Incubation in the Starling, Sturnus Vulgaris: Resolution of the Conflict Between Egg Care and Foraging

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Complementary studies on Starlings nesting on the Frisian island Schiermonnikoog included estimation of total energy expenditure of the parent birds, evaluation of the cost of incubation, experiments on the effect of manipulated nest temperature on incubation rhythm, and observation of prey intake rates during foraging trips. Typically the eggs were covered 95% of the time, and experimentally it could be shown that a fall in nestbox temperature elicited an increase in sitting activity. It is argued that this relation is brought about by responsiveness to egg temperature at the outset of the sitting spell, as this will determine the shape of the warming curve and hence the time until equilibrium incubation temperature has been restored. In years of poor food supply the parents compensated by making longer foraging trips and extending their active day. An inter-season comparison suggests that the length of the foraging trip is set to collect a given amount of food. In years of high densities of the principal prey (larvae of Tipula paludosa) and hence high intake rates, the parents spent more time in alternate feeding sites thereby constituting a more varied diet. In the year of poorest food supply the sitting bout was extended to allow the off-duty partner sufficient time to collect food. In rich years some females managed to incubate unassisted. The energy cost of incubation involves a modest increment in the daily energy budget. Flight at this time is greatly reduced and this savings more than offsets the cost of heating the eggs, such that during incubation the parent birds probably enjoy the lowest demand of any phase of the breeding cycle. The main problem posed by incubation is thus how to collect enough food in the time available, rather than coping with excessive energetic costs.