Understanding the patterns and processes that create differences among individuals in components of fitness, like the probability of survival or reproductive rates, is essential to our knowledge of population dynamics and for informing conservation efforts. For organisms in seasonal environments, early-breeding individuals regularly attain higher fitness than their late-breeding counterparts. Two primary hypotheses, related to quality and date, have been proposed to explain lower reproductive success of late breeders, but the veracity of these ideas has not been fully resolved. I tested predictions associated with these hypotheses to assess the effects of indices of parental and environmental quality on nestling quality and survival in an insectivorous passerine, the tree swallow (Tachycineta bicolor), at two widely separated breeding locations in western Canada. I combined experiments and statistical modelling of observational data to evaluate two mechanisms proposed to contribute to seasonal decline in environmental quality: an increase in nest parasite abundance and a decrease in food abundance with later breeding dates. A parasite reduction experiment revealed a disproportionate benefit of parasite removal on length of primary feather for early-hatched nestlings, suggesting greater energetic constraints early in the breeding season. Furthermore, late-hatched nestlings from parasite-reduced nests had longer head-bill lengths than their control counterparts, and developed head-bills of similar length to those of early-hatched nestlings. Other than these findings, there were few detectable effects of parasites on nestling size, growth and immunity, as has been reported from several previous studies. Indeed, negative effects of parasites were only apparent when food (i.e., insect) biomass was considered. In a second series of experiments in which parental quality was controlled, I also tested whether food abundance declined during the breeding season, as predicted if environmental conditions deteriorate seasonally (i.e., date). Reduced reproductive success of late-breeding individuals was causally related to a seasonal decline in environmental quality. Declining insect biomass and enlarged brood sizes resulted in nestlings that were lighter, in poorer body condition, had shorter head-bills, shorter and slower growing ninth primary feathers and that were less likely to survive to fledge. Next, I asked whether results obtained from long-term mark-recapture data corroborated findings of short-term manipulations. I examined seasonal variation in first-year apparent survival to investigate the relative influence of large-, small- and individual-scale factors associated with the quality and date hypotheses. Although parental quality was an important predictor of first-year apparent survival of tree swallows, my results further suggested that quality of parents was not the primary factor influencing seasonal variation in first-year apparent survival. Rather, findings were most consistent with the date hypothesis. The relationship between apparent survival and a direct measurement of environmental quality indicated that annual variation in moisture had important consequences for first-year apparent survival of tree swallows in Saskatchewan. First-year apparent survival probabilities were higher during wet years and wetter conditions are generally linked to greater insect abundance. In British Columbia, nestlings from larger broods were less likely to survive, possibly as a result of receiving less food. Apparent survival probabilities were also higher when food was more abundant. I demonstrated that both parental and environmental quality influenced seasonal variation in fitness-related traits of tree swallows. However, the strongest evidence suggests that environmental quality, and in particular food abundance, had the greatest effect on seasonal variation in nestling quality, reproductive success and first-year apparent survival in tree swallows. My results highlight the importance of considering regional precipitation trends when projecting effects of climate change on demography of aerial insectivores.