In resource defence mating systems, males monopolize a resource that is of primary importance for breeding females. For secondary cavity nesters, the availability of suitable nesting sites is important in determining the strength of intrasexual competition, whereby phenotypic and behavioural traits will be favoured that enable individuals to gain access to these sites. The traits that are important in male competition may additionally affect mate choice decisions and a female's investment in the current brood. In a field study on blue tits, Cyanistes caeruleus, we increased intrasexual competition by experimentally limiting nest sites in experimental plots and compared these plots to control plots. Birds breeding in experimental plots did not differ phenotypically from birds in control plots. However, females that bred in the nest site-limited plots fed their offspring at a higher rate than control females. This result indicates that increased competition for limited resources led to more investment in current reproduction, either because successful females were of higher intrinsic quality or because they adjusted their investment in relation to superior territory or male characteristics.