Are great tits (Parus major) really optimal foragers?

In this study, we test the classical prey model of optimal-foraging theory with great tits (Parus major) feeding on two types of mealworms presented on a conveyor belt. Contrary to the results of some previous experiments, prey types were given to birds in random order, therefore birds could not predict their next prey item. We tested birds' diet choices at four different prey-encounter rates. Our results show that in 95% of cases great tits consumed the more profitable prey type upon encounter. On the other hand, consumption of the less profitable prey type did not differ statistically from the "always-attack" strategy in 77% of cases when the rate of encounter with the more profitable prey was below a critical value, and did differ from that strategy in 67% of cases when the rate of encounter with the more profitable prey was above that critical value. Contrary to predictions of the classical prey model of optimal-foraging theory, our birds never completely excluded the less profitable prey type from their diet. We also estimated the functional responses of individual birds with respect to the more profitable prey type; birds' diet changes occurred too slowly to make these functional responses stabilizing.