BRANTA — Humphrey P. Sitters
The role of night-feeding in shorebirds in an estuarine environment with specific reference to mussel-feeding Oystercatchers
Institution: University of Oxford, UK
Supervisors: CM Perrins, JD Goss-Custard (CEH)
Details: DPhil 2000 (Completed)
Current Address: Limosa, Old Ebford Lane, Ebford, Exeter EX3 0QR, UK (Jul 2007) Email
Subject Keywords: Optimal foraging, night feeding, feeding specialisation, Common Mussel, Mytilus edulis
Species Keywords: Eurasian Oystercatcher Haematopus ostralegus
Abstract
The shorebirds (Charadrii) that are adapted for foraging in intertidal habitats can only do so when their feeding sites are exposed by the tide. Most have been recorded as feeding at night but the importance of this has rarely been evaluated. This thesis explores the role of night-feeding in those Eurasian oystercatchers Haematopus ostralegus that specialise in depredating the common mussel Mytilus edulis.
Radio-tracking showed that oystercatchers normally feed on the same mussel-beds at night as by day. Between-bed movements occur more often during neap tides than springs. Higher bird densities on neaps (leading to greater intraspecific interference and therefore, for many, less profitable foraging) may be the reason for the tendency to try alternative sites.
One oystercatcher, whose time-budget was monitored by radio-telemetry, fed for similar periods on day and night tides. This suggests that it was indifferent to the changed conditions and showed no choice for feeding more by day or by night.
Observations during August-February using video (with infra-red illumination at night) showed that the three groups of mussel-feeders - ventral hammerers, dorsal hammerers and stabbers - behaved in much the same way by night as by day. Ventral hammerers and stabbers changed from sight-location of prey by day to touch-location at night whereas dorsal hammerers used sight-location day and night. Each group took mussels from different microhabitats: ventral hammerers from among weed (probably because they are more easily detached), dorsal hammerers from among mussels exposed on the surface (probably because they are more easily hammered in situ) and stabbers from under water (probably because they are more likely to have gaping valves).
Ventral hammerers achieved similar instantaneous intake rates day and night. Dorsal hammerers had lower intake rates at night when they showed positive correlation with illuminance (probably because they search by sight). The intake rates of stabbers were lower by night than by day in autumn but the reverse in winter. This arose because the time it took stabbers to find mussels at night decreased from autumn to winter. This may be linked to changes in the feeding behaviour of mussels.
Previous studies in daytime had shown higher intake rates in both of the hammerers than in stabbers. It was therefore thought that stabbers are less fit. This study reveals that the fitness of stabbers may not be so different because of the high intake rates they achieve in winter.
It is concluded that, for oystercatchers, night-feeding in intertidal environments is an essential part of their strategy for fulfilling their daily food requirement for which they are well-adapted.