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Last summer, a team of researchers put tags on five cuckoo birds at Khurkh Bird Ringing Station in northern Mongolia, says Chris Hewson, senior research ecologist at the British Trust for Ornithology. While one bird, the only oriental cuckoo that was tagged, migrated to the northern part of central Siberia, the common cuckoos went much farther.
The four common cuckoos hung around the ringing station until last autumn when they started migrating southward through China. Two of the birds ended up 90 degrees of latitude away in southern Africa, he says.
“It was one of the longest animal migrations which has been recorded for a land bird, certainly,” he says. “It begs the question, I suppose, as to why a bird from Eastern Asia goes all the way to Africa for the winter.”
To ensure survival, birds should migrate as short a distance as possible, he says. But these cuckoos overcame the costs of how much energy and time it takes to fly a long distance, while also surviving any hazards along the way.
“Obviously, flying 2,000 or 3,000 kilometers over the ocean is quite a dangerous thing to do,” he says.
On why the cuckoos fly so far
“The common cuckoo is basically an African species, which has expanded its breeding range all the way across Eurasia. So from the far west, we've got the British Isles all the way to far eastern Russia. But until we did this project at its predecessor, when we tagged some birds in China in 2016, no one knew where the Asian birds would end up wintering. And it was thought that they may end up in southern Asia. But there was a possibility that they'd go to Africa.
“So we were quite surprised to find that they did actually do that. It showed that they were able to overcome the costs in terms of the energy it takes to fly there, the time it takes them to fly there and also exposure to hazards along the way. … And of course, you're moving to a new place all the time, so you're exposed to predators. But spring conditions as rainfall with the tropics moves from north to south in the autumn, and then from south to north back in the spring, so they're able to follow the rains and the rains produce the food the birds depend on. So actually, the energy is not a problem for them at all.”
On how often the birds stop along the way
“They have what's called a stage migration, and they actually stop quite a few times. They have two or three major stopovers. But other than that, they stop most days when they're not over the desert. The major stopover for a Mongolian cuckoo before they fly over to Africa is in India, where they'll put on quite a lot of fat in autumn. And then they wait for what's called the Intertropical Convergence Zone, a frontal system which takes rain from north to south. And behind that, they have gentle easterly winds taking them across the ocean. So it's quite safe and probably even serene passage for them. And once they're fattened up and they've got these tailwinds, it's something they could do quite comfortably. And then they gradually move south within Africa following the rains, which leaves in its path a sort of fresh spring-like condition, which produces lots of food for the cuckoos to feast on.”
On how the birds know to fatten up before the journey
“I think one of the triggers for them to fatten up are things like changes in day lengths. So certainly in the UK, where we actually had a captive cuckoo and it wasn't putting on any fat until we put it outside and it was exposed to the daylight. As soon as we did that, you can see that the days were getting shorter. It put on a lot of fat and was ready for its migration. So that's one of the cues.”
On how long the cuckoos stay in one place
“They'll stay in their final destination for a couple of months before coming back. If they find decent conditions, they'll certainly stay for a couple of months. But if the conditions they find deteriorate, they'll start to move once again from southern Africa. So the bird that we just tracked back to Mongolia spent the middle of the winter in eastern Zambia. So yeah, they do have some stopovers, but they're pretty much on the move throughout the year. So maybe six weeks to two months at each location is about the maximum that they'll do.”
On oriental cuckoo, which migrates to Siberia
“The first bird referred to there was an oriental cuckoo. That's a different species, again. And unlike the common cuckoos, which were all breeding in the area that we caught them, this bird migrated on for 2,000 to 3,000 kilometers up into the northern part of central Siberia, where it started to move south after the common cuckoos had already left Khurkh. And we were hoping to see where it was going to go because we think this species, the oriental cuckoo, probably winters in southeastern Asia and even as far as Australia. Unfortunately, we lost contact with that bird in central China and we're not sure exactly why that is, whether that was due to the tag failing or the bird itself dying. But nonetheless, unfortunately, we weren't able to track it down to its winter location, which would have been a first for that species.”
On longer migrations in the animal world
“The longest bird migration is the Arctic tern, which as its name suggests, breeds in the Arctic and winters all the way down in the Antarctic. But certainly, as far as land birds go, the common cuckoo is actually proving to be probably the longest migrant. So this particular migration isn't the longest. But our colleagues from Copenhagen and their Russian collaborators tracked a common cuckoo two years ago from the far eastern part of Russia, not far from Alaska. In fact, in Kamchatka. And that went all the way down to South Western Africa in Namibia. And that, in fact, is the longest land bird migration which has so far been documented.”
On if climate change is impacting migration patterns
“It's something that we're looking at with respect to the common cuckoos that we have tracked from the U.K. So most birds are getting back to the U.K. earlier in spring, as the springs are warmer and therefore earlier. But the common cuckoo is one of those species: It's only that and the common swift, which aren't getting back much earlier. And they're also two species which have undergone quite severe declines. So we wondered if there's something linking the two there.
“So we've been looking at the migrations of common cuckoos from their stop over in West Africa before they moved northwards over the Sahara and looking at it in quite a lot of detail in relation to the rainfall patterns. And it seems that the rainfall patterns, when they do arrive, they arrived just in time for the birds to fatten up because the rain brings productivity, which means there's lots of insects for them to eat. They arrive just in time for these birds to fatten up in time to come to Britain. And it may be that habitat loss rather than climate is more of a problem for them at the moment.”
On if flying thousands of miles makes sense from an evolutionary standpoint
“The common cuckoo is very closely related to a bird called the African cuckoo, which stays within the tropics of Africa. It does move around a bit, but nonetheless, it's a tropical species all the time. What's happened is that the common cuckoo has evolved to migrate north into Europe and Eurasia to exploit spring conditions there. In fact, they're a brood parasite. So they actually lay their eggs in the nests of other species and let them bring up the young. But the same applies that they're basically exploiting those hosts which are exploiting the abundant insects and things in the spring in Eurasia.
“Why all cuckoos, even from as far east as far eastern Russia and China, come back to Africa is a different question. I think it relates to the fact that the cost of the migration is not as great as we thought. So there's plenty of food for them all the way. The breeding season is much later but a month later than it is in Western Europe in the Far East. So they've got plenty of time to make that journey as well. And they seem to have really good strategies for, for instance, flying over the Indian Ocean. So they don't seem to incur much of a sort of mortality cost for that. And as I mentioned before, is another species called the Oriental cuckoo, which probably winters, although it's not yet been documented very thoroughly in terms of tracking, in sort of southeastern Asia, northern Australia. So it may be that even if they did try and find another wintering ground in Asia, they would find competition from the Oriental cuckoo. Overall, there may not be very much selection for them to try and find a shorter migration in a wintering grounds that's closer to their breeding ground.”
On what questions he still has about cuckoos
“What we don't know very much about is what's happening to the populations of common cuckoos in eastern Asia. We know that in many, many, many populations of common cuckoos and other migratory species across the globe are in rapid decline. But we don't know what's going on with the Asian populations. But certainly in Europe, we found some evidence that cuckoos from the U.K., for instance, take two different routes to get down to Africa. Birds which migrate via Spain rather than Italy seem to do less well than those that migrate via Italy. And there's a quite strong correlation between use of this less successful route via Spain and the degree of population decline on the breeding grounds. But trying to understand exactly what's driving the population declines and therefore what we can do to try and reverse or stem the declines is something that we're really focusing on at the moment.”
This segment aired on June 16, 2020.
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