Foraging Behaviour of Migrant pied Flycatchers, Ficedula hypoleuca, on Temporary Territories

Abstract

Pied flycatchers F. hypoleuca on a migration stop-over in Portugal were accumulating reserves at a rate of 0.34 g d[day]-1 for a trans-Saharan flight. Marked birds occupied limited feeding ranges which were defended against conspecifics but not other species. The flycatchers foraged for active arthropods with the predominant items of diet being ants, wasps and beetles. Capture attempts were clumped in time which was taken to mean that prey were clumped in space. Within a tree a bird''s feeding rate declined exponentially. For one bird the time taken to find a new feeding place varied 4-fold between observation sessions. This could not be explained by time of day or temperature and was taken to reflect prey availability. In poor feeding conditions the bird stayed longer in each tree or nearby after encountering food. To this extent variations of feeding strategy appeared consistent with optimal foraging theory though direct measurements of prey dispersion were lacking. The recovery of prey patches following a bird''s departure was such that most occurred within 10-15 min. Birds did not move round their territories on a random walk but tended to proceed in a circuit (albeit with many reversals). Probably as a result, trees were revisited after intervals of about 12 min. Territory circuiting behavior spaced repeat visits to trees in such a way as to enhance the bird''s rate of encounter with prey. It was not possible to assess the net benefit of exclusive territoriality and circuiting to the flycatchers since the costs of defense could not be estimated.