We investigated the relationship between male nest defence and female breast patch size in an alpine population of rock sparrow (Petronia petronia) in northern Italy. We presented a mounted weasel (Mustela nivalis), a common nest predator, to 28 pairs breeding in nest boxes, with 12-13-d-old nestlings, and measured the intensity of male and female defence reaction. We measured the frequency of attack flights, intensity of alarm calling and total time spent in view, and then combined these for each individual, in a single defence factor by principal component analysis. All the females arrived to defend the nest while only 21 males arrived, and females defended the nest more intensely than males. We analysed, by stepwise regression, the relationship of male defence factor to female behaviour and phenotype (breast patch size, a measure of quality) and brood properties (size, mass, phenology). Male defence factor was significantly related only to female breast patch size. We argue that male rock sparrows apparently make parental investment decisions according to their mate's quality, and examine possible alternative hypotheses.