Dearest neighbors: Adaptive nest clustering and density-dependent nest survival in dabbling ducks

Background/Question/Methods

Predation is the primary source of nest mortality in waterfowl, and is a central factor driving nest-site selection.  Though predation risk is difficult for birds to assess directly, nesting hens may use habitat characteristics (e.g. vegetation height) and the presence of active conspecific nests to choose safe nest sites.  These nest-site selection cues could lead to nest clustering at small spatial scales, especially if habitat quality is heterogeneous.  Understanding local nest dispersion is especially important because some predators may be able to detect and respond positively to nest density.  However, despite the importance of understanding density-dependent predation in waterfowl, there has been little research on how duck nests are clustered or dispersed at small spatial scales, and whether local nest density affects predation rates.  In this study, we used three years of data on breeding waterfowl in California to study nest dispersion within our study site.  We then tested for density-dependent predation risk using temporally-refined nest clustering as our measure of density.

Results/Conclusions

We divided each nesting season into weekly intervals and then used Ripley’s L to assess the spatial association of nests versus complete spatial randomness.  In each year, nests were strongly and significantly clustered after the first 2-3 weeks of the season at spatial scales between 50-800 m.  Nests were significantly clustered at the scale of 400 m in 11 out of 12 weeks in 2008, 12 out of 14 weeks in 2010, and in all 14 weeks in 2011.  To assess the adaptive significance of nest clustering, we calculated the distance to the nearest-neighboring (active) nest and included this distance as a covariate in a logistic exposure analysis of nest survival.  In all years, nests with closer nearest neighbors were more likely to successfully hatch, a result which challenges the commonly-held view that nest predation is positively density-dependent.  Furthermore, we found that nearest neighbor distance was a better predictor of nest success than either nest vegetation or nest age.  We suggest that the adaptive benefits of nesting in a cluster may depend on the local predator community.  In California, the most common nest predator (Striped Skunk) likely depredates duck nests opportunistically while searching for small mammal prey, and nest clustering may dilute predation risk from this type of predator.