The Role of Urban Climate and Land Cover in Phenology, Nest Success, and Habitat Use
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Urban development is the fastest growing global land use and it increasingly brings drastic environmental change to natural systems. Climatic variability and spatial habitat heterogeneity within a city could influence species in habitat remnants. The objectives of this project were to investigate the relative importance of temperature variation and urban land cover for nest survival, the function of thermal refuges for habitat use by breeding birds, and the relationship between urban climate and phenology. Temperatures, leaf phenology, invertebrate abundance, nest habitat, and nest fate were monitored over a four-year study period at nine sites along an urban to rural gradient. The sites were located in a riparian corridor passing through the cities of Reno and Sparks, Nevada. Study area temperature anomalies from the 20-yr normal strongly impacted nest survival rates of American Robins (Turdus migratorius), Mourning Dove (Zenaida macroura), and Black-headed Grosbeaks (Pheucticus melanocephalus). Nest survival increased at warmer than normal temperatures and to a lesser extent at colder than normal temperatures. Canopy cover within a 20m radius of the nest and pedestrian traffic at a site were positively associated with nest survival. Microsites available to nesting birds that had more vegetation were cooler at night and warmer in the day when they had more bare ground, less impervious surfaces, and less canopy cover. The nests of doves and robins were constructed at cooler locations when air temperatures were warmer. Temperature varied between sites with the warmest temperatures recorded in rural sites, followed by the more urban sites, and with coolest temperatures in suburban sites. The timing of nesting by robins occurred earlier in the warmer sites and at sites with earlier bud break by Populus spp. Peaks in foliar invertebrate abundance were also weakly associated with leaf out, while the sequence of peaks by aerial invertebrates was related more to the level of urbanization at a site. The results from these studies demonstrate the ability of urban development to differentially affect phenology across trophic levels, have an additive effect with temperature variation on nest survival, and offer opportunities to provide thermal refuges to buffer the impacts of regional climate change.