Primary and secondary sex ratio manipulation by zebra finches.

Wild zebra finches, Taeniopygia guttata, breed opportunistically when there is sufficient food available, often rapidly mobilizing their reproductive systems in response to an ephemeral boom in grass seed production. For females in captivity, fecundity, attractiveness to mates and survival to reproduction are all correlated with their fledging weight. By contrast, for males, only attractiveness is related to fledging weight; the relationship between fledging weight and male mortality is much weaker and that for male fecundity is unknown. Previous work thus suggests that how much food nestlings receive will have a profound impact on their reproductive success, and that this effect may be more marked for females than for males. I manipulated the food available to domesticated breeding zebra finches to test Trivers & Willard's (1973, Science, 179, 90-92) hypothesis of adaptive sexual investment. When food availability was restricted, clutch sex ratios were significantly more male biased than when food was available in excess. Within clutches, daughters hatched sooner than sons and first-hatched chicks fledged at higher weights than those that hatched last. Chick mortality was female biased when food availability was low but male biased when food availability was unrestricted. I compared the song output of brothers of differing weight at independence, but found no significant difference between them. These data suggest that zebra finches manipulate both their primary and secondary sex ratios in relation to food availability to invest adaptively in sons and daughters, and support Trivers & Willard's hypothesis. Copyright 1998 The Association for the Study of Animal Behaviour.