Although developing birds are vulnerable to extreme and erratic temperature conditions associated with climate change, parents have some ability to buffer these effects via incubation and postnatal behavior. However, parents are constrained by their own physiology and ecology. In this thesis, I sought to determine which factors (seasonal thermal profile, consistency of ambient temperature and/or parental behavior) drove traits linked to fitness across ontogeny in free-living house sparrow nestlings (Passer domesticus). I found that the effects of these factors were context-dependent; seasonal thermal profile and average temperature were important in shaping body size across ontogeny, but variance in nest temperature and female postnatal visits better predicted hatching and day 10 survival, respectively. Future studies should seek to answer these questions in other populations and explore hypotheses surrounding interactions between developmental environments to better our understanding of climate change and thermoregulation in response to increasingly warm and erratic global temperatures.