Pj. Rey et Je. Gutierrez, PECKING OF OLIVES BY FRUGIVOROUS BIRDS - A SHIFT IN FEEDING-BEHAVIOR TO OVERCOME GAPE LIMITATION, Journal of avian biology, 27(4), 1996, pp. 327-333
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.