CLUTCH PREDATION, COVER, AND THE OVERDISPERSION OF NESTS OF THE WILLOW PTARMIGAN

Clutch predation is generally considered a selective force in nest site selection by ground-nesting birds. We examined characteristics of vegetation and cover around nest sites and inter-nest spacing for Willow Ptarmigan (Lagopus lagopus) to determine whether choice of nest location influenced probability of predation. Types of vegetation around successful and depredated nests and random points were similar. However, hens chose sites that had more lateral and overhead cover than that found at random points. Amount of cover was similar for successful and depredated nests. We suggest that hens choose cover to deter predation on themselves or to reduce energy costs of incubation. Inter-nest distances were larger and nests were located closer to the centroid of the territory than if nest sites had been selected at random. Hens that renested placed their nests farther from their first nests but not farther from neighboring nests than expected if they had located them at random. Probability of a clutch being preyed upon was not related to its distance from a clutch that had previously been depredated. We conclude that, for the breeding densities that we encountered, nests were spaced far enough apart to deter area-restricted search by predators. Predation may have been a factor causing over-dispersion of nests in the past, but current selection may be masked by other factors such as the hen's behavior or stochasticity.