Sex-specific growth rates in zebra finch nestlings: a possible mechanism for sex ratio adjustment

Wild and captive zebra finches (Taenopygia guttata), like several other species, produce a male-biased sex ratio at fledging when food is scarce. This is due to primary sex-ratio adjustment and female-biased nestling mortality. Given that young females fledging at low body masses have been shown to have low fecundity as adults, lower returns to parents from producing female offspring in conditions of restricted food has been raised as a functional explanation (Trivers and Willard's hypothesis of adaptive sexual investment; 1973). However, an alternative, mechanistic hypothesis is that under restricted conditions female chicks are more costly to produce. In consequence, lower returns to parents under these conditions would happen earlier in the life of female offspring rather than later. To test this hypothesis, I hand-reared chicks on a food gradient. In the absence of parent-offspring and sib-sib interactions, final body mass and growth rates for females were lower in conditions of restricted food. For males, final body mass and growth rates did not differ with food condition. Low female growth rates in food-restricted conditions might be one potential mechanism causing female-biased mortality in birds. More importantly, this result is the strongest evidence yet of female offspring experiencing higher marginal fitness benefits from additional food than males and it has implications for primary and secondary sex-ratio adjustment. Also, as this mechanism has been shown in the absence of parent-offspring interactions, significant questions can now be raised as to how parental and offspring behavior interact in their effects on secondary sex-ratio adjustment.