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Natural selection penalizes individuals that provide costly parental care to non-relatives. However, feedings to brood-parasitic fledglings by individuals other than their foster parents, although anecdotic, have been commonly observed, also in the great spotted cuckoo (Clamator glandarius) – magpie (Pica pica) system, but this behaviour has never been studied in depth. In a first experiment, we here show that great spotted cuckoo fledglings that were translocated to a distant territory managed to survive. This implies that obtaining food from foreign magpies is a frequent and efficient strategy used by great spotted cuckoo fledglings. A second experiment, in which we presented a stuffed-cuckoo fledgling in magpie territories, showed that adult magpies caring for magpie fledglings responded aggressively in most of the trials and never tried to feed the stuffed cuckoo, whereas magpies that were caring for cuckoo fledglings reacted rarely with aggressive behavior and were sometimes disposed to feed the stuffed cuckoo. In a third experiment we observed feedings to post-fledgling cuckoos by marked adult magpies belonging to four different possibilities with respect to breeding status (i.e. composition of the brood: only cuckoos, only magpies, mixed, or failed breeding attempt). All non-parental feeding events to cuckoos were provided by magpies that were caring only for cuckoo fledglings. These results strongly support the conclusion that cuckoo fledglings that abandon their foster parents get fed by other adult magpies that are currently caring for other cuckoo fledglings. These findings are crucial to understand the co-evolutionary arms race between brood parasites and their hosts because they show that the presence of the host's own nestlings for comparison is likely a key clue to favour the evolution of fledgling discrimination and provide new insights on several relevant points such as learning mechanisms and multiparasitism. [ABSTRACT FROM AUTHOR]

The post-fledging period is a critical phase for juvenile survival, and parental care provided during this period is a key component of avian reproductive performance. Very little is known about the relationships between foster parents and fledglings of brood parasites. Here, we present the results of a 5-year study about the relationships between fledglings of the non-evictor brood parasitic great spotted cuckoo ( Clamator glandarius) and its magpie ( Pica pica) foster parents. Sometimes, great spotted cuckoo and magpie nestlings from the same nest can fledge successfully, but most often parasitic nestlings outcompete host nestlings and only cuckoos leave the nest. We have studied several aspects of cuckoo post-fledging performance (i.e. feeding behaviour, parental defence and fledgling survival) in experimental nests in which only cuckoos or both magpie and cuckoo nestlings survived until leaving the nest. The results indicate that great spotted cuckoo fledglings reared in mixed broods together with magpie nestlings were disadvantaged by magpie adults with respect to feeding patterns. Fledgling cuckoos reared in mixed broods were fed less frequently than those reared in only cuckoo broods, and magpie adults approached less frequently to feed cuckoos from mixed broods than cuckoos from only cuckoo broods. These results imply that the presence of host's own nestlings for comparison may be a crucial clue favouring the evolution of fledgling discrimination; and furthermore, that the risk of discrimination at the fledgling stage probably is an important selection pressure driving the evolution of the arms race between brood parasites and their hosts. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]