Avoidance by Birds of Insect-Infested Fruits of Vaccinium ovalifolium

Abstract

This study investigates the interaction between the insects (a sawfly and a moth) that infest the fruits of Vaccinium ovalifolium (Ericaceae) and the birds that act as seed dispersers of this plant in a temperate rainforest of Southeastern Alaska. Experiments and observations were performed in the field and with captive birds in an aviary. Intact fruits are round whereas those that are attacked by an insect usually acquire a deformed shape; thus, birds have a cue to discriminate among the blueberries. Infested berries are not necessarily larger or smaller than intact berries, so fruit size cannot be used as a cue to discriminate between them. Results from the aviary experiments showed that the two avian species examined, varied thrushes and American robins, consumed significantly less insect-infested fruits than uninfested ones. In the field, birds also showed a preference for round fruits over deformed ones. Here, the probability of removal of an insect-infested berry depends on the type of habitat where the plant is located. Avian fruit removal was higher in an open site than inside the forest while the incidence of insects was greater in the latter. The presence of deformed fruits did not decrease the removal of uninfested fruits in either site. Not all seeds in an infested fruit are killed by the larvae, so animals may still be dispersers of the viable seeds if the infested fruit is consumed. The larger the fruit the higher the number of seeds within it. The two species of birds preferred large over small fruits in the aviary. Therefore, a seed in a large berry - infested or not by insects-might have a greater probability of being dispersed than one in a smaller fruit (although to assess this we need to know which other factors affect fruit selection). There is no apparent competition between birds and insects that results in a nonoverlapping use of the resources (fruits). The two insect species risk mortality by vertebrate frugivores, as larvae develop within ripe fruits. The evolutionary implications of the bird-insect interaction for plant fitness are apparently negligible, mainly because insects do not seem to kill many seeds and also because the bird-insect encounters are rather few.