JOIN USNature Scientists from the University of Cambridge have found that cuckoos use multiple disguises to fool other birds. That the cuckoo is devious is well known, but new studies show that the birds are experts in the art of deception too, writes S Ananthanarayanan.
The female cuckoo, like all mothers, wishes a fine upbringing for her children, but, unlike human mothers, wants none of the drudgery. She builds no nest, does not brood and hatch her eggs and she does not make the great effort it takes to feed the young – she just lays her egg in the nest of another bird, and leaves it all to the foster mother.
And to help her carry out this scheme, she has adapted her eggs to resemble those of other birds and also her own looks so that she can be mistaken, as if she were not a cuckoo. Scientists at the University of Cambridge have found that she goes one step further: even when she is being found out, some of her sisters artfully do not don that disguise, and keep other birds guessing!
Brood parasite
The method of the cuckoo is to lay one egg among the other eggs already there in another bird’s nest. The other bird is usually the reed warbler or a dunnock and the egg the cuckoo lays is deceptively like the other eggs. In fact, there are seven different egg types that cuckoos lay and a cuckoo that lays one kind will choose a host nest of just the right kind, so that the egg stays undetected and cared for.
The baby cuckoo is energetic and ungrateful and pushes the foster sibling’s eggs and chicks out of the nest, to be the sole recipient of the mother’s attention and is none the poorer for being in a foster home. But how does the mother cuckoo know how to choose the right nest?
The answer is that the mother cuckoo was herself hatched among the similar looking eggs and finding the right nest again to lay her own egg is really a homecoming for her!
There is the question, of course, of why the foster mother bird does not catch the trick being played on her? Well there has been adaptation for this as well. In some birds where there is brood parasitism, the parasite babies have evolved cries and ‘mouth gape’ patterns, or colouration of the open mouths that both resemble the foster mother’s natural young and also strongly propel the mother to feeding behaviour. It seems the foster mother’s genetic programme to feed whatever there is in her nest is so strong that the deception is effective. Genetics has been successful in generating adapting variations among cuckoos but not so among host birds – one more of instances of parasite species not necessarily repaying the victim species in an obvious way.
But getting back to the colouration of the eggs – how is it that female cuckoos always lay the kind of eggs they were themselves hatched from? Does the egg variety of the cuckoo mate not have an effect? While a female cuckoo has no way of knowing the egg variety of a male cuckoo before mating, it is also a fact that the choice of mate is really random. The reason for consistent egg colouration seems to lie in the nature of the cuckoo chromosome, or the genetic material that the male and female contribute to the egg.
In mammals, like in humans, the females have two chromosomes of the same kind – they are XX. But the males have one ‘Y’ and one ‘X’ and they are ‘XY’. The offspring can thus have either get a ‘X’ from both parents and end up as ‘XX’ and female, or he may get the father’s Y chromosome and end up as ‘XY’. But in birds, it is the females who have the different kinds of chromosomes. The two kinds are called ‘Z’ and ‘W’ – females are ‘ZW’ and males are ‘ZZ’. Female offspring, which are ‘ZW’, have then received the ‘W’ chromosome from the mother, and the colouring genes that this chromosome possibly carries is passed on from the mother – the father does not influence egg colouration.
Stealth and disguise
While this is the way the cuckoo egg prospers once it is laid, the female cuckoo has first to sneak up to the host bird’s nest and lay that egg. It seems that the marked resemblance of the cuckoo, in size, shape and plumage, to the hawk, especially the sparrowhawk, a bird of prey, has a role to play in this. Resemblance of one species to another, more powerful and fearsome species, for protection from enemies, is not uncommon in nature.
Conversely, species that prey on other creatures may evolve to resemble harmless species, to more easily get within striking range of their victims. The cuckoo and sparrowhawk could be such a mutually benefiting pair. The dull grey colour, which merges with foliage could also help conceal, the hawk for the attack, the cuckoo to induce the host bird to leave the nest unguarded.
Studies have shown that many smaller birds do associate cuckoos with sparrowhawks. Feeders were hung up by Cambridge University scientists, in a study published in 2008, to attract Great tits and blue tits. When stuffed and mounted cuckoos or sparrowhawks were placed near the feeders, it was seen that the attendance dropped, of both kinds of tits. But this did not happen if models of other birds, like the collared dove or the teal were presented.
The alarm response to cuckoos was found to rise from resemblance to the hawk – if the underparts of the cuckoo model were not ‘barred’, like the hawk’s, they were treated as if they were doves! Nor was it only the underparts that caused the alarm, for models of hawk’s with white underparts also evoked fear, while doves with barred underparts did not. And as the teal is not at risk of parasite egg laying by the cuckoo, the fear was because she was mistaken for a hawk!
But the deception does not last long and the reed warbler, a popular host parasitized by the cuckoo has developed a defense. When a female cuckoo is spotted, the reed warblers attack, by ‘mobbing’ the cuckoo, to reduce the chance that she should lay in one of their nests. As we have seen, the cuckoo has evolved to be grey and hawk-like to reduce the chance of being mobbed.
And still, there are some cuckoos that are bright and brownish-red. Such alternate colours are rare among birds, but are seen among parasitic cuckoo species. The current work of the Cambridge group, reported in the journal, Science, indicates that this is a second-order ruse that the cuckoo has developed.
The study shows that the ‘mobbing’ behaviour of reed warblers is strongly influenced by the behaviour of their neighbours – if they find that a grey, cuckoo-like bird has been mobbed, they are likely to attack another grey tone cuckoo. If there are a number of grey tone cuckoos, the flock of warblers would all go for the grey tone variety, allowing the red-brown kind to slip in and lay eggs in the nests! This is then an instance of a species developing alternate forms, called ‘polymorphism’, to overcome limitations of mimicking features alone.
“ …Our research shows that individuals assess disguises not only from personal experience, but also by observing others. However, because their learning is so specific, this social learning then selects for alternative cuckoo disguises and the arms race continues,” says Rose Thorogood, co-author of the paper.