PECKING OF OLIVES BY FRUGIVOROUS BIRDS - A SHIFT IN FEEDING-BEHAVIOR TO OVERCOME GAPE LIMITATION

This paper analyzes the change in feeding behaviour of frugivorous bir ds (from swallowing the whole fruit to pecking the pulp) in olive orch ards. The frequency of olive consumption by pecking was indirectly ass essed by comparing the appearance of seeds in diet samples of frugivor ous birds captured in olive orchards and in a wild olive shrubland. Al though cultivated and wild olives are varieties of the same species, t he size of cultivated olives is twice that of wild olives. The proport ion of olives of a size adequate for swallowing by Blackcap Sylvia atr icapilla was very low in olive orchards (0-2.0%, four plots and two ye ars) and much higher in a wild olive shrubland (58 and 88% in two diff erent years). For a larger frugivorous bird, the Song Thrush Turdus ph ilomelos, more olives fitted its gape width (2-74% in olive orchards, 100% in the shrubland). Seeds appeared in 58.1% of the diet samples of Blackcaps in wild olive shrubland, but in only 4.9% of diet samples i n olive orchards; however, in both habitats olives were the most impor tant component in the diet and appeared in most of the analyzed sample s. This suggests that fruit swallowing is typical in the shrubland and that fruit pecking, which implies that the single large seed is not s wallowed, is a common behaviour of this species in olive orchards. For Song Thrushes seeds appeared in 49.4% of the diet samples in olive or chards and in all of the samples in the shrubland, suggesting a lower frequency of pecking behaviour than in Blackcaps. Therefore, pecking b ehaviour was caused by the large size of the cultivated olives. In lab oratory trials. olive pecking was much less profitable than swallowing the whole fruit. For Blackcaps the benefit/handling time ratio for pe cking was approximately four times lower than for swallowing and for S ong Thrushes it was 12 times lower. The high energy content of olives allowed the ingestion of sufficient energy by fruit pecking to make th is feeding behaviour cost-effective, despite long handling time. Due t o the scarcity of fruits that fit the gape of small frugivorous birds in olive orchards, pecking of olives seems necessary for the establish ment of these birds in this habitat. In a broader context, this study shows that some frugivorous birds are able to overcome gape limitation by changing their feeding behaviour from swallowing to pecking the fr uit. This is possible because, although pecking is more costly than sw allowing in terms of handling time, it may still be cost-effective, in particular with large fruits.