Preferential allocation of food by magpies Pica pica to great spotted cuckoo Clamator glandarius chicks

Abstract

Adult magpies Pica pica provide parasitic great spotted cuckoo Clamator glandarius nestlings with a diet very similar to that fed to their own chicks. In both naturally and experimentally parasitized nests, great spotted cuckoo chicks were fed at a higher rate than magpie chicks in the same nest. This preferential allocation of food by magpie parents to great spotted cuckoo chicks is consistent with the supernormal stimulus hypothesis, because this result implies that cuckoo chicks provide stronger stimuli for parental care than host chicks. Great spotted cuckoo chicks receive most of the food brought to the nest by the foster parents, because they exploit a series of stimuli which jointly (or sometimes individually) operate as a supernormal stimulus. This hypothesis predicts that if any stimulus is masked, the efficiency of the cuckoo in eliciting parental care will decrease. Here, we analyze experimentally the effects of two of these stimuli, preferential feeding of large nestlings and of nestlings with conspicuous palatal papillae. Firstly, when we experimentally introduced one medium-sized (7-9 days) cuckoo chick into an unparasitized magpie nest where the largest magpie chick was 12-15 days old, the cuckoo did not receive significantly more food than the average or the largest magpie chick. Secondly, when unparasitized nests were experimentally parasitized with a cuckoo chick that had its gape painted to mimic that of magpie chicks, the parasitic cuckoo received less food than the average magpie chick.

Nestling competition, rather than supernormal stimulus, explains the success of parasitic brown-headed cowbird chicks in yellow warbler nests

“…Their success seems to stem from a combination of intense begging behaviour, conspicuous palatal papillae, and larger size (in natural situations) compared with their host's young (Soler et al 1995a). The palatal papillae seemed to play a key role in their success, as chicks with their gapes painted received less food than the average magpie chick (Soler et al 1995a). In contrast to the system studied here, great spotted cuckoos are specialist parasites, and magpies exhibit some chick discrimination (Soler et al 1995b).…”

“…Great spotted cuckoo chicks (Clamator glandarius) were fed more than smaller and similar sized magpies (Pica pica), and equally to much larger host young (Redondo 1993). Their success seems to stem from a combination of intense begging behaviour, conspicuous palatal papillae, and larger size (in natural situations) compared with their host's young (Soler et al 1995a). The palatal papillae seemed to play a key role in their success, as chicks with their gapes painted received less food than the average magpie chick (Soler et al 1995a).…”

“…Parasites may act as a supernormal stimulus by providing stimuli that hosts would be unable to resist (Dawkins & Krebs 1979). This is supported by studies on great spotted cuckoos (Clamator glandarius), which were favoured by magpies (Pica pica) over their own chicks (Redondo 1993;Soler et al 1995a).…”

Benefits associated with escalated begging behaviour of black-billed magpie nestlings overcompensate the associated energetic costs

“…Nestlings wore the neck collar for a period of 2 to 2.5h. This period is ~1h shorter than that previously used in magpies (Soler et al, 1995). Although this shorter experimental period may reduce the hypothetical differences between nestlings of different treatments, it may also reduce the possibility of some nestlings regurgitating boluses of food.…”

“…after their fifth dose), we placed neck collars on all the chicks in a group of parasitized and natural broods. The ligature wire around the neck was tight enough to hinder the swallowing of food and loose enough to avoid strangling the chicks [see Soler et al (Soler et al, 1995) for a further description of the neck-collar method in magpies]. Nestlings wore the neck collar for a period of 2 to 2.5h.…”