Male‐biased sex ratio in nestlings of the Turquoise‐fronted Amazon ( Amazona aestiva ): evidence for bias established at or before hatching

Sex ratio theory predicts that parents may adjust the sex ratio of their offspring when the costs of producing each sex differ. We report sex ratios at hatching and fledging for the Turquoise‐fronted Amazon Amazona aestiva , a species with documented sexual size dimorphism, in which males attain greater body mass and larger skeletal dimensions, despite the genus being classically described as sexually monomorphic. Over three breeding seasons (2004–2006) at Parque Provincial Loro Hablador, Chaco, Argentina, we molecularly sexed 104 nestlings from 38 broods at or shortly after hatching and 76 fledglings from 29 broods. Both datasets showed a consistent male bias (hatching: 58.7%, fledging: 63.2%), with the fledging sex ratio significantly deviating from parity (exact binomial test: P = 0.029). To identify whether this bias was generated after hatching, we examined sex‐specific outcomes across four fate categories: fledging ( n = 48 sexed), brood reduction or nestlings dying in the nest ( n = 12), legal harvesting for the pet trade ( n = 13) and predation/other mortality ( n = 19). No fate category differed significantly from a 1:1 sex ratio, and the sex ratio of nestlings dying from brood reduction did not differ from that of successful fledglings (Fisher's exact test: P = 0.746). Within the subset of nestlings with known fate, the proportion of males was virtually unchanged between hatching and fledging (0.587 vs. 0.583), indicating no detectable net sex‐differential survival during the nestling period. These results suggest a male bias established at or before hatching, rather than generated primarily by post‐hatching survival, legal harvesting, or predation. Weak statistical evidence suggested that later‐hatching nestlings within a brood may be more likely to be male (GEE: P = 0.052), whereas later‐initiating broods did not produce fewer males, contrary to a simple Trivers‐Willard prediction. This study provides the first formal evidence of a male‐biased nestling sex ratio in an Amazon parrot and highlights the value of combining hatching‐stage sexing with individual fate data to infer mechanisms of sex‐ratio bias.