Growth and sex of nestling Merlins in Orkney

Seventeen Merlin nestlings in six broods on Mainland Orkney in 1981 were weighed and measured regularly until they fledged. Nine chicks grew more rapidly and attained greater weights than the remaining eight irrespective of when they hatched in relation to their siblings. They also had longer beaks, hind claws, middle toes and tarsi, and thicker tarsi, and were therefore assumed to be females. It is suggested from measurements of these birds that weight in relation to wing and/or tenth (outer) primary length are sufficient to judge the sex of the young from about 13 days old. It is also suggested that the results may be generally applicable in northern Britain.