MEASURING THE IMPACT OF PARENTAL FORAGING BY STARLINGS (STURNUS-VULGARIS) ON SOIL INVERTEBRATE PREY AVAILABILITY - AN EXCLOSURE EXPERIMENT

Depletion of soil invertebrate availabilities by starling foraging dur ing the breeding season was investigated by using exclosures to create areas where the birds could not feed. A field 'preferred' by the wild birds was compared with a 'non-preferred' field. Prey biomass was mea sured from soil cores and the foraging success of individuals feeding in experimental enclosures gave a measure of prey availability. Foragi ng by these birds resulted in significant short-term depletion after a few hours of foraging. Trials conducted after a 24-h interval showed that this depletion was due to removal of prey by the birds rather tha n invertebrate movements. Comparison with exclosure sites al the end o f the chick-feeding period showed that foraging by the wild birds caus ed significant long-term depletion. This depletion was greatest in the 'preferred' field. which offered a higher availability of leather jac kets Tipula paludosa. The 'non-preferred' field yielded lower capture rates, despite a higher earthworm biomass in the soil cores. These fin dings suggest that soil core analysis alone is insufficient to predict the foraging success of starlings and other birds feeding on soil inv ertebrates. The significant reduction in prey availability to parent s tarlings is discussed in relation to the number of parental foraging t rips made to the two fields.