Habitat selection is in part a function of predation pressure, which drives the evolution of compensatory mechanisms. These mechanisms in turn direct predator life history and thereby structure trophic networks. We studied strategies that grassland birds use to compensate for juvenile mortality. Nest contents are a dietary component for many grassland snakes, mammals, and birds. Nest predation is the leading cause of nest failure in birds and thus is an important evolutionary driver. Grasslands are relatively simple ecosystems, with few opportunities for niche divergence. Most North American grassland songbirds nest on the ground, are short-distance migrants, and have substantially similar diets. However, they have diverged with respect to nest-site habitat selection and the timing of breeding. We investigated the extent to which nest-site vegetation and breeding synchrony offset nest predation risk in four ground-nesting songbird species. During 2014-2016, we monitored nests in northeastern Colorado, measured the composition and vertical structure of nest-site vegetation, and estimated nest density. To assess the relative influence of these explanatory variables, we developed logistic regression models based on a priori hypotheses. We fit models using the RMark package in R, and we assessed model parsimony based on ΔAICc and weight.
Over three years that varied widely in precipitation patterns and abundance of breeding songbirds, we estimated daily survival, and we measured vegetation at 730 nests (Lark Bunting: 573; Horned Lark: 69; McCown’s Longspur: 58; Western Meadowlark: 30). Preliminary analyses based on 2014 data revealed that nest-site vegetation differed among species, but variation in nest vegetation was not strongly correlated with nest survival. Nest-site specialization may minimize variance that would otherwise reveal the adaptive importance of nest-site vegetation. Alternatively, nest-site attributes may be a relic of past environments in an ecosystem that has undergone rapid change since Pleistocene deglaciation. For all species combined, the number of active nests increased linearly over the course of each season, but the daily number of predation events did not increase with active nest density. The temporal distribution of nesting efforts was bimodal or unimodal for each species, but these peaks occurred at different times and magnitudes. Synchronized breeding within each species could be a strategy to maximize breeding potential and minimize predation risk under a fixed predator population. Asynchrony among species may be the product of an unpredictable environment, in which species track different resources with shifting spatial distributions.