Rejection Behavior by Common Cuckoo Hosts Towards Artificial Brood Parasite Eggs

--We studied the rejection behavior shown by different Norwegian cuckoo hosts towards artificial Common Cuckoo (Cuculus canorus) eggs. The hosts with the largest bills were grasp ejectors, those with medium-sized bills were mostly puncture ejectors, while those with the smallest bills generally deserted their nests when parasitized experimentally with an artificial egg. There were a few exceptions to this general rule. Because the Common Cuckoo and Brown-headed Cowbird (Molothrus ater) lay eggs that are similar in shape, volume, and eggshell thickness, and they parasitize nests of similarly sized host species, we support the puncture resistance hypothesis proposed to explain the adaptive value (or evolution) of strength in cowbird eggs. The primary assumption and prediction of this hypothesis are that some hosts have bills too small to grasp parasitic eggs and therefore must puncture-eject them, and that smaller hosts do not adopt ejection behavior because of the heavy cost involved in puncture-ejecting the thick-shelled parasitic egg. We compared our results with those for North American Brown-headed Cowbird hosts and we found a significantly higher proportion of rejecters among Common Cuckoo hosts with grasp indices (i.e. bill length x bill breadth) of <200 mm 2. Cuckoo hosts ejected parasitic eggs rather than accept hem as cowbird hosts did. Among the Common Cuckoo hosts, the cost of accepting a parasitic egg probably always exceeds that of rejection because cuckoo nestlings typically eject all host eggs or nestlings shortly after they hatch. Received 25 February 1990, accepted 23 October 1990. THE EGGS of many brood parasites have thicker shells than the eggs of other bird species of similar size (Lack 1968, Spaw and Rohwer 1987). Several hypotheses have been developed to explain this phenomenon, the most recent being that of Spaw and Rohwer (1987). Spaw and Rohwer (1987) and Rohwer and Spaw (1988) argued that the thick eggshell of the parasitic American cowbird (Molothrus) species has evolved so as to resist puncture ejection by small host species. They tested an assumption of this hypothesis by measuring the length and the width of the bill (the product of these two measurements they termed the "grasp index") of Brown-headed Cowbird (M. ater) hosts which had been classified as acceptors or rejecters. They concluded that some small-sized hosts are more or less forced to be acceptors because of heavy cost involved in getting rid of the thick-shelled Brown-headed Cowbird egg. This hypothesis has received support from Picman (1989) and Rohwer et al. (1989). Rothstein (1975, 1976, 1977) showed that, although many parasitized species accepted (or did not remove) introduced nonmimetic artificial eggs of Brown-headed Cowbirds, some species ejected them. He observed and inferred that these potential hosts ejected eggs from their 348 nests either by grasping the eggs or by puncturing the eggs before removal. Rohwer and Spaw (1988) used this distinction in ejection type when they considered the possibility of physical constraints to ejection for those species parasitized by Brown-headed Cowbirds. They compared the characteristic type of ejection (grasp or puncture) or lack of ejection response (acceptance) with the bill size for each of 40 parasitized passerine species. They suggest hat small bill size constrains some species from grasping the cowbird eggs for ejections, and that the strength of the cowbird eggs limits successful puncture ejections for most of these species. Rohwer and Spaw (1988) propose that the costs associated with these constraints have selected for acceptance. It is not clear from their indirect test which acceptor species are capable of successfully puncturing cowbird eggs for ejections (and would do so, given sufficient selective pressure) and which acceptor species cannot puncture the cowbird egg because the eggshell is too strong. So far only one host species of the North American Brown-headed Cowbird, the Northern Oriole (Icterus galbula), has been shown to be a true puncture ejector (Rothstein 1977). To test whether Northern Orioles experience any The Auk 108: 348-354. April 1991 April 1991] Parasite-Egg Rejection i Cuckoos 349 cost in puncture-ejecting the thick-shelled Brown-headed Cowbird egg, Rohwer et al. (1989) added Brown-headed Cowbird and control eggs into oriole nests, and found that the host species occasionally damaged some of its own eggs in the process of ejecting the cowbird