In the UK, the common cuckoo is famous for heralding the start of spring. It is also famous as the only British bird not to rear its own young. Instead, it lays its eggs in the nest of another species, allowing the host to take care of incubation and feeding and rearing duties.
Cuckoos begin arriving back in the UK from their wintering grounds in Africa in late March. They time their arrival to coincide with the hatching of hairy caterpillars, such as the fox tail, cinnabar moth, and drinker moth caterpillars, which are their preferred foods.
Hairy caterpillars are toxic, and most other birds avoid them. To avoid being poisoned, the cuckoo bites the head off the caterpillar, slices it open with its bill, and then shakes it to extract the poison before swallowing it.
About a month later, and when they have recovered the energy expended during the long migratory flight, cuckoos are ready to start laying their eggs.
In the UK the most common hosts for cuckoos are the meadow pipit, dunnock, pied wagtail, robin, and reed warbler, although dozens more foster species have been recorded, and there are hundreds worldwide.
Although cuckoos as a species lay eggs in many different colours and patterns, female cuckoos have evolved to favour one particular host species over others, laying eggs in just one colour or pattern.
As female cuckoos evolve to lay eggs that better mimic those of the host species, so the host species adapts to better identify the intruder’s egg. This means there is an ongoing arms race between the cuckoo and the host bird, and one in which the male plays no part.
A male cuckoo can mate with several female birds that all lay eggs in different colours, but his genes have no impact on the colour of the eggs produced. If he did, then there would be a chance that the eggs containing his offspring or his daughter’s offspring were a different colour from the favoured host’s eggs and therefore a greater risk of the egg being identified and rejected.
Some groups of female cuckoos are better at mimicry than others. For example, the pale blue eggs laid in the nest of the redstart are a close match, and to the human eye almost identical, whereas cuckoos who lay in dunnocks’ nests don’t seem to make much effort at all.
Dunnocks also lay blue eggs, but the intruding cuckoo lays a white egg with brown speckles. Despite this, dunnocks happily accept the forgery and incubate it as their own, which would suggest that cuckoos have been invading redstarts’ nests for much longer than dunnocks who are yet to evolve the ability to distinguish the egg, unlike redstarts.
Other host species, such as bramblings, have evolved to lay eggs with complex patterns that may even vary from female to female to make it easier for them to detect a foreign egg. But those highly patterned eggs are also the most closely mimicked by cuckoos.
Once a female cuckoo has chosen a nest, she will spend some time watching it from a nearby vantage point. This is to ensure that she times the egg switch just right. Too early and she risks the host’s eggs hatching before her own. If she’s too late, then she’ll sometimes eat the entire clutch of eggs to force the parent hosts to produce a replacement brood.
Unlike most birds, cuckoos lay their eggs in the afternoon or early evening. The host bird will have laid her eggs in the morning and will stay close to the nest for some time after laying, which doesn’t give the cuckoo an opportunity to make the secretive swap.
When the host leaves the nest in the afternoon to feed, the cuckoo approaches, pushes one of the existing eggs out or eats it, and replaces it with her own egg, before quickly leaving again, in a process that takes as little as 10 seconds. If another cuckoo has already got there before her, she seems unable to differentiate between the host’s eggs and the other cuckoo’s egg, and birds may end up hosting multiple eggs.
If for some reason she is unable to lay in the selected nest, she will lay on the ground or in the nest of an unsuitable species, rather than find another suitable nest.
It turns out that cuckoos aren’t just masters of disguise when it comes to egg production. During the observation period and just after laying in the nests of reed warblers, female cuckoos give a conspicuous rapid ‘chuckle’ or bubbling call that is very similar to that of the sparrowhawk.
It may seem strange that after going to all that effort to not give herself away when depositing the forged egg she would draw attention to herself in this way. But the call is actually a distraction technique.
Sparrowhawks are one of the main predators of reed warblers so while the reed warbler is busy ensuring her own safety and seeing off a potential threat, she is not paying attention to her clutch and the newly deposited egg. Reed warblers do not respond to the male’s more familiar ‘cuck-oo’ call in the same way.
Each year, cuckoos can lay up to 50 eggs in different nests. But not all of these hatch. As birds have evolved to lay better-patterned eggs, so too have they evolved to recognised imposter eggs.
It’s not fully understood how hosts can recognise a cuckoo’s egg in the nest, but it’s thought to be down to three factors; visual clues, including UV appearance, the presence of a cuckoo near the nest site, and mobbing behaviour by other birds nearby.
Interestingly, species whose population is threatened, such as the reed warbler, are much less likely to eject an egg. It seems the risk of ejecting one of their own eggs or even deserting the nest is higher than playing host to a potential cuckoo egg.
Cuckoos’ eggs need to be incubated for 11-13 days, which correlates with the incubation period for their hosts’ eggs. Meadow pipits’ incubation period is 13-15 days, dunnocks incubate for 14-15 days, and pied wagtails for 13 days. But to ensure that her egg hatches first, the cuckoo employs another strategy.
Most birds lay eggs 24 hours after ovulation. After the ovum is fertilised, it travels through the oviduct where the albumen and shell are added to form an egg, and by the time the egg is laid the microscopic embryo contains about 10,000 cells.
Cuckoos, however, retain the eggs and internally incubate them for up to 24 hours before laying them. The body temperature of a cuckoo is about 40°C, whereas eggs outside of the body are incubated at about 36°C. This higher temperature means that when the cuckoo lays her egg it has the equivalent of a 31-hour head start on its foster siblings.
The idea of internal incubation had been put forward by egg collectors in the 18th and 19th century who had been surprised to see the advanced embryo development of cuckoos when blowing the eggs. But it was rejected by the scientific community for almost 200 years until they were able to conduct experiments with the eggs of zebra finches.
After hatching, the cuckoo chick pushes the host’s eggs and any chicks out of the nest using its back and wings, so it alone can be fed by its foster parent. This is instinctive behaviour. Its biological parents are not around to teach it and in fact it will never see or know who its mother is.
It was Edward Jenner, the English physician and scientist, better known for creating the smallpox vaccine, who demonstrated how the cuckoo chick is able to hoist the eggs and chicks over the edge of the nest. Prior to this it was believed that the adult cuckoos pushed them out of the nest, but Jenner showed that the baby cuckoo has a small cup-like depression in its back which disappears after 12 days and that has specifically evolved for this purpose.
Writing in a paper published by the Royal Society he said,
“The singularity of its shape is well adapted to these purposes; for, different from other newly hatched birds, its back from the scapula downwards is very broad, with a considerable depression in the middle. This depression seems formed by nature for the design of giving a more secure lodgement to the egg of the Hedge-sparrow, or its young one, when the young Cuckoo is employed in removing either of them from the nest. When it is about twelve days old, this cavity is quite filled up, and then the back assumes the shape of nestling birds in general.”
Cuckoos are much larger than their hosts and at just 14 days old after they have fledged, the chicks are about 2 times the size of their foster mother.
One hypothesis suggests that by evicting the eggs and chicks, they do not have to compete for food. With siblings present in the nest cuckoos may get overlooked because their foster parents recognise that they an imposter, either from sight or because of their begging calls. Even if they are able to understand there is someone in the nest that shouldn’t be there, they may distribute the food evenly between all chicks, meaning the cuckoo doesn’t get a large enough share for its growing requirements.
However, other studies have shown that although cuckoo chicks raised alongside the host’s chicks are much smaller when they fledge than cuckoos raised alone, after a couple of weeks cuckoos raised with siblings grew faster than cuckoos without siblings.
Although hosts have evolved to recognise cuckoo’s eggs, even those that look very much like their own, they are surprisingly bad at recognising the giant chick taking up all the room in their nest.
Studies have shown that cuckoo chicks make a begging call that sounds remarkably like a full brood of chicks, which tricks the parents into feeding them.
Despite this, cuckoos still struggle to receive as much food as they need while in the nest, and so to compensate remain with the host parent for much longer than host chicks would.
Adult cuckoos begin to return to their wintering grounds in sub-Saharan Africa from the middle of July. Juveniles do not accompany them, but set off on their journey about a month later.
It appears they possess an innate ability to find their way, although they leave the breeding sites more slowly and follow different routes. For example, they make more sea-crossings than adults which could be a way for them to avoid predators.
After spending their first winter in Africa they return to their natal sites, albeit slightly later than more mature birds. Most do not breed during this first year, although they may engage in some sexual activity, responding to calls of the opposite sex and following each other around.
3 Responses
wow. just wow
i cant believe such small creatures have so much power and are so smart.
I just came across a video of a cuckoo chick pushing out host mother’s eggs and chicks. I found it strange that a newborn could posses such strength. I thought the video was fake and came to do my research. Thanks for providing this information. Nature is beautiful but sometimes scary.
I have not not heard Cuckoos calling where i live for the last three years, in North East Lincs, but this year i have heard them in three different places, all close to the coast, i try to spot them but have only ever seen one once before.