Biological organisms have evolved a plethora of tactics to survive and reproduce in a wide range of habitats. While some species are what you might call ‘honest’ in their approach to life, many species are rather ‘dishonest’ and take advantage of other species. In particular, parasites exploit their hosts in a number of ways to cheat their way through life. Cuckoos are one such parasite: they escape the energetically costly investment of rearing young by laying their eggs in the nests of other birds. This is very detrimental for the hosts, as many, if not all, of their own young do not survive.
While cuckoos are quite adept at sneaking their own eggs into the nests of their apparently unsuspecting hosts, they are not always successful. Many birds actively try to prevent this parasitism, often by attacking the offending cuckoos or by evicting any eggs in their nests that look strange. This enticed Jenny York and Nicholas Davies from the University of Cambridge, UK, to try to discover whether cuckoos have any other clever tricks to enhance the success of their own offspring.
York and Davies chose to focus on reed warblers as the host species, as their nests are often the preferred choice of cuckoos. Firstly, they played four different calls to nesting reed warblers, including a female cuckoo, a male cuckoo, a Eurasian sparrowhawk and a collared dove. Theoretically, the reed warblers should not treat the calls of the male cuckoo or the dove as threats; the female cuckoo, while a threat to their clutch, should not pose an immediate threat to the adults; whereas the sparrowhawk would necessitate instantaneous vigilance. Indeed, the reed warblers did not react to the male cuckoo and the dove calls; however, they responded with increased alertness to the female cuckoo and the sparrowhawk calls.
However, York and Davies wanted to confirm whether this similarity in response was due to similarities between the calls or whether they are individually perceived as threats. Therefore, they repeated the same experiment on tits, birds which are often preyed upon by sparrowhawks, yet are not typically used as hosts by cuckoos. So, one could expect that the tits’ response to the calls of female cuckoos and sparrowhawks would vary if they do indeed sound different. However, their responses to all four calls were identical to those of the reed warblers, suggesting that while the calls of the female cuckoos and sparrowhawks sound different to us, they sound similar enough to birds.
But how does having a similar call to a sparrowhawk help female cuckoos? Well it's the perfect distraction, of course. York and Davies conducted one final experiment where they painted one of the eggs in a reed warbler nest solid brown, ensuring it looked different to their usual splotchy eggs, and then presented the parents with the four varying bird calls. The reed warbler parents that were distracted by either the sparrowhawk or the female cuckoo calls were less likely to reject the odd egg, in contrast to the parents presented with the male cuckoo or dove calls, which often removed the foreign-looking egg from their nest.
By diverting the attention of the hosts with a false predatory call, female cuckoos increase the chance of their own offspring being accepted and surviving. While this may be a rather ‘dishonest’ way of ensuring your genes live on, we have to admit that it is a very clever tactic.