Time and energy budgets of Oystercatchers <i>Haematopus ostralegus</i> occupying territories of different quality

The effect of territory quality on the time and energy allocation of Oystercatchers was investigated on the island of Schiernonnikoog in the Dutch part of the Wadden Sea. Low quality leapfrog territories required a higher level of parental effort for successful reproduction than high quality resident territories. The time budgets of residents and leapfrogs differed slightly, the main difference occurring during the chick-rearing stage when leapfrogs spent on average 8.2% of the time available during low water in flight in order to transport food to the chicks, compared to only 2.9% in residents. In both residents and leapfrogs, inactivity (including preening) constituted a considerable part of the time budget, comprising almost 50% of the low water period during the pre-laying stage and decreasing to 23% during the incubation stage when each bird spent almost hat: the time on the clutch. Analysis of individual time budgets during the incubation stage revealed that time spent inactive must be considered 'surplus time' since this can be reallocated to foraging when the proportion of time devoted to incubation increased. Obligatory inactivity required less than 10% of the rime available during low water. The persistence of 'surplus' time during the chick-rearing stage suggests that parental effort is not constrained by a shortage of time. Energy expenditure, estimated from time-budgets, agreed well with energy consumption, calculated from observed food intake, and with measurements using doubly labelled water. It varied from 2.1 x BMR during the incubation stage to 2.7 x BMR during the chick-rearing stage. These values are much lower than the maximum energy expenditure reported for many other bird species during the reproductive cycle. During the chick-rearing stage, daily energy expenditure of leapfrogs was 6-7% higher than that of residents due to the additional costs involved in the transportation of food. Despite the fact that the reduced reproductive success of leapfrogs, as compared to that of residents, is mainly caused by the failure of the parents to provision their chicks with a sufficient amount of food, leapfrogs did not increase their parental effort to a considerable extent. This would fit the interpretation that long-living species like the Oystercatchers are reluctant to work very hard during one reproductive cycle.