Maternal investment during egg laying and offspring sex: an experimental study of zebra finches

To study whether offspring sex is related to the amount of resources invested in eggs we performed an experiment on zebra finches, Taeniopygia guttata. By manipulating their food supply, we forced two groups of females either to increase or to decrease investment in subsequent eggs. Since zebra finches are sexually dimorphic and the reproductive value of the sexes may vary with maternal nutritional status, we predicted that females would adjust the sex of their offspring to egg quality. Females that received poor-quality food for 7 weeks before egg laying, then food supplemented with proteins after they laid the first egg, significantly increased the mass of subsequent eggs. An increase of egg mass with laying sequence was less pronounced in females that received high-quality food before laying and experienced food deterioration after starting to lay. The proportion of sons in subsequent eggs tended to increase in the latter group (although this was marginally significant) but was not related to laying sequence in the other group: these patterns differed significantly between the groups. Offspring sex was not related to egg mass, but newly hatched male chicks were heavier than female chicks. Furthermore, the hatching success of male eggs was lower than that of female eggs. We suggest that differential hatching success of the sexes and sex differences in mass at hatching may constitute important factors shaping brood sex ratios. (C) 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.