Female birds incur costs associated with increased egg production, including reductions in chick provisioning ability, in future fecundity, in survival, and in egg and chick viability. It should be possible to identify the components of the physiological system underlying reproduction, or the specific reproductive traits themselves, that explain these costs, but this has proved to be difficult, in part because of marked, but unexplained, individual variation in these traits. Resolving the physiological and evolutionary consequences of this individual variation represents an exciting challenge for the future. Several mechanisms have been proposed for the cost of egg production (e.g., protein depletion and impaired flight muscle function; immunosuppression), which assume relatively simple resource-allocation trade-offs. I argue that such mechanisms provide an unsatisfactory explanation for costs that can occur over months or even years. A more productive approach for future research will be to focus on hormonally mediated, non-resource-based costs of egg production caused by pleiotropic effects of reproductive hormones that can operate over the longer time scales at which costs of reproduction are expressed.