Nest predation among vegetation layers and habitat types: revising the dogmas.

Greater nest predation rates on ground-nesting birds than on off-ground-nesting birds have long been assumed and used as an explanation for patterns such as greater cryptic and monomorphic coloration of ground-nesting birds and for area sensitivity and population decline of many Neotropical migrant species. I use three independent data sets to show that this assumption is not true in forest habitats, where nest predation is instead least on ground-nesting birds. Larger clutch sizes and longer nestling periods of ground-nesting species in forest habitats are indirect evidence that ground-nesting species in forest habitats have suffered lower nest predation over evolutionary time. In contrast, ground-nesting birds seem to suffer greater predation than off-ground-nesting species in shrub and grassland habitats, but evaluation of predation is complicated by habitat disturbance in many studies. Nesting mortality in general appears to be greater in shrub and grassland habitats, and species in these habitats are showing some of the most consistent long-term population declines. Additional examination of nesting mortality of coexisting species in various ecological conditions is needed to uncover patterns that may influence evolution of life-history traits and population demographies.