The cost of reproduction: A new link between current reproductive effort and future reproductive success

Although a negative trade-off between current reproductive effort and future reproductive output has for long been hypothesized and in some cases empirically shown, no mechanism with the potential to mediate such a reproductive cost from one season to another has been demonstrated. Here we suggest such a mechanism: the time and/or energy constraints during moult. We manipulated current reproduction of blue tits (Parus caeruleus) by delaying their breeding attempts. This resulted in reproductive costs both in terms of decreased survival and lower subsequent reproductive success in the form of delayed start of breeding (females) or a smaller clutch (males). We found that delayed birds expended about 15% more energy in thermoregulation during the subsequent winter than did control birds. Thus, more time and/or energy directed to reproduction, may constrain moult which results in the production of feathers with low insulating capacity. These feathers are retained during the following winter and breeding season. Thus, this mechanism has the potential to link reproductive activities in one season to future reproductive success.