Nature’s greatest strategists

Myths and legends concerning the cuckoo’s singular and oddly fascinating behaviour have been deep-rooted in our culture for millenia. Back in Ancient Greece, Aristotle noticed that cuckoos “laid their eggs in the nest of smaller birds after devouring these birds’ eggs”. Indeed, cuckoos and their hosts are constantly evolving tricks in order to ensure the success of their offspring. Biologists refer to this as an ‘evolutionary’ arms race. If the cuckoo manages to successfully deceive its host, through mimicry for instance, the chances that its chicks survive is greatly increased. The same goes for the host (e.g: the Great reed warbler). If the Great reed warbler catches the cuckoo out, it ensures the success of its own clutch. What are the cuckoo’s tricks and strategies to beat host defence? Nick Davies and Mike Brooke have investigated several factors that could help answer this question, and what better way to do so than to ‘play cuckoo’ themselves?

Using some handmade model eggs, they started to explore the possibility that cuckoos used egg mimicry to fool their hosts. Egg mimicry involves the colour and spotting of the egg not only in human vision but also in bird vision. Unlike humans, birds can perceive ultraviolet wavelengths as well as the visible range, as we know it. Their experiments showed that the hosts were less likely to reject the mimetic eggs. Although this result may seem obvious, the reason for egg mimicry was much debated on in the past. Alfred Russel Wallace, a contemporary of Darwin, suggested that cuckoos might have evolved mimetic eggs as a camouflage independently in order not to draw attention to the host’s clutch. However, as Nick Davies later explains in his book “Cuckoo: Cheating by Nature”, his experiments did not support this theory as he found no tendency for the predators to take more of the cuckoo eggs if they were not camouflaged. The second hypothesis is that the cuckoos might use egg mimicry to fool other female cuckoos. When the female cuckoo visits the nest, she removes one of the eggs in order to make room for her own. If a female cuckoo visited a nest which had already been parasitised by another beforehand, it would be logical to assume that it would be advantageous to eliminate the previous cuckoo’s egg before laying her own. Surprisingly though, the female cuckoos do not appear to be selective when discarding one of the eggs. Therefore, the only possible reason for egg mimicry is to deceive the hosts.

What about egg size? Cuckoos lay eggs around three times smaller than other species the same size. Another trick? In order to test this, Davies and Brooke placed larger model eggs, painted to resemble the host's eggs, in various reed warbler nests. They found out reed warblers were actually more likely to reject a bigger egg, so egg size does play a role in this arms race. Cuckoos have evolved to lay smaller eggs in order to trick their hosts into accepting them. This has the added benefit for the cuckoo that each egg is less costly for the female to produce. In the process of making their eggs less detectable, they have managed to shift even more of the parental investment onto their hosts. 

The timing and the speed of laying also play a very important role in the cuckoo’s strategy, as Davies and Brooke very quickly discovered. They began by placing their model eggs in completed but empty reed warbler nests and as expected, all were rejected. This explains why cuckoos wait until the hosts have begun laying to start parasitising the nest. However, when the reed warblers start laying, the number of eggs in the nest have no effect on the number of rejections. Most songbirds lay between 5 and 6 in the morning. Cuckoos lay around 10 to 12 hours later than that, and following their experiments, the two researchers were able to determine that reed warblers were less likely to reject the egg. Why? Despite a few theories, no one knows for certain yet. 

One of the most extraordinary aspects of the cuckoo’s behaviour is the speed at which it lays its eggs. Whereas most songbirds lay their eggs in around 20 minutes to an hour, the cuckoo can do so in just 4 seconds. Rapid laying must be another trick. Perhaps the reed warblers are alerted by the sight of a cuckoo at the nest. Davies confirmed this idea using a stuffed cuckoo, that he placed at a reed warbler nest. To his delight, the percentage of rejection was higher. This explains why there would be positive selection for rapid egg laying, but how do they actually control the timing of the laying? Most birds would spend the entire process sitting on their nest as the egg went down their oviduct in preparation for the laying. Female cuckoos spend this time on a branch carefully observing the host’s behaviour to time their laying perfectly. The ability of the cuckoo goes beyond just timing the laying from a branch close to the host’s nest. According to a unique account by Edgar Chance, early 20th century pioneer of cuckoo research, cuckoos are capable of controlling the very moment of extrusion of the egg, allowing them to delay the laying for two to three hours, if necessary. The way they do this is still unknown.

In order to win the arms race with their hosts, cuckoos have evolved smaller eggs which convincingly mimic their host's eggs. They've also gained the ability to lay eggs strategically with devastating efficiency. In the next blog post, I will explore what happens after these eggs hatch, and how cuckoo chicks take part in this conflict between cuckoos and their hosts.