat. Using nest locations from five breeding seasons and their relative distance to other nests, leks, the shoreline, and meadow edge, we tested whether Ruff nests are randomly distributed across the suitable breeding habitat or show some level of spatial association. We first compared average nearest neighbor (ANN) distances between Ruff nests (observed and simulated) and spatial features in univariate models. Then, we examined the effect of all spatial features on nest location in a multivariate generalized linear mixed model (GLMM) using a Bayesian framework. Our results show that nest distribution is nonrandom; nests of reeves are found closer to leks of male Ruffs and other nests than expected by chance. In some years, we found nests further away from the meadow edges and shore than expected by chance. Overall, our results suggest that nesting females may use social cues and the distance to habitat boundaries when choosing a nest site. We suggest that understanding the social and environmental factors affecting female nest choice can help to improve the management and conservation routines at the breeding sites of these threatened waders. Our results indicate that lekking sites may be used to identify nesting areas of conservation management value.