Habitat selection and prairie drought in relation to grassland bird community structure and the nesting ecology of Sprague's pipit, Anthus spragueii.

摘要

While plants and animals of North American grassland are adapted to withstand drought, little is known about how they might be effected if severe prairie drought starts to occur more frequently because of global climate change. The aim of this study was to determine whether responses to drought affect habitat selection in grassland songbirds. In chapter one, I test whether spatial patchiness affects the ability of a habitat to buffer drought effects by comparing the bird, plant and arthropod communities of native and introduced (Eurasian) vegetation at sites that represent moderate (Last Mountain Lake) and more arid conditions (Matador). Native vegetation was richer and more diverse than introduced vegetation in terms of plant species, but there was no evidence that one habitat was acting as a better buffer at the bird-community level. Sprague's Pipit (Anthus spragueii) was significantly more common in native prairie at both sites, however, and appeared to be attracted to habitat with intermediate levels of cover. In chapter two, I test whether Pipit nests are located in relatively dense vegetation to maximize the amount of shelter available and whether the nest entrance is oriented to offset the effects of day-time heating. Compared to random location, Pipit nest-sites were generally associated with dense vegetation, but there was no strong directionality in nest orientation, suggesting that the well-concealed nests of this species may offset selective pressure associated with heat stress. In chapter three, I test whether Pipit nest attentiveness depends on nest temperature, and estimate energy costs associated with artificially alevated nest temperatures. Pipits did not adjust their incubation rhythm in response to heat stress, but they paid a clear energetic penalty for tending hot nests. They followed a "Goldilocks" strategy by defending the nest against both heat and cold, and taking recesses when nest conditions were between 30-35{dollar}spcirc{dollar}C. These results support the view that drought can act like an ecological crunch through increased heat stress at the nest site. They also indicate that physiological and behavioural traits can provide a "crunchable" range of phenotypic variation.