Equivocal Responses of Feeding Parents to Experimental Brood Sizes in a Hawk–Cuckoo Host: Brood Size as a Reference for Parental Provisioning Decisions?

The number of offspring surviving until independence is the fundamental drive in the evolution of parental care. Because of the related costs, parental investment must be balanced with essential resources for parents themselves, among the resources available in the environment under the current parental condition. It is advantageous for parents to adjust their level of investment to the number of offspring; however, there is little evidence that parents employ numerical competence in adjusting their investment level. We investigated how parents respond to experimentally manipulated brood sizes in a passerine species, known as a host of a brood parasitic cuckoo whose chicks presumably deceive their hosts numerically. Parents reduced their provisioning to broods of reduced sizes, whereas parents did not increase provisioning to enlarged broods compared to that in the control condition. These parental responses can be attributed to the response of chicks to the experimental treatments compared to that in the control: chicks lowered begging intensity in the reduced condition, while they did not intensify being in the enlarged condition. Further analyses revealed that eagerness of parents to respond to chick begging intensity differed between the experimental treatments: strong parental response was detected toward begging chicks only in the reduced condition. We propose that the detected equivocality of parental responses might be related to the difference in the number of chicks between the unmanipulated and experimentally manipulated broods, the former reflecting the initial parental decision on the amount of resources to allocate to the brood.