Disparity between adultand juvenile American robins Turdus migratorius foraging for ground invertebrates and cherry fruits

摘要

Although it has long been known that juveniles often have foraging skills inferior to those of adults, it has generally been assumed that animal prey are more difficult to capture than fruit, and thus that juveniles foraging on fruit should be similar to adults in their efficiency. To examine these ideas, we investigated the abilities of juvenile and adult American robins Turdus migratorius to forage for ground invertebrates and fruits of the black cherry tree Prunus serotina. We hypothesized that juveniles, lacking the experience of adults, would not have the skills of adults and therefore would be less proficient invertebrate and fruit foragers. Juveniles captured 69% of invertebrates at which they struck compared with 80% of adults' strikes that ended in capture. However, juveniles made more strikes than adults, so mean prey capture per minute was the same. Juveniles were also less skilled fruit foragers. Juveniles were twice as likely as adults to fail to pick a cherry (55% of adults' vs. 28% of juveniles' attempts ended in success). However, there was no significant difference in the proportion of juveniles and adults that dropped a cherry once it was picked. As a result of their low levels of success, juveniles consumed about half the number of cherries per minute as did adults. Contrary to prior assumptions, skills involved in fruit foraging may not be so easily acquired and many omnivorous species, like the American robin, must learn both invertebrate and fruit foraging skills.