Sex allocation theory states that parents should adjust their offspring sex ratio according to the expected fitness returns from sons and daughters. Several recent studies indicate that such adaptive manipulation of offspring sex ratio is achievable, and that it may be influenced by e.g. morphological characters. Here we manipulate behaviour through interspecific cross-fostering of great tits (Parus major) and blue tits (Cyanistes caeruleus), and investigate its effect on the offspring sex ratio of adults that were themselves cross-fostered as chicks. The experience of being raised by a different species has previously been shown to result in aberrant species assortative behaviour and song, and a lowered dominance status during winter. Brood sex ratios of conspecifically breeding pairs with and without cross-fostered members were compared. Broods with at least one cross-fostered parent contained significantly more males than did control broods. Sex of cross-fostered parents did not influence the brood sex ratio. We conclude that female great tits and blue tits seem to be able to adjust the sex ratio of their broods, and that changes in their own or their partners’ behaviour may elicit such adjustments.