Do Great Tit Parus major parents gear their brood defence to the quality of their young?

Abstract

Parental investment theory suggests that animals should increase their reproductive success by investing more heavily in higher‐quality offspring than in lower‐quality offspring. Great Tit Parus major nestlings of higher body weight have been shown to enjoy a higher survival rate after fledging. We examine for the first time whether Great Tit parents invest more heavily in heavier nestlings as measured by defence (six variables) against a Pygmy Owl Glaucidium perlatum, similar to G. passerinum, which is a predator of both adult and nestling birds including hole‐nesters like the Great Tit. Thirty‐two parent Great Tit pairs in a wood‐ and parkland study area in Essen‐Stadtwald did not vary their antipredator behaviour in relation to nestling weight; between‐brood variation of weight was similar to that found in different populations and in different years. However, parental defence behaviour varied, as expected, with parent's sex and nestling number, though overall responsivity of parents to many other independent variables appeared lower than found previously.