Prenatal reproductive success of an aerial insectivore, the tree swallow (Tachycineta bicolor)

,Worldwide, many avian species are declining as the effects of large-scale, anthropogenic pressures continue to rise. Prenatal mortality is potentially an important driver of demographic processes in animal populations, yet little is known about embryonic mortality in birds, despite extensive research into factors driving mortality in the nestling, fledgling, and adult life stages. Hatching failure in avian species is common, and because many species are globally declining, assessing the processes that operate during all life stages of individuals, including prenatally, is important to fully understand demographic changes. My thesis investigated hatching failure and prenatal reproductive success in tree swallows (Tachycineta bicolor), a member of the aerial insectivorous guild which is rapidly declining across North America. Using fluorescence microscopy to identify embryonic and sperm nuclei, I first determined that although 10.1% of eggs failed to hatch (excluding abandoned, broken, and depredated eggs), 98.3% to 100% of eggs from a population of tree swallows breeding on three sites near Prince George, British Columbia, Canada, were fertile, and that the majority of hatching failure was due to embryonic mortality, not infertility. Embryonic mortality was most likely to occur during early incubation before development was visible macroscopically. I then assessed extrinsic (e.g., environmental conditions) and intrinsic (e.g., female age) factors that influence egg mass and embryonic viability (i.e., survival of an embryo to hatching). Egg mass was positively correlated with laying order and the maximum temperature two days before egg laying, but was not predicted by female age; however, the assumption that heavier eggs are of higher quality may fail to account for the composition and concentrations of key resources for embryos in yolks. Embryonic viability increased with later clutch initiation dates, and was not significantly predicted by temperature, female age, egg mass, or laying order, and did not differ between study sites or years. Finally, I opportunistically examined maternal plasticity and prenatal reproductive success in response to a precipitous decrease in ambient temperature and associated rainfall (i.e., cold snap) during the egg laying period in one of the two years examined. Most females did not initiate incubation during the cold snap and nearly half of the females laying during the cold snap skipped at least one day of egg laying. While individual quality did not predict whether a female would have a laying skip, females with laying skips laid heavier eggs and larger clutches. Laying skips were not directly related to embryonic viability; however, laying skips and delayed incubation increase the lay-to-set interval of earlier-laid eggs, which may decrease hatching success. This study is the first to quantify fertility and developmental stages of embryonic mortality in tree swallows and provides opportunity for further investigation into factors contributing to hatching failure in aerial insectivores, especially as climate variability increases.

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