We found clear differences in tit responses depending on the mounted species to which they were exposed. During the presentations of the two harmless controls, a familiar collared dove and a novel teal, the tits often continued to visit the feeders, and afterward their attendance returned to pre-exposure levels. By contrast, they avoided the feeders during and after both sparrowhawk and cuckoo presentations. The most striking result from experiment 1 was that the response was similar to sparrowhawks and cuckoos, even though cuckoos are of no threat to adult tits.
The plumage manipulations in experiment 2 suggested that the strong alarm response to cuckoos depended on their resemblance to hawks because when their hawk-like underpart barring was obscured, the tits treated them as no more of a threat than doves. This supports the idea that the evolution of barring in parasitic cuckoos, revealed by the comparative analysis (Payne 1967; Kru¨ger et al. 2007), enhances their resemblance to hawks.
However, underpart barring cannot be the only feature inducing an alarm response because the tits showed equally strong alarm to barred and unbarred hawks. Furthermore, little alarm was shown to barred doves. Therefore, the underpart barring must combine with other cuckoo features, for example, their grey upperparts and elongated wings and tail, to cause hawk resemblance. We found no significant effect of the specimen, which suggests that these results cannot be attributed to any peculiarities of the particular mounts we used.
We also found no effect of the study site, so the tits on Wicken Fen, which would have experienced daily encounters with cuckoos during the previous summer, had equally strong responses to cuckoos as the tits in Cambridge and Madingley Wood, which were unlikely to have encountered cuckoos. This suggests that the strong effect of the cuckoo at both sites was not simply one of alarm to a novel stimulus. Tits attending rich food sources are especially vulnerable to attack because sparrowhawks learn that these are good locations for finding prey (Hinsley et al. 1995).
Because sparrowhawks make surprise attacks (Newton 1986; Cresswell 1996), alarm to any hawk-like stimuli is likely to be adaptive, despite the loss of feeding time from frequent false alarms. Nevertheless, the 5 min exposure of the specimens gave the tits ample opportunity for close inspection, so it is remarkable that a cuckoo caused a strong alarm response, given that it lacks a hawk’s lethal weapons, namely talons and a hooked beak. If the inspection of a potentially dangerous predator is costly, then even a slight resemblance through shape, grey upperparts, and underpart barring may be sufficient to deter approach.
Other studies have shown that mimics do not have to resemble the model perfectly to gain protection, especially when signal receivers regard the model as highly noxious or dangerous, or if the model is relatively common (Ruxton et al. 2004). Perhaps the tits’ response depends not only on the stimulus but also on the context; cuckoos are absent in winter so hawk-like stimuli at this time are more likely to be hawks.
The motivation of the signal receiver (value of the resource it is exploiting) may also influence responses to models and potential mimics (Barnett et al. 2007; Cheney & Coˆ te´ 2007). For example, it may pay a more hungry tit to risk a closer inspection of hawk-like stimuli when there is the potential for the stimulus to be a harmless mimic. Previous work on egg discrimination has shown that both great and blue tits, like other species with no history of cuckoo parasitism, will accept eggs unlike their own. This suggests that the egg rejection exhibited by cuckoo hosts has evolved specifically in response to cuckoo parasitism (Davies & Brooke 1989; Moksnes et al. 1991).
Our results here show that, at least in one context, great and blue tits respond to adult cuckoos as if they were hawks. This raises the possibility that the discrimination by cuckoo hosts of the adult cuckoo as an enemy distinct from hawks, which can be attacked (Moksnes et al. 1991; Duckworth 1991;Welbergen & Davies in press), is also an evolved response to cuckoo parasitism.
Experiments have revealed that in response to brood parasitism, hosts pay closer attention to the features of their own eggs so they are better able to discriminate against foreign eggs (Rothstein 1982; Lotem et al. 1995). Similarly, hosts may pay closer attention to multiple features of hawks so they can better discriminate cuckoos. Further studies are now needed to test the features used by cuckoo hosts to distinguish cuckoos from hawks and to test whether, despite some host discrimination, parasitic cuckoos still gain from hawk resemblance. Just as host improvements in egg discrimination have been selected for better cuckoo egg mimicry (Brooke & Davies 1988), so perhaps have improvements in their plumage discrimination selected for better cuckoo–hawk mimicry.
The study followed the guidelines for the treatment of animals in behavioral research and teaching (Association for the Study of Animal Behaviour). The mounted specimens were obtained from licensed taxidermists. We thank Chris Thorne and the Wicken Fen Group, Nancy Harrison, Julia Mackenzie, and Camilla Hinde for color-ringing tits; Jan Davies for making the barred/unbarred underparts for the mounts; John Parker, the director, for permission to work in the Cambridge University Botanic Garden; two anonymous referees for their helpful comments and the Natural Environment Research Council for funding.
N. B. Davies* and J. A. Welbergen
No comments:
Post a Comment