Resource allocation between reproductive phases: the importance of thermalconditions in determining the cost of incubation

Changes in the resources allocated to particular stages of reproduction are expected to influence allocation to, and performance in, subsequent reprod uctive stages. Experimental manipulation of individual investment patterns provides important evidence that such physiological trade-offs occur, and c an highlight the key environmental variables that influence reproductive co sts. By temporarily altering the thermal properties of starling nests, we r educed the energetic demand of first-clutch incubation, and examined the ef fect of this manipulation on performance during the same and the subsequent reproductive attempts. Compared with controls, starlings investing less in incubation were more successful in fledging young, and were more likely to hatch all their eggs if a subsequent reproductive attempt was made. Our re sults show that incubation demands can limit reproductive success, and that resources saved during incubation can be reallocated to later stages of th e same reproductive attempt and to future reproductive attempts. This study also shows that small changes in thermal environment can affect breeding s uccess by altering the energetic demands imposed on incubating parents, ind ependently of the effect of temperature on other environmental variables su ch as food supply.