Ardea
Official journal of the Netherlands Ornithologists' Union

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Bunskoeke E.J., Ens B.J., Hulscher J.B. & de Vlas S.J. (1996) Why do Oystercatchers Haematopus ostralegus switch from feeding on Baltic Tellin Macoma balthica to feeding on the Ragworm Nereis diversicolor during the breeding season? ARDEA 84 (A): 91-104
Oystercatchers Haematopus ostralegus breeding on the isle of Schiermonnikoog in the Dutch Wadden Sea, switched from a diet dominated by the bivalve Macoma balthica in late spring to a diet dominaČted by the annelid worm Nereis diversicolor in early summer. Although all Oystercatchers switched, the timing and the magnitude of the switch differed between individuals. Since searching for Macoma appeared incompatible with searching for Nereis, we expected individuals to search for the prey species yielding the highest intake rate for a given period in the season. Some analyses clearly supported this suggestion, while the results of others were at best ambiguous. Although the density of large Macoma did not change, the intake rate of Macoma declined during the summer, due to a decline in the capture rate of Macoma. This may have resulted from an increase in the burying depth and a decline in the condition of Macoma, as this forced the Oystercatchers to prey on an increasingly smaller fraction of the Macoma population. Intake rate on Nereis was not related to the density of Nereis, which tripled during the course of the study in 1986, while intake rate seemed to decline in late summer after having reached a peak in early summer. This peak was possibly due to a high surface activity of Nereis. Thus, the diet switch may have been due to an increase in the burying depth and a decline in the condition of Macoma, or to an increase in the surface activity of Nereis, or both. Problems of interpretation arose primarily from the consistent differences between individuals in prey choice and the lack of independent measures of prey availability.


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