Do great spotted cuckoo nestlings beg dishonestly?

It is generally assumed that begging signals provide parents with reliable information on the state of their young, allowing them to decide accordingly how much to invest in their nestlings. Avian brood-parasitic nestlings exhibit begging displays that are highly exaggerated. In most species, they also increase their begging level in relation to their level of hunger, as predicted by models of honest signalling. However, it has been suggested that great spotted cuckoo, Clamator glandarius, nestlings are an exception to this general rule because they spit out food when satiated and continue begging at a high intensity. We tested this assertion by using both natural observations of food delivery by foster parents and a laboratory experiment in which we hand-fed both cuckoo and host magpie, Pica pica, and carrion crow, Corvus corone, nestlings while controlling for the degree of food deprivation. First, we found that cuckoo nestlings did not throw out food because they are satiated. Second, as in host magpie nestlings, during a period of food deprivation the cuckoo nestlings begged at a significantly higher frequency and intensity than when fed ad libitum (a period of food abundance) and begging signals increased with an increased level of hunger in both species. These results indicate that in this cuckoo species the frequency and intensity of begging are in accordance with food requirements, and, thus, that begging is not dishonest in the sense of being unreliable.