Nest-site preference and maternal effects on offspring growth

Maternal preferences for oviposition sites are assumed to be adaptive, but offspring fitness is not always higher at preferred sites and, thus, further study of the selection pressures that influence oviposition behavior is warranted. Among birds, predation is regarded as the primary agent of selection on nest-site microhabitat preferences, but alternatives are rarely considered. We tested the hypothesis that avian nest-site preferences are an adaptive response to fitness costs imposed by variation in nest-site microclimate. We documented that Chestnut-collared Longspurs (Calcarius ornatus) strongly preferred to orient nests towards the southeast and showed that this preference influenced microclimate: nests facing southeast had the highest midday temperatures. Yet, preferences were not adaptive because nestlings in nests with the preferred orientation gained mass at a slower rate, had retarded skeletal growth, and reached a smaller final size. We experimentally tested this result by altering orientation of nests and confirmed, for the first time, that variation in nestling growth was causally linked to variation in nest microclimate arising from nest-orientation preferences. Adults responded to the high temperatures at preferred southeast-facing nests by spending more time shading young from the sun, apparently attempting to ameliorate heat costs. This response, however, resulted in parents spending less time feeding young, potentially explaining slower growth in these nests. Direct effects of higher temperatures may also play a role in slower growth. Although we lack an explanation for this apparently maladaptive preference, these results demonstrate that nest-site choices of birds can yield fitness costs imposed by variation in nest microclimate.