Rejection of cuckoo (Cuculus canorus) eggs by meadow pipits (Anthus pratensis)

Meadow pipits (Anthus pratensis) normally accept mimetic cuckoo (Cuculus canorus) eggs laid in their nests. In field experiments in which a mimetic or a nonmimetic model cuckoo egg was placed in meadow pipit nests, those hosts that were simultaneously presented with a cuckoo dummy mounted beside the nest showed a significantly higher ability to recognize and reject (normally by desertion) the parasite's egg than those that were not presented with the dummy. In the present study we sought to answer the question of why meadow pipits do not always (i.e., even when no cuckoo was visible) reject cuckoo eggs. The results from the field experiments provide support for a theory that two “brood parasite” stimuli (a cuckoo egg in the nest and a cuckoo near the nest) are necessary to induce the meadow pipits to reject cuckoo eggs. When the threshold for rejection had been reached, the hosts reacted in a relatively short time after they were faced with the parasite stimuli. The results of these experiments also showed that a significantly higher rate of rejection occurred earlier rather than later in the incubation period. A possible explanation for the higher rate of acceptance of model cuckoo eggs among meadow pipits in Norway compared to those in Britain is discussed.