[show abstract][hide abstract] ABSTRACT: Nest-defence behaviour of passerines is a form of parental investment. Parents are selected, therefore, to vary the intensity of their nest defence with respect to the value of their offspring. Great tit, Parus major, males were tested for their defence response to both a nest predator and playback of a great tit chick distress call. The results from the two trials were similar; males gave more alarm calls and made more perch changes if they had larger broods and if they had a greater proportion of sons in their brood. This is the first evidence for a relationship between nest-defence intensity and offspring sex ratio. Paternal quality, size, age and condition, lay date and chick condition did not significantly influence any of the measured nest-defence parameters.
Proceedings of the Royal Society B: Biological Sciences 04/2000; 267(1443):535-8. · 5.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Life-history theory predicts skewed offspring sex ratios in a range of situations in which the costs and benefits of producing the two sexes differ. In recent years, many studies have demonstrated biased sex ratios in a variety of bird species. However, many of these investigations have been based on small sample sizes, on data from a single year, or both. Using a recently developed polymerase chain reaction-based molecular DNA technique, 912 great tit (Parus major) nestlings from 118 broods in 5 different years were sexed. As found in a number of previous studies on the same species, there were significant predictors of offspring sex ratio in individual years. However, there were no consistent trends across years, and none of the measured variables signif- icantly predicted sex ratio over all years combined. Furthermore, brood sex ratio of the population did not depart from the expected binomial distribution. Although there are theoretical advantages to manipulating the sex ratio in this and other species, the physiological mechanism by which it is achieved in birds remains obscure. We argue that data from several years are needed to confirm whether facultative sex ratio manipulation is a consistent breeding strategy used by birds. Key words: great tits, molecular sexing, Parus major, sex ratios. (Behav Ecol 11:294-298 (2000))