Colors and the Kids

I’ve been addicted to Serial as of late, and if you are also following along with it, I want you to now cue in your head the theme song.

Except that this post will not resolve the mystery surrounding the murder of a teenage girl. Rather, the crime in question here involves the questionably mixed contents of a bird’s nest.

Recently, I told you about brood parasites, birds that will drop their eggs into the nests of others to have other individuals, even from other species entirely, raise their offspring.

However, this is not your typical babysitting service. Hosts are receiving nothing in return for their dutiful parental care. In fact, because raising baby birds requires a lot of energy and time, these hosts are paying hefty costs by caring for parasitic offspring. Because what they provide these parasitic young necessarily takes away from what they provide to their own young.

So what is a parasitized host species to do?

One obvious solution to the problem is to evolve a more discriminatory egg. Some kind of signal that is a secret code that will allow you to tell the difference between your egg and that of a parasite.

So let’s have a look, shall we?

Eggs laid by different host-races of the Common Cuckoo are on the top row, with eggs laid by their target hosts shown directly below. Photos by M. C. Stoddard and copyright the Natural History Museum.

Photos by M. C. Stoddard and copyright the Natural History Museum.

What you’re looking at are the eggs of parasitic cuckoos and their hosts. The top row are the eggs of different Common Cuckoo host-races, while shown directly below them are the eggs of their target hosts.

It appears that over time cuckoos have evolved eggs that look more and more like the patterned eggs of their hosts. Those that did a better job mimicking their host eggs were the ones that stayed and passed on their genes to future generations. Those that did not do such a good job duping their host may have been tossed over the nest. Over time, you get cuckoo eggs like those above. Save the first pair of eggs, aren’t these cuckoos’ eggs amazingly convincing!?!

Clearly these hosts have no chance in hell of outdoing their cuckoo parasites. Or do they?

Before you think that these hosts have been defeated, take a closer look at those egg pairs. The cuckoos have made pretty good replicas, but are they perfect? Might the hosts have recognizable patterns in their eggs that they have learned and that allow them to better discriminate eggs in their nest?

Next time on The Birds.



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