Dealing with parasites

海伦-It's time to turn our ears to behavioural ecology. How do animals interact with their environment to improve their lot? Well, it seems that cuckoos across the world have found ways of exploiting the nests of the other birds, but not all hosts are so easily exploited. To tell us more about which birds know best, we have Rebecca Kilner from Cambridge University with us today in the studio. Hi, Rebecca.

Rebecca - Hello.

海伦-Thanks for being with us. Well cuckoos, they are crazy creatures really, but first of all, where do they live? Where do we find cuckoos living? Where in the world?

Rebecca - Well cuckoos are a member of a sort of super family of birds and there are about 140 species of them all together. But it's really only sixty of the species that are well-known and they're well-known for their habit of stealing child care from other species. So what they do is they lay their eggs in the nests belonging to another bird and they leave their poor victim with the hard job of bringing up their baby and those brood parasitic birds, as we call them, are scattered throughout the world, so we can find individuals representing this family on every continent.

海伦-And they exploit the nests of all sorts of other birds as well. I take it there are lots of poor birds out there who are having to look after things that really are nothing to do with them at all, not their babies at all.

Rebecca - That's, right. Yes, so individuals from all of the songbird family really, can be found being exploited by cuckoos all over the world.

海伦-It is a crazy thing to see. We sort of get pictures and footage on the television of a tiny songbird rearing this enormous cuckoo. Sometimes the size difference is almost comical, isn't it? It looks ridiculous. How did cuckoos go about being able to get away with this? How do they do this?

丽贝卡-很明显,这个故事开始于the cuckoo adds her egg to the host nest and she does this very, very secretively. We know best probably about the European cuckoo or the common Cuckoo which exploits song birds in Britain and what we know is that she's incredibly secretive. It's very, very hard for even bird watchers to see cuckoos. It's certainly very hard for the host birds, the potential victims of the cuckoo to see this malevolent stealer of child care. So, what they do is they lurk around potential host victim nests and then when the timing is right, once the host has started laying her eggs, they very quickly glide down to the host nest in the afternoon. And they will add a single egg of their own to the clutch, having first removed one of the host's own eggs and they do all this in 10 seconds. They quickly fly away again. So there's very little chance that the host bird will spot them at the nest.

海伦-They only take one of the other eggs out. They leave the other ones there and sort of hide their one egg between all the other ones that were there.

Rebecca - Yes, that's right. So, they might possibly take two eggs but they never take every other egg from the nest and the reason they don't take all the other eggs is because it's been shown by experiment that if you reduce the clutch to a single egg then the host will simply give up on the nest because it's not longer profitable. So it's in the cuckoo's interest to leave as many host eggs in the nest as possible but she takes out one presumably because it's more - the incubation of the clutch is more efficient if there aren't too many eggs in the nest. So by taking one out and adding her own, she keeps the clutch size, the number of eggs in the nest is constant.

海伦-Okay and so, the cuckoo's got her egg into this other nest. What happens next? So presumably if it works then the host mother doesn't realise and father don't realise that they've got an impostor and what happens next?

Rebecca - Well, you hit on a very important point there. So it's possible that the host will realize they have an impostor egg in the nest.

海伦-Right.

Rebecca - So, for example, in the reed warblers which nest at Wicken Fen near Cambridge, roughly 20% of cuckoo eggs that are laid in the host nest are spotted by the host owner, the nest owner, as foreign and they throw them out. So more for than not, the cuckoo gets away with it.

海伦-And then presumably, they eventually will hatch, what happens then?

Rebecca - Yes, so the cuckoo egg is incubated along with all the other eggs in the host nest. But the cuckoo egg will hatch, possibly one or two days earlier than the host's own young and then at this point, the cuckoo egg sets about - the cuckoo chick rather, sets about instinctively killing off all the members of the host clutch.

海伦-I've seen that on TV in a documentary and that's extraordinary that it knows as well to, as soon as it hatches, get rid of all the other eggs around me.

Rebecca - Yeah, that's right.

海伦-It's amazing.

Rebecca - It's an extraordinary behaviour and it's an instinctive behaviour. So, it will - basically, in response to any sort of pressure on the small of its back, the cuckoo chick will walk backwards up beside to the nest and try tip that whatever is pushing on its back over the edge to destroy it.

海伦-So you've just got a nest with a single cuckoo left in it.

Rebecca - That's right. So, the common cuckoo within say, 48 hours of hatching, the cuckoo chick has destroyed the host's own offspring and it is by itself in the nest.

海伦-And then the - by that stage, the mother and father of, what they thought was their own child - their offspring, do they then just keep on feeding it as if it was their were own offspring? Do they actually have any way of figuring out, "Hang on, this isn't what I expected to see coming out."

Rebecca - Well, it's quite remarkable with the reed warblers at Wicken Fen will carry on feeding the cuckoo chick and they feed the cuckoo chick at roughly the same rate as they would provision a brood of their own chicks.

海伦-And that is extraordinary. But are there other species that have figured out ways or evolved ways really, isn't it, of defending themselves against cuckoos? Presumably, there must be quite a strong pressure to not allow a cuckoo to come in to your nest because by that - because you've really lost a whole breeding opportunity.

Rebecca - Yes.

海伦-So how have they evolved ways of getting around that?

Rebecca - Well, so there are several different lines of defence the host can mount to defeat the cuckoo who's trying to steal the child care. The first line of defence is that the host can mob the adult cuckoo as she lurks near the nest, preparing to lay her egg.

海伦-So she just screams and sort of tries to get them to...

Rebecca - Yeah, in much the same way as birds would mob a predator, for example, potential predator, they mob a cuckoo.

海伦-But they have to have seen them in the first place to be able to do that so they can be able to do that.

丽贝卡——没错,是的。这是第line of defence. Now obviously, cuckoos, we know that they can breach that line of defence because we know that cuckoos get their eggs into the nest. So the second line of defence that hosts can mount is to spot the foreign looking egg in their nest and so this in turn has driven the evolution of cuckoos that can lay mimetic eggs because they're much more likely to escape that line of defence...

海伦-So it means they look very similar to the eggs of the host.

Rebecca - That's right. They very closely resemble the host's own eggs. It's almost impossible to tell them apart. It's extraordinarily masked.

海伦-So it is one species of cuckoo only go and parasitise one of the species of bird or can they do different eggs? Do they have different tricks?

Rebecca - Well, the single species of the common cuckoo has split into genetically distinct host races and each host race can specialize on a different host species and they're recognizable by the fact that they lay different eggs appropriate to each host species.

海伦-That's amazing. That really is. So they can recognize. Some species can recognize the eggs and presumably, some can recognize; "This isn't my chick. This is not what I was expecting to see." Can they?

Rebecca - Well extraordinarily, it seems that the hosts of the common cuckoo can't do that. So, we think this might be because their first two lines of defence are relatively robust. They are especially good at recognizing foreign chicks, foreign eggs added to the nest. And so, because they've got this very secure initial lines of defence, they seemingly have no lines of defence at the chick stage. So if the cuckoo makes it to that point then it's home and dry, the cuckoo chick will be fed.

海伦-So they almost say, you know, we should be good at being able to figure this out early on and we're not going to worry about, by the time eggs have hatched then we expect it to be our own birds. That's always the case.

Rebecca - Yes, there's another constraint on the reed warblers as well which is that their breeding season is very short and so, even if they were to give up at that point, it's very unlikely that they would be able to get another brood produced that season.

海伦-It really is just fantastic stuff. One final question just for now. Do birds that get parasitized, that get tricked, duped by these cuckoos, do they do it over and over again or do they learn a lesson and next time, you know they were even more careful to not let the cuckoos come in near their nest or do you see reed warblers that every season, every attempt at breeding, a cuckoo comes along and takes away all of their efforts to continue their own species?

Rebecca - Well I can't speak for reed warblers but I've studied a host species in Australia and I've seen one poor victim bird, be struck three times in a row by a cuckoo.

海伦-Bad luck, isn't it?

Rebecca - Very bad luck indeed, yes.

Later in the show, we put some more questions to our guest:

海伦-Is it just cuckoos that do this?

Rebecca - No, you're quite right. It's not just confined to the birds and that's because universally across the animal kingdom providing child care is costly. So, that means it's a tremendous incentive for individuals to steal childcare from other individuals and we know that that happens in fish, for example. There's a cat fish that steals parental care from fish that brood their eggs and fry in their mouth. So, it takes care to get its host to swallow its fertilized egg and then the cat fish develops within the mouth of the host fish and eats all the other eggs and fry in there and grows spikes so that the host doesn't swallow it and then finally, it is spat out when it's completed development.

海伦-Great! That's amazing!

丽贝卡,同样也有一些可怕的insects that steal parental care from ant colony. Say for example, the cuckoo butterfly, so named because it induces ants to pick up its caterpillar and carry it back to their colony where they treat it like a giant member of their own colony and they carefully nurture it between 11 months and 2 years until it's grown large enough to pupate and then it becomes the adult butterfly. So yes, it's widespread in the animal kingdom.

海伦-And just a really quick one, we were talking earlier about the various strategies that animals have to recognize when they're being parasitised by a cuckoo. But what happens when a bird kind of looks at their chick and goes "Hang on a sec, you're not mine." When birds do actually recognize that they've got a cuckoo, what happens?

Rebecca - Yes, there are some - we didn't talk about this early but there are some host species that are confined to Australia, as far as we know at the moment, who can recognize the cuckoo chick in the nest. And what happens here is that, recognition typically takes place four days after hatching and the bird simply stop feeding the chick in the nest, even though it's alive and well and healthy and begging for food. Something happens that makes them realize it's not their own and they give up on the chick and they take the nest apart even thought the chick is still sitting in there and still alive and they build a fresh nest next door and eventually, the cuckoo chick starves to death and themeat ants move in and dismember the corps and carry it back to their own nest.

海伦-Absolutely brutal! And it - just rounding up this whole idea of cuckoos and what they do, it does seem quite extraordinary and that you go to such an effort to have babies and you think looking after them is very important and to trust that to another animal seems really quite bizarre, doesn't really make so much sense. You're obviously putting a lot of trust in the host animal to look after your babies well, aren't you?

Rebecca - Yes, that's right. You're leaving them with the responsibility of bringing them up but at the same time you're avoiding all the costs that they incur in rearing children. So, it makes sense that these parasites have evolved.

海伦-Excellent, well thanks so much Rebecca