Cuckoos are nest parasites. That means they lay eggs in the nests of other birds, which then try to raise the chicks. So you would think they would be quiet about it. However female cuckoos tend to make a bubbly, chuckoo call when they are laying their eggs. That’s a strange thing for them to do, because the host birds don’t like cookies very much.
That chuckle does not sound at all like a male cuckoo call. But it bears similarities to the sparrowhawk’s call, which led researchers Jenny York and Nicholas Davies to wonder if the call might be a purposeful decoy—a ruse to distract the hapless host birds when their nests are over. killing York and Davies’ book Nature Ecology & Evolution This week provides evidence that the cuckoo chuckle does not really seem to distract war birds by making them afraid of a sparrowhawk attack.
Faking it
Many species have evolved to imitate predators, often for their own protection. For example, an insect mimics the harmful waste, and this causes the lepers to move. Cuckoos seem to be fans of that trick: some species of cuckoo like hummingbirds, and this seems to protect them from being attacked by other birds.
Does the common female cuckoo have a cry that mimics a bird? Both of them cuckoo chuckle and the sparrowhawk cry has a rapid succession of notes with similar pitch and similar rhythm. They sound different to the human ear. But, write York and Davies, “Cooperation through imperfect imitation is common in the natural world, and similarity to artificial calls in certain key features may be sufficient to deceive hosts.” The way to find out is to determine if the call seems to be effective in attracting the host birds.
War birds like reed warblers are sensitive to the possibility of cuckoo parasitism. They collect the adult students, and sometimes they destroy the eggs they put in their nests by selling them or leaving the nests entirely. They also “monitor cuckoo activity in their nesting area and vary these defenses in relation to the risk of local parasitism,” write York and Davies. Because of this, female chicks are fast and furious when they lay their eggs—from the chuckle call.
Parental stress
York and Davies conducted three studies to compare the effects of the chuckle call and the sparrowhawk call. They photographed reed warblers in their nests and used loudspeakers placed near the nests to play many different bird calls: the behavior of the male cuckoo cuckoo-clock-like callthe female chuckle call, the sparrowhawk cry, and (as a control) a very non-threatening call. pigeon collar. The birds do not respond to male cuckoo calls or dove calls, but they get a cautious response to hawk calls and female cuckoo chuckles: they start looking around their nest for threats.
But warblers are a target species for cuckoos, so maybe they react to both as a threat even if they can tell the difference between them. To test this, York and Davies conducted a similar experiment with birds that are prey for the sparrowhawk but not targeted by cuckoos: the great tit and the blue tit. The birds responded vigilantly to both the chuckle call and the sparrowhawk call, but not the dove or male cuckoo call.
It is possible to explain cuckoo mimicry in evolutionary terms if it buys them more reproductive success—if it increases the possibility that eggs laid with a false hawk call are not noticed by the host birds, who are too busy worrying about eating to pay close attention. to their throne. Therefore, York and Davies tested whether reed warblers disturbed by hawk or chuckle calls could lower their nest defenses.
The researchers placed an alien egg in the nests of 75 different types of reeds by extracting an egg, dyeing it red, and returning it. Then they sound one of four calls near each nest. The majority of foreign eggs — 66 percent — were laid in nests that heard male calls or dove calls, but only 26 percent of eggs in hawk and chuckle call nests were laid. That suggests that the chuckle call helps to distract the reed warblers long enough for a strange egg to go unnoticed.
The results are the best example of an evolutionary arms race: cuckoos whose eggs are taken by soldiers have more babies and are better in terms of evolution; Meanwhile, the twenty birds that laid the cuckoo eggs left each other. So while the host birds, over time, adapt to the cuckoo’s tricks, a cuckoo adapts to become a trick too.
Nature Ecology & Evolution2017. DOI: 10.1038/s41559-017-0279-3 (About DOIs).