Further evidence for Emlen’s hypothesis from two parrot species

Abstract

Recent studies have demonstrated that chromosomal sex determination does not necessarily prevent facultative sex ratio adjustment in birds. However, it is not known how the sex ratio adjustment is achieved. One hypothesis states that birds might adjust the sex of the first‐laid egg only, but published data from Eclectus parrots indicated that the sex ratio of second‐laid eggs was also biased. In broods where two young were fledged together, the two nestlings were often of the same sex. According to a re‐analysis, this finding, however, was not significant. In addition, new data from German breeders are presented and, according to these data, there is no evidence for a biased sex ratio in second‐laid eggs either. Furthermore, I analyse recently published data from kakapo. In this species there is a significantly biased sex ratio for the first eggs, but again not for later eggs. Thus, the sex ratio adjustment in both parrot species could arise through post‐ovulation adjustment according to Emlen's hypothesis. © 2003 Taylor & Francis Group, LLC.