Cuckoos have multiple adaptation to achieve their nefarious objective, from laying quickly to mimicking the appearance of hawks but their mimicry of their hosts eggs is spellbinding. Females of the common cuckoo typically lay eggs in the nest of only one host species, and so their eggs need to look like the other in the nest or they risk being thrown out of the nest. It appears that cuckoos fall into different ‘races’ and each one specialises in a single host. However, scientists are still unsure exactly how this system works.
One of the prevailing ideas about how cuckoo races work in that egg mimicry is passed down almost exclusively along the female line. This line of thought posits that males can mate indiscriminately and this won’t impact on the ability of females to mimic the eggs of their host species. This is theoretically possible because of the way sex is determined in birds. Unlike mammals, where males are the heterogametic sex (XY), in birds it is the female who possesses two different sex chromosomes (ZW) while males are homogametic (WW). If genes that specify egg patterning are on the Z chromosome then it doesn’t matter who the father is because the female offspring will still inherit the ability to deceive a specific host species.
However, male genes may still play a role in all of this. An alternative idea of how brood parasites maintain their host specificity comes from work carried out on the indigobird, which parasitizes the red-billed firefinch (Payne et al. 2000). In some cool aviary experiments, indigobirds wwere fostered by different host species and then the female offspring were given the choice of which species’ nest to lay in when they matured. Females chose the species that brought them up, so host species that brought them up, so host species is likely to be learned, but interestingly they also chose to mate with males who had been raised by the same host species. The reason for their choice is that males learn their song from their host species, and females exposed to that same song in the nest develop a preference for it. Now this is unlikely to be a direct analogue for how the races of common cuckoo are maintained but it has been shown that males in different habitats, where some host species are more common, have different songs but it remains to be seen if this is just an ecological adaptation or a true signal for female mate choice.
Currently we still don’t know exactly how it works, which is great because it mean that there is still a great reason to wonder around places like Wicken Fen and study these amazing birds!