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Redrawn from Moskat and Hauber 2007.
The importance of intraclutch variation in affecting the costs of mistaken recognition explains the success of another type of host bird. One species that shows great success against cuckoo eggs is the blackcap ( Sylvia atricapilla ). Within clutches, the birds’ own eggs show very little variation, so rejection decisions are based on the degree of mimicry of cuckoo eggs when compared to own eggs. In this case, there is very little cost to learning to distinguish own eggs and foreign eggs, so learning to recognize and reject is adaptive (Honza et al. 2004).
The likelihood that a host will be parasitized will result in differences in frequency of egg rejection. If the threshold for accepting eggs is too low, then many of the host’s own eggs will be rejected along with the parasitic eggs. However, if the threshold is set too high, then the host will accept many of the parasitic eggs, but retain its own eggs. Among reed warblers, this threshold is set depends on the stage of egg laying as described above as well as their perceived probability that the nest has been parasitized. Birds that recognized a cuckoo at their nest usually rejected an egg afterward because the probability of benefitting from rejection was much greater than if no cuckoo had been observed at the nest (Davies et al. 1993) (See also [link] ). On the other hand, the rejection behavior of reed warblers that did not observe a parasite at the nest was best explained by the parasitism level in the area. As parasitism level decreases overall, the benefit of rejecting eggs diminishes since it is more likely that a bird will mistakenly reject its own eggs. Therefore, lower parasitism levels results in increased acceptor behavior.
While species such as reed warblers will reject eggs based on whether they see a parasite at the nest, it is unclear how host birds learn to recognize parasites. One study performed on yellow warblers ( Dendroica petechia ), a species that suffers heavily from brown-headed cowbird parasitism, found several cues that elicited defensive responses such as sitting in the nest. Female yellow warblers responded strongly to stuffed cowbirds, which had shorter bills than models that had been modified with a longer bill. The authors suggest that recognition of shape confers an advantage since parasitism usually occurs in low light conditions.
Additionally, yellow warblers respond differently to male and female cowbirds even though their physical appearances are quite similar. When female cowbird chatter and male cowbird songs were played, female yellow warblers responded much more intensely to the female vocalizations, suggesting that difference in sexes is recognized by sound.
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