Influence of Insect Damage in Fruits of Hawthorn on Bird Foraging and Seed Dispersal

Relationships between fruit pests (larvae of Blastodacna hellerella Dup.), avian seed dispersers (mainly blackbirds, Turdus merula) and hawthorn fruit (Crataegus monogyna Jacq.) were examined in England during early winter, 1981. Insect damaged fruits were rejected by birds foraging on the crown of the bushes. This causes a decrease in bird foraging efficiency in heavily infested bushes. The increase in rejections slowed down the removal of good undamaged fruits and prevented nearly 15% of good fruit being removed by avian dispersers. Hence the exposure time of these uneaten fruits to the attack of other noxious agents is increased, further decreasing the probability of their seeds being dispersed. Insect damage thus causes an even greater detrimental effect to highly infested bushes over that caused by the insect damage per se to the seed and flesh. Evolution of possible defensive mechanisms of hawthorn fruits against predators is discussed in relation to coevolutionary interactions between plant and seed dispersers.