Pecking of Olives by Frugivorous Birds: A Shift in Feeding behaviour to Overcome Gape Limitation

Abstract

This paper analyzes the change in feeding behaviour of frugivorous birds (from swallowing the whole fruit to pecking the pulp) in olive orchards. The frequency of olive consumption by pecking was indirectly assessed by comparing the appearance of seeds in diet samples of frugivorous birds captured in olive orchards and in a wild olive shrubland. Although cultivated and wild olives are varieties of the same species, the size of cultivated olives is twice that of wild olives. The proportion of olives of a size adequate for swallowing by Blackcap Sylvia atricapilla was very low in olive orchards (0-2.0%, four plots and two years) and much higher in a wild olive shrubland (58 and 88% in two different years). For a larger frugivorous bird, the Song Thrush Turdus philomelos, 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 in olive orchards; however, in both habitats olives were the most important component in the diet and appeared in most of the analyzed samples. This suggests that fruit swallowing is typical in the shrubland and that fruit pecking, which implies that the single large seed is not swallowed, is a common behaviour of this species in olive orchards. For Song Thrushes seeds appeared in 49.4% of the diet samples in olive orchards and in all of the samples in the shrubland, suggesting a lower frequency of pecking behaviour than in Blackcaps. Therefore, pecking behaviour was caused by the large size of the cultivated olives. In laboratory trials, olive pecking was much less profitable than swallowing the whole fruit. For Blackcaps the benefit/handling time ratio for pecking was approximately four times lower than for swallowing and for Song Thrushes it was 12 times lower. The high energy content of olives allowed the ingestion of sufficient energy by fruit pecking to make this feeding behaviour cost-effective, despite long handling time. Due to the scarcity of fruits that fit the gape of small frugivorous birds in olive orchards, pecking of olives seems necessary for the establishment 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 fruit. This is possible because, although pecking is more costly than swallowing in terms of handling time, it may still be cost-effective, in particular with