Maternal preference for nest sites is predicted to be an adaptive consequence of selective pressures acting on parents and young at the nest site. Nest predation risk has been linked to nest-site placement in birds, but microclimatic extremes can impose fitness costs on both adults and young, and these two factors may conflict. I used the temporal and spatial variation in microclimatic conditions and nest predation risk generated by variation in wildfire severity to examine the relationship between nest-site preference, nest microclimate, and fitness costs to parents and young in the Dark-eyed Junco (Junco hyemalis), a facultative cavity-nester. Adults preferred to select exposed nest sites oriented toward the north—sites that consistently had the most moderate thermal regimes. Nestlings reared in burrow-type nests gained mass more slowly and experienced retarded skeletal growth compared with exposed nests, but slower growth was not explained by suboptimal nest temperatures, nestling provisioning rates, or nest attentiveness. Although young raised in the warmest nest sites (exposed and south-facing) did not experience reduced growth rates as predicted, incubating females reduced their nest attentiveness and parents increased their nestling provisioning rate at these nest sites. The results suggest that nest microclimate can affect thermoregulatory costs to parents and offspring that can influence parental care decisions. Variation in nest predation was unrelated to microclimatic characteristics of nest sites, which suggests that parents do not make tradeoffs between microclimate and risk of nest predation when locating nests.
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Vol. 126 • No. 3