Prudent Females? Effects of Food Availability and Predation Risk on Female Investment in Offsprin

Life history theory posits a trade-off between investment in self and reproduction depending on environment. Because of their increased investment in prenatal offspring, females in particular may exercise prudence if environmental conditions are unfavorable. Changes in female investment can alter offspring phenotype, resulting in non-genetic plastic changes known as maternal effects, which can have lasting effects into adulthood. We asked how predation risk and food availability alter female investment in offspring in a migratory passerine bird, the black-throated blue warbler (Setophaga caerulescens ; BTBW). In chapter 1, we used an experimental approach to assess the effect of nest predation risk on female contributions to eggs. We found no difference in mass or yolk hormones between treatment and control eggs, suggesting that nest predation risk does not affect female investment during egg formation. In chapter 2, we explored how variation in food availability during egg formation may limit or alter prenatal investment in offspring. Surprisingly, we did not find a relationship between energetic investment in clutches and food availability but did find a negative relationship between two anabolic yolk androgens, testosterone and androstenedione, and food availability. The negative relationship between yolk androgens and food availability suggests that females may decrease androgens when food is ample to avoid the costs of maintaining high levels of androgens, and instead promote increased growth of their nestlings through increased feeding. In chapter 3, we used a 14-year data set to evaluate the effect of predation risk on female investment in prenatal and postnatal offspring, as well as offspring condition. These data included surveys of nest predators in a subset of BTBW territories, as well as clutch size, female feeding rates, nestling mass, and nestling age at fledge. We found no relationship between predation risk, female investment in offspring, and nestling phenotype. Together these chapters highlight the persistent investment in reproduction of female BTBWs, as they did not decrease investment in response to increased predation risk and had limited response to changes in food availability. This may be because the probability of successfully breeding in the future is too low to decrease current investment.