Among bird species where only one parent constructs the nest, the “sexual display hypothesis” predicts that building behaviour and the structure of the completed nest is a post-pairing, sexually selected signal that informs the non-builder of her/his partner's quality and/or willingness to invest in reproduction. Moreover, the “differential allocation hypothesis” predicts that an individual's investment in parental behaviours, such as nest building, will vary in relation to the partner's quality. These hypotheses were examined in the socially monogamous, hole-breeding Blue Tit, a species in which the female alone builds nests. Parental quality was quantified by recording body size, feather mite load and age. The weight of nests was found to correlate positively with female head-bill length and feather mite load, but not with any indicators of male quality. This result is in accordance with the “sexual display” hypothesis, and demonstrates that nest size could be a form of intra-specific communication that helps inform the non-building partner of the builder's reproductive quality.