Frugivorous birds are expected to maximize pulp ingestion and, hence, prefer large fruits. Experience and learning may play a role in fruit-size choices, and species may also have innate preferences. We investigated preference for fruit size of Oneseed Hawthorn (Crataegus monogyna) in wild and naive Eurasian Blackbirds (Turdus merula). The preference patterns of wild and naive birds were similar, although greater differences were found among fruit-size classes in wild birds. When presented with a similar number of fruits per size class, both naive and wild birds preferred large fruits. However, when the same fruit mass per size class was offered, they reduced overall fruit consumption, and preferences for large fruits became weaker. The results suggest that Eurasian Blackbirds behave as energy-intake maximizers when feeding on Oneseed Hawthorn fruit and that they are driven, to some extent, by innate preferences for fruit size. However, birds in aviaries may not always behave in the same way as their wild counterparts, and further work is needed to verify that wild birds behave similarly under natural conditions.