We studied the flocking and foraging behaviour of the chough Pyrrhocorax pyrrhocorax and the Alpine chough P. gruculus coexisting in the south-western Italian Alps in order to evaluate the costs and benefits of foraging in single- and mixed-species flocks.
In the single-species context, 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, probably because of the small number of individuals per flock.
The propensity for mixed-species flocking was rather low. The observed frequencies of single-species flocks of choughs and Alpine choughs were significantly higher than those expected on the basis of random flocking, whereas the observed frequencies of mixed-species flocks of the two species were lower than those expected. The stay times became significantly shorter for the chough in the presence of the Alpine chough. Moreover, feeding rates of the Alpine chough were significantly lower in the presence of the dominant chough.
The present study does not confirm the hypothetical foraging advantages of flocking. In single-species flocks, the benefits for the Alpine chough (higher feeding rates in larger flocks) were roughly compensated by the costs (shorter stay times in larger flocks), whereas the chough apparently neither gained benefits nor endured 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 flocking gives no evident foraging advantages to either the chough or the Alpine chough, whereas mixed-species flocking provides some disadvantages for both species.