THE INFLUENCE OF FLOCKING ON THE FORAGING BEHAVIOR OF THE CHOUGH (PYRRHOCORAX-PYRRHOCORAX) AND THE ALPINE CHOUGH (PYRRHOCORAX-GRACULUS) COEXISTING IN THE ALPS

We studied the flocking and foraging behaviour of the chough Pyrrhocor ax pyrrhocorax and the Alpine chough P. graculus coexisting in the sou th-western Italian Alps in order to evaluate the costs and benefits of foraging in single- and mixed-species Becks. In the single-species co ntext, flock size significantly affected the foraging behaviour of the Alpine chough; in larger flocks, the birds stayed for a shorter time in a patch and fed more quickly than in smaller flocks. Flock size did not significantly affect the foraging behaviour of the chough, probab ly because of the small number of individuals per flock. The propensit y for mixed-species hocking was rather low. The observed frequencies o f single-species Becks of choughs and Alpine choughs were significantl y higher than those expected on the basis of random flocking, whereas the observed frequencies of mixed-species flocks of the two species we re lower than those expected. The stay times became significantly shor ter for the chough in the presence of the Alpine chough. Moreover, fee ding rates of the Alpine chough were significantly lower in the presen ce of the dominant chough. The present study does not confirm the hypo thetical foraging advantages of flocking. In single-species hocks, the benefits for the Alpine chough (higher feeding rates in larger hocks) were roughly compensated by the costs (shorter stay times in larger f locks), whereas the chough apparently neither gained benefits nor endu red costs. In mixed-species flocks, the Alpine chough sustained costs due to a reduction of feeding rates and the chough suffered costs due to a reduction of stay times. Hence, on average, single-species flocki ng gives no evident foraging advantages to either the chough or the Al pine chough, whereas mixed-species flocking provides some disadvantage s for both species.