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 volume among individual seeds) and fruit color in determining feeding preferences of American Robins (Turdus migratorius). Experiments were conducted using artificial fruits with either 8 small plastic beads or a single large one with equivalent volume. Other fruit characters were held constant. As predicted, large seeds were voided rapidly by regurgitation, resulting in higher pulp consumption rates for large-seeded fruits than for small-seeded ones, whose seeds were passed through the gut. Most birds apparently used this difference in profitability as a choice criterion: four of seven preferred large-seeded fruits. That three individuals did not do so suggests that birds may differ in their ability to perceive minor differences in fruit profitability, or to use them as choice criteria. Pulp color was also important: blue fruits were preferred by all seven birds. This preference was surprising, since Robins commonly feed on red fruits in the field.