The impact of maternal condition and brood sex composition on parental care and fitness in the zebra finch, Taeniopygia guttata castanotis

Although sex allocation in birds has been widely studied, many questions remain concerning the relative costs of rearing sons and daughters, the effects of brood sex ratio on variation in parental care, and long-term fitness consequences of natal brood composition to offspring. To explore these factors, I performed two sets of experiments that investigated maternal condition and brood sex ratio effects on nestling care and offspring fitness in zebra finches.;In the first "maternal effects" experiment, I manipulated the condition of one cohort of females by trimming their flight feathers; control females and all males were left intact; birds were allowed to breed in a large aviary for six months. Manipulated females focused their parental effort on rearing sons, while control females allocated effort to rearing daughters. Males mated to manipulated females tended to focus effort on daughters, while control males showed no consistent response. Manipulated females displayed lower reproductive rates, primarily due to producing fewer daughters, laid smaller clutches, and had longer inter-clutch intervals than controls. Although the manipulation did not affect offspring survival, results from companion experiments indicate that brood sex composition and maternal reproductive rate had different impacts on offspring mating attractiveness and reproductive performance in the two treatments.;In the "brood sex ratio" experiment, I manipulated sex composition to create broods consisting of four males, four females, or two offspring of each sex. Parents spent more time assessing offspring need/quality at nests that had low survivorship, especially all-female broods, suggesting that assessment is a non-trivial component of parental time budgets, and showed treatment differences in the timing of assessment activities. Results suggested that males are better competitors as nestlings and that, as a result, females have been selected to fledge early and receive care out of the nest. Brood sex ratio influenced male (but not female) mating attractiveness: in mate choice trials, males reared in mixed-sex broods were preferred to those reared in all-male broods. Different sets of early life traits predicted mating attractiveness of males and females; two traits, hatch order and brood age range, had opposing fitness effects on the sexes.