Changes in body mass of breeding Northern Wheatears, Oenanthe oenanthe, on the island of Öland, south Sweden, were monitored during 1985-1987 in the context of a food provisioning experiment. Individuals were weighed periodically by attracting them to feeders which were placed on electronic digital balances. Rations of 7 g of mealworms per visiting individual were offered daily on certain territories (in 1987, some individuals were offered rations of 4.5 and 9 g). Unprovisioned birds were captured and weighed during the study. Males maintained constant body masses throughout the season. Females increased in mass up to laying, maintained relatively large masses during incubation, and dropped in mass by 14% during the first week after the young hatched. Mass then attained constant levels until the time of offspring independence. There was a positive correlation between brood size and the masses of unprovisioned females during the late nestling phase. Extra food had a positive effect on the general mass level of females but not of males. However, provisioning had no significant effect on the masses of females feeding thermoregulating nestlings, indicating that reproductive stress is not important in determining posthatching mass losses. All females lost mass after hatching, and the heaviest lost proportionally more. Extra food was delivered almost exclusively to the chicks a few days after hatching. Females apparently maintain reserves during incubation and transfer them to the young after hatching by a voluntary shift in the allocation of collected food. Males do not store reserves and have therefore no scope for significant mass changes.
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Juan Moreno (1989) studied this question.