“Intensity of Nest Defence Is Related to Offspring Sex Ratio in the Great Tit Parus Major”, 2000 (; backlinks; similar):
Nest-defense behavior of passerines is a form of parental investment. Parents are selected, therefore, to vary the intensity of their nest defense with respect to the value of their offspring.
Great tit, Parus major, males were tested for their defense 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-defense intensity and offspring sex ratio. Paternal quality, size, age and condition, lay date, and chick condition did not influence any of the measured nest-defense parameters.