EVIDENCE OF INCIDENTAL NEST PREDATION AND ITS EFFECTS ON NESTS OF THREATENED GRASSLAND BIRDS

In this study we document the effect of nest predation on a grassland bird community that includes habitat for regionally endangered or rare bird species. Predation on bird eggs and nestlings on this sandplain grassland in southern Maine, USA was positively correlated (P < 0.01) with foraging for invertebrates by striped skunks (Mephitis mephitis) in plots of primary skunk usage. The relationship between rates of predation and vegetation physiognomy was analyzed on three spatial scales, but no strong correlation was found at any level. The hypothesis that skunk predation at this site was incidental, rather than targeted towards birds' nests, was supported by the significant correlation between skunk invertebrate-foraging activity and nest predation, the absence of skunk foraging behavior consistent with active nest searching, the absence of a relationship between nest success and vegetative cover or proximity to a forest/grassland edge, the low density of breeding birds (< 2.0 territories ha-1) on these grasslands, and the small proportion of avian material, less than 1.5%, reported in the summer skunk diet. Although nest predation was high (58.0% overall), its unpredictability probably limits the utility of certain antipredator behaviors by birds nesting at this site. Such incidental nest losses can severely limit recruitment, and thus populations, of rare or endangered species.