The sex ratio of nestling great tits

Abstract

A method is described of sexing nestling Great Tits by the colour of the outer fringes of the primary coverts. At least 85% of the young sexed by this method proved to have been sexed correctly. The method can be used both for the Continental Parus m. major and for the British P.m. newtoni. It was found that both in areas and during periods with a high nestling mortality, the percentage of males amongst nestlings of 15 days was significantly higher than in other periods and areas. It is suggested that in conditions of food shortage females are at a disadvantage and suffer a higher mortality. Evidence is presented to support this hypothesis. The influence of this differential mortality on the sex ratio of the adults is briefly discussed. It is concluded that it might be partly responsible for a surplus of unmated males. © 1970 Taylor and Francis Group, LLC.