THE INFLUENCE OF SEED PACKAGING AND FRUIT COLOR ON FEEDING PREFERENCES OF AMERICAN ROBINS

We investigated the role of seed packaging (division of total seed vol ume among individual seeds) and fruit color in determining feeding pre ferences of American Robins (Turdus migratorius). Experiments were con ducted using artificial fruits with either 8 small plastic beads or a single large one with equivalent volume. Other fruit characters were h eld constant. As predicted, large seeds were voided rapidly by regurgi tation, resulting in higher pulp consumption rates for large-seeded fr uits than for small-seeded ones, whose seeds were passed through the g ut. Most birds apparently used this difference in profitability as a c hoice criterion: four of seven preferred large-seeded fruits. That thr ee individuals did not do so suggests that birds may differ in their a bility to perceive minor differences in fruit profitability, or to use them as choice criteria. Pulp color was also important: blue fruits w ere preferred by all seven birds. This preference was surprising, sinc e Robins commonly feed on red fruits in the field.