The ability of oystercatchers Haematopus ostralegus to compensate for lost feeding time: field studies on individually marked birds.

Wintering shorebirds are often disturbed by people and raptorial predators, causing them to loose feeding time and making it more difficult for them to meet their high energy demands. Previous work on captive oystercatchers, Haematopus ostralegus, suggests that they may be able to compensate for the lost time by increasing the rate at which they feed, but it is not known whether free­living oystercatchers can do so. This paper tests this possibility on five individually marked wild oystercatchers feeding on mussels Mytilus edulis. Each marked bird was mentioned continuously throughout tidal exposure periods between August 1993 and February 1994 on days during which disturbance either did or did not occur. The disturbance was either natural (raptors) or human, this either arising from shellfishermen or being experimentally induced. A multiple regression model was built for each using data only from undisturbed days to establish trends in intake rate associated with environmental factors, such as competitor density, the season and state of the tide. These were used to predict the intake rate that would have been expected to occur, had the disturbance not taken place. These expected ‘undisturbed rates’ were then compared to the actual rates after a disturbance had occurred. The ‘disturbed’ and ‘undisturbed’ rates were not significantly different, either in the immediate post­disturbance period or over the remainder of the exposure period. Plots of cumulative biomass consumption than on undisturbed birds on disturbed days took longer to achieve a given consumption than on undisturbed days. Contrary to the hypothesis, there is no evidence that these birds increased their rate of feeding to compensate for lost feeding time. Instead, they extended their feeding time by remaining longer on the mussel bed. When time losses due to disturbance are rather short (30–60 min), the results suggest that there is some ‘slack’ in the system which oystercatchers are able to exploit to compensate for time lost through disturbance. Studies on three sites with differing frequency of disturbance by people suggest that the birds are able to habituate to the frequent presence of people and to reduce the distance at which they take flight, thus reducing the amount of time lost to disturbance in the first place. [ABSTRACT FROM AUTHOR]