Plaintive Cuckoo Nestling Adaptations and Development in Common Tailorbird Nests - Costly Child Custody
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- Institutt for biologi 
Avian brood parasitism represents a suitable system for studying coevolution, as parasite and host engage in an escalating arms race. The parasite develops adaptations to overcome host defences, and in addition develops traits that convince host parents to invest in the parasitic nestlings. A restricted number of study species have built much of the body of knowledge on brood parasitism, but as selection pressures vary between parasite-host systems, generalization between systems may vary in validity. This study aimed at investigating development, behaviour and adaptations of parasite nestlings in a poorly known parasite-host system consisting of the plaintive cuckoo Cacomantis merulinus and its common tailorbird Orthotomus sutorius host. Following the hypothesis that parasite nestlings solicit more parental care than host offspring, applying the derived understanding of the common cuckoo Cuculus canorus developmental characteristics and adaptations as a fundamental basis, three predictions were deduced: 1) the parasite nestlings would gain more weight than one tailorbird nestling, 2) the parasite nestlings would receive more provisioning than a host nestling, and 3) given that the previous predictions were met, the parasite nestlings would possess an extravagant begging display measured as begging intensity. The first prediction was redeemed, as the cuckoo nestlings gained more weight faster than host progeny. On the contrary to predictions 2 and 3, cuckoos did not solicit higher provisioning rates and they begged at rates equal to that of one host nestling during the first part of the nestling period. Differences in physiology between the two species where cuckoo nestlings allocate parental resources differently from host nestlings may provide an explanation as to why they gain more weight but fail to appropriate higher provisioning. Selection pressures may have favoured slower development during the first part of the nestling stage, where an intensification of the begging display during the latter half may convince parents to provide prolonged care to the cuckoo nestling and it may benefit from elevated feeding rates. The acquirement of a warning display may facilitate the exaggerated begging display. Cuckoo nestling mimicry of one tailorbird nestling s begging calls may be an adaptation that secures equal provisioning to one host nestling during the first period in the nest. The findings from this study are integrated in the body of knowledge on this parasite/host system, and coevolutionary adaptations of the system as a whole are discussed on the basis of previously attained knowledge of brood-parasitic systems.