Angry Birds Fight Back
I know, the title is misleading. But if you have seen the video of the fairy wrens’ response to a freeze dried cuckoo specimen, you would definitely agree with me.
According to Dr. Naomi Langmore, at whose seminar presentation I had the privilege of viewing the video, the fairy wrens literally tore the cuckoo specimen to shreds. It is an extreme but justified response.
First of all, it should be noted that not all cuckoos are brood parasites but for convenience, any mention of a cuckoo in this article will refer to a parasitic cuckoo.
What are parasitic cuckoos?
For those who are new to the term, brood parasites are birds that do not make their own nests but instead remove an egg from the nest of another bird(host) and lay their own in it. The host ends up taking care of the cuckoo chick. And there’s more: the cuckoo chick hatches before the host’s eggs do. It then proceeds to move each egg onto its slightly hollow back, pushes and shoves until the unfortunate egg is dropped over the edge of the nest. Even if the host chick manages to hatch before the cuckoo chick does, the cuckoo chick will do the very same actions and shove the helpless host chick over the edge to its death. This behaviour has to be a genetically passed instinct because the cuckoo chick has no time to learn this behaviour.
Cuckoos vs Hosts
Both the cuckoo and its hosts have been embroiled in an evolutionary arms war where their survival is at stake. When host birds recognise, destroy or tip out cuckoo eggs from their nests, some cuckoo species have evolved so that their eggs mimic the host eggs’ shape, size and colour. What makes this amazing is that the eggs are host specific. A single species, the Common Cuckoo Cuculus canorus , parasitizes a wide variety of hosts but individual female Common Cuckoos specialize in their own preferred host. The females will lay eggs that resemble the eggs of their chosen host. While effective, this method is not foolproof if hosts learn to recognise pattern differences or changes in clutch size.
The picture above shows nests of different species with both host and parasitic eggs. Parasitic eggs are identified by the black arrows showing near perfect mimicry. If it were not for the slightly larger sizes of the cuckoo eggs, it would be near impossible to distinguish between them.
Most of the time, hosts just take care of the cuckoo egg/chick and it makes one wonder why they do that. The difference between a chick that looks like you and a grey monster of a cuckoo chick should be obvious.
One of the more interesting hypotheses relating to it is the so-called ‘Mafia Hypothesis’
My offspring or nothing
Some cuckoo species like the Great Spotted Cuckoo re-visit nests that they have parasitized.If the cuckoo’s eggs have been ejected, the cuckoo will proceed to destroy the nest together with any eggs or chicks present.
If their eggs are still present in the nest, the nest is left alone. If this isn’t enough to convince you that the cuckoo is evil, I don’t know what will.
Not only do they commit infanticide, they are the bird equivalent of the mafia.
Another way hosts can defend against parasitism is by chick discrimination where host birds recognize and reject cuckoo chicks. This is where Australian bird species come into the picture. The large-billed Gerygone and the superb fairy wren are native Australian species which both display chick discrimination abilities.
Now you see me, now you don’t – oh wait, you can hear me
In November 2012, it was revealed that researchers have accidentally discovered that superb fairy wrens were teaching their unhatched offspring a secret password by singing a special tune, often as many as fifteen times per hour, to their eggs for a limited period of time. When their offspring are born, the special tune will be incorporated into their begging calls (a call asking to be fed).
A cuckoo egg is laid too late in the cycle and hatch too early for the chick to pick up on the secret password. In most cases when the host returns with food for the chick and hears a begging call that does not contain that one secret note, they abandon the nest and make a new one leaving the spawn of evil to starve to death.
While abandoning the nest helps the host bird evade wasted parental investment, they are unable to save their eggs or chicks. This is where the large billed Gerygone comes into the picture.
Forced eviction notice for unwanted tenants
The large-billed Gerygone however, displays a new anti-parasitism behavior. Like the superb fairy wren, it can discriminate between chicks, but instead of abandoning the nest, it takes it one step further : it ejects the cuckoo chick from its nest.
It’s offspring will have a higher chance of surviving into adulthood and their parental investment has not gone to waste. Also in the seminar with Dr. Langmore, we were shown a video where the large-billed Gerygone dragged a kicking, screaming cuckoo chick out of its nest and cheerfully bumped it over the edge. I won’t lie, I wanted to cheer for the victorious bird.
Heads or Tails: Light or Darkness
This behavior also highlights studies which found that host birds will display only one of two discrimination behaviors: egg and chick discrimination. Light and visibility in the nests seem to be the driving factor in determining the two discrimination behaviors.
The European Robin has a cup-shaped nest and light is abundant. While it is able to distinguish foreign eggs, when the cuckoo chick has hatched, the European Robin senses no difference. In the case of the superb fairy wrens and the large billed Gerygone, they build dome shaped nests which hardly let any light in. They display no egg discrimination behavior, but they can identify imposter chicks.
Find the Difference
There is now evidence that the cuckoos have gone one step ahead in this evolutionary war against chick discrimination behavior. Instead of producing mimetic eggs, they are now producing mimetic chicks.
If you weren’t told which were the cuckoo chicks and which were the host chick in the picture beside, would you be able to tell the difference? I wouldn’t. And the host birds would definitely have a field day trying to find out.
I don’t know what the host birds will do in the next step of the evolutionary war as mimetic chicks are overkill (in my opinion). But considering the crazy things that both sides have managed to come up with so far, it is definitely something to look forward to.
If you want to give them a read yourself
: and wikipedia!