I documented offspring sex ratios in 242 burrowing owl broods from 1997 – 2004 in the Snake River Birds of Prey National Conservation Area in southwestern Idaho. Mean (95% CI) proportion of males per brood over eight breeding seasons was 0.51 0.04 and 0.52 0.1 at time of banding and fledging, respectively. I investigated whether patterns of offspring sex ratios were in accord with equal parental investment in the sexes (Fisher’s hypothesis), differences in dispersal patterns between the sexes (Local Resources Competition hypothesis or Local Resource Enhancement hypothesis), differences in female parent condition (Trivers and Willard hypothesis), and whether females altered sex ratio to maximize productivity when brood reduction is possible (Brood Reduction hypothesis). Offspring sex ratio did not differ from the 0.50 proportion male that would be expected through random segregation of chromosomes at meiosis, and none of the predictions of the above hypotheses were supported to explain long-term trends in burrowing owl offspring sex ratios. Thus, selective pressures may act in opposite directions and stabilize the population offspring sex ratio close to parity, or burrowing owls may be unable to facultatively manipulate offspring sex ratios to increase fitness.
Chapter 2: Burrowing Owl Offspring Sex Ratios: a Test of the Trivers and Willard Hypothesis
Sex allocation theory predicts that females should bias the production of offspring towards the sex that will maximize maternal fitness. The Trivers and Willard (1973) hypothesis suggests that when one sex has greater variance in reproductive success, mothers in good condition may have greater fitness by producing more of that sex, whereas mothers in poor condition may have greater fitness by producing more of the reproductively stable sex. Through access to burrowing owls nesting in artificial burrows within the Snake River Birds of Prey NCA of southwestern Idaho, I attempted to both increase and decrease adult female body condition over two breeding seasons, 2003 – 2004, to determine effects on nestling sex ratios. I provided supplemental food to increase condition at nests (N = 14) and clipped 3-cm from the tips of four primary feathers on adult females to decrease condition (N = 10). Nests where females were banded and visited at the same frequency as other nests but not clipped or supplementally fed served as controls (N = 13). Offspring at supplemented nests tended to be in better condition than offspring at control nests, but variation was high both within and among broods. There was no difference in condition between offspring in decreased and control nests. Despite some difference in nestling condition and contrary to predictions of the Trivers and Willard hypothesis, offspring were no more likely to be male at supplemented nests, and no less likely to be male at handicapped nests. Burrowing owls may lack the differences in reproductive variance and sexual size dimorphism to induce development of a mechanism for offspring sex ratio variation.