Demographic and behavioral responses of breeding birds to variation in food, nest predation, and habitat structure across multiple spatial scales

Birds show huge spatial variation in breeding behaviors and demography, from the global scale to the local. Yet the causes of this variation remain poorly understood. We reviewed the literature and document a worldwide pattern that southern-latitude passerine birds show consistently lower nest attentiveness during incubation than related northern species. We experimentally tested the hypothesis that greater food limitation may be responsible for low nest attentiveness in southern birds, by providing supplemental food during incubation to the karoo prinia (Prinia maculosa), a southern-hemisphere species with low attentiveness. Attentiveness was significantly higher in food-supplemented females than controls, but was still substantially below that of other related northern species. We therefore reject the hypothesis that food limitation is the main cause of latitudinal variation in incubation behavior.;Habitat selection is another critical behavior with substantial fitness consequences. A main assumption of habitat selection theory has been that habitat preferences are adaptive. Yet, mismatches between habitat preferences and reproductive performance have been prevalent across a wide variety of taxa. We take an integrative and comprehensive approach to the evaluation of habitat selection in a breeding songbird, the Brewer's sparrow ( Spizella breweri), by examining habitat preferences and a suite of fitness components across multiple spatial scales. We show that habitat preferences and resulting fitness consequences vary across spatial scales, and different environmental factors (e.g., food versus nest predation) may be more important at different scales. At the largest scale, birds preferred landscapes with higher shrub cover, which was associated with greater offspring size and the ability of parents to attempt more nests within a season. At the smallest scale, parents chose nest patches containing higher densities of shrubs, which was associated with higher nest success. We experimentally tested two hypotheses (total-foliage and potential-prey-site) for why microhabitat structure influences nest predation rates, and found strong support for the potential-prey-site hypothesis that predators may abandon search efforts sooner in areas where more potential nest sites must be searched prior to finding actual sites. Together, our results demonstrate the utility of conceptually thorough approaches and experimental analyses towards a better understanding of habitat selection and other important behaviors.