‘Cuckoos – do cheats ever prosper?’, a talk by Matt Collis
Matt Collis is a free-lance ecologist and wildlife educator based in Torbay. He became interested in birds in his teens and it was through stories that his Gran told him that he became particularly interested in the cuckoo. Unlike the vast majority of birds, the cuckoo does not rear its own young but is an obligatory brood parasite. How does it manage to do it and what are the obstacles?
As Matt reminded us, the common cuckoo, with its familiar call, is seen as a harbinger of spring right across its range from the UK east to Japan and it features in poetry and folklore in many countries. Aristotle, writing in 350BC, described the cuckoo’s lifestyle. Some people thought that other species felt honoured to play host to the egg and chick of this unfortunate bird which was unable to rear its own young but others considered the cuckoo’s behaviour to be unnatural. To Gilbert White it was “a monstrous outrage on maternal affection…”. It was Darwin, in The Origin of Species, who proposed an evolutionary process which had led to this behaviour. Originally cuckoos reared their own young but occasionally a cuckoo laid an egg in the nest of another species. If these young survived they might well do likewise and benefit from the habit, so it spread till by natural selection it became the norm. The common cuckoo is not the only brood parasite. Such behaviour has evolved at different times and in different species. While our cuckoo chick ejects its host’s eggs or young shortly after hatching, the crested cuckoo chick begs so vigorously that it is given most of the food and the other chicks starve while the American striped cuckoo chick has a hook on its beak and slashes and kills its nest mates. Of the 150 Cuculiformes just 59 are brood parasites. The remainder build nests and raise their own chicks.
Matt showed us fascinating extracts from a video made by Edgar Chance in 1922 of a cuckoo laying in meadow pipit nests. Both male and female cuckoos have multiple partners, the male mating with as many females as possible. The female focuses on finding suitable nests to lay in and watching the behaviour of the potential hosts. Although the cuckoo has been found to lay in the nests of some 150 species there are only 40 regular hosts and some of these are used only in a very restricted area. Each female cuckoo belongs to a particular gen, which has evolved to lay in the nest of a particular species, the one which reared her. In most cases her eggs are a close match to those of the host. Timing is critical. If the cuckoo egg is laid in a nest with no eggs or just one, it will almost certainly be rejected by the host. If it is laid too late and the host chicks hatch several days before the cuckoo, they will be too big to be ejected from the nest and may well out-compete the cuckoo. Unlike most species, the cuckoo lays her egg in the afternoon. She holds it ready to lay, flies to the nest, removes a host egg in her beak and quickly lays. It pays to be quick as if the host returns and finds the cuckoo it may attack her. If a female produces an egg and there is no appropriate host nest or she cannot access the one she has chosen, the egg may be laid in a “wrong” nest and will most likely be ejected or fail. Cuckoos have evolved to resemble sparrowhawks which may make some hosts more wary of attacking them, another example of trickery.
Matt explained that trickery is also involved at the egg stage. A large egg in among the small host eggs would be very conspicuous. The cuckoo therefore produces very small eggs for its size. Some host species learn to eject eggs which differ from their own in background colour and markings. To combat this, the cuckoos in each gens produce eggs which mimic the eggs of their host species. Some are very accurate, others less so. If the host species has a standard egg design and the cuckoo egg is a good match it is likely to be accepted. However some host species produce very variable eggs and there is a greater chance that the cuckoo egg will stand out and be rejected. This is an arms race between cuckoo and host. The dunnock is unusual in showing no rejection defence and accepts a cuckoo egg however unlike its own it is. This is a mystery. It could be that the dunnock is a relatively new host which has not had time to develop a defence mechanism but this seems unlikely as Shakespeare mentions the hedge sparrow (i.e. dunnock) feeding the cuckoo [King Lear Act 1 Scene 4]. A possible explanation is that the dunnock has a long breeding season so only a limited number of its breeding attempts are vulnerable.
In some cuckoo species, trickery is again required at the chick stage. The chicks of an Australian bronze-cuckoo have evolved to sound like the chicks of the host. This has not been necessary for our cuckoo. Species like the reed warbler which frequently eject a cuckoo egg will accept a cuckoo chick. In fact they will accept the chick of a robin or dunnock both of which are quite unlike their own.
How many cheats are there worldwide? Matt said that there are about 100 obligate brood parasites. However, many more are facultative brood parasites, laying eggs in their own nest and sometimes in the nest of other birds of the same species. These include coots, moorhens, starlings and many ducks.
So, do cheats ever prosper? The short answer is “yes”. They have evolved and survived, so the benefits must outweigh the costs. Cheats must and do prosper. However, their lifestyle does entail costs with the risk of failure and an on-going the need to adapt. Putting all your eggs in another bird’s nest is risky.
We thank Matt for this most informative and fascinating talk which was much appreciated by those present.
Helen Baker
