How should cuckoo chicks signal in different host nests?

Abstract

We welcome Soler and Soler's comments1xSoler, M. and Soler, J.J. Trends Ecol. Evol. 1999; 14: 294–295See all References1 on our paper2xKilner, R.M., Noble, D.G., and Davies, N.B. Nature. 1999; 397: 667–672Crossref | Scopus (189)See all References2 in this issue of TREE, and agree that only by understanding both links in the chain: chick need–begging signals–host provisioning3xGodfray, H.C.J. Nature. 1991; 352: 328–330CrossrefSee all References3 can we fully understand cuckoo chick (Cuculus canorus) begging displays. Our Nature paper dealt exclusively with the second link, whereas a forthcoming companion paper4xSee all References4 focuses on the first link.In asserting ‘the cuckoo chick needs the same provisioning rate as an entire brood of reed warblers’ (Acrocephalus scirpaceus), Soler and Soler imply that what the cuckoo chick gets is what it wants. But the two are not necessarily the same4xSee all References4.Turning to the behaviour of cuckoo chicks in other host nests, we agree that it would be fascinating to compare their begging displays. However, predictions cannot easily be made simply from our work on reed warbler hosts. We believe that it is impossible to understand how cuckoo chicks should signal to their host parents without understanding: (1) how the needs of a cuckoo chick compare with those of host young, and (2) how host parents integrate begging signals to determine provisioning rates. There are good reasons for supposing that both will differ between hosts, but precisely how they do remains to be determined.For example, apart from the reed warbler (which weighs ≈12 g), other common British hosts are the dunnock Prunella modularis (≈20 g) and the meadow pipit Anthus pratensis (≈18 g). The larger the host, the more similar the needs of a cuckoo chick and a single host young will be. In a dunnock nest, therefore, a cuckoo might have to call fast enough to compensate for the visual difference between its own gape and that displayed by two to three dunnock nestlings, rather than four nestlings in a reed warbler nest. This assumes, of course, that its optimal growth rate is the same in both nests. However, predictions about the cuckoo's begging displays are further complicated by the fact that the begging call rate of dunnock nestlings is far more rapid than that of reed warbler nestlings (R.M. Kilner and N.B. Davies, unpublished), which might be because dunnock nestlings typically compete with half-sibs rather than full-sibs5xBriskie, J.V., Naugler, C.T., and Leech, S.M. Proc. R. Soc. London Ser. B. 1994; 258: 73–78Crossref | Scopus (110)See all References5. By contrast, meadow pipit young are far less vocal (R.M. Kilner and N.B. Davies, unpublished), perhaps because they occupy ground-nests and so are more vulnerable to predators6xHaskell, D. Proc. R. Soc. London Ser. B. 1994; 257: 161–164CrossrefSee all References6. All of this suggests that cuckoos will have to tune into different offspring–parent communication systems in the nests of different hosts. This presents a fascinating developmental problem, given that cuckoo host races are restricted to the female line7xMarchetti, K., Nakamura, H., and Gibbs, H.L. Science. 1998; 282: 471–472Crossref | PubMed | Scopus (85)See all References7, hinting that either begging is maternally controlled or that cuckoos learn how to beg in different host nests.