Nestling growth and development in the brown falcon, Falco berigora: an improved ageing formula and field-based method of sex determination

Abstract

The accurate determination of nestling age and sex is an important tool for studies that examine life-history traits and ecological interactions. Despite the widespread distribution of the brown falcon, Falco berigora, in Australia, morphological criteria for sexing nestlings of this species have yet to be published and nestling development has not been intensively studied. While an ageing formula for the species exists, the small sample from which it was derived precluded appropriate statistical assessment of independence problems and other potentially confounding variables such as hatch order and sex. This study used a larger sample of free-living nestlings to account for these factors and found the most reliable measure for ageing nestlings to be wing length. Wing length increased linearly with chick age, independent of seven other potentially confounding factors examined. Ageing formulae based on wing length before and after remiges emerge are presented. In addition, an accurate test for determining nestling sex at banding age, based on tarsus width, is proposed. Nestling chronology of this species is also described in detail for the first time. Nestling development was similar to that described for other Falconiformes; however, the chronology of nestling development was too variable to be useful in assigning chick age. Despite this, within 6 days of the eldest chick hatching the relative brightness of down and the degree to which chicks' eyes had opened were useful in assigning hatch order amongst nestlings with similar wing lengths.