Good afternoon ladies, and welcome. I will be your 'host'...

General groaning (or what counts as groaning¹ in the language of Cuckoos).

...alright, alright. I will be giving the lecture today. If you would all be kind enough to find yourselves a nice, comfortable branch and settle down, I will begin. Are you ready?

Cuck-oo cuck-oo cuck-oo² from the back of the woodland auditorium

Gentlemen, you were not invited. This is a ladies only tutorial. Kindly remove yourselves or be quiet. We don't want any challenging calls or fighting here.

Good. I'll begin then.

Now, I dare say all you young ladies think you know your business well enough and wonder why you should waste your time coming to a lecture on things you know by instinct.

Times are changing. Many bird populations are dropping alarmingly, including yours. You are now on the RSPB's "Amber List"³ and we wouldn't want you to find yourselves on their "Red List", now, would we?

Babble of alarm

No. Well, quite. So, it is my purpose, this afternoon, to remind you of what works and reinforce the importance, in particular, of egg shell mimicry.

Some of you have cracked this problem...

Groan

...and some of you have no sense of humour.

Tut tut tut

Yes. Right. So, as I was saying, some of you have solved this problem, but possibly, only in the short term. Your eggs are so similar to those of your host species, that the host cannot tell the difference. The problem is, some of the host species are also in sharp decline. In the case of the Marsh Warbler Cuckoo Gens⁴, your host is already on the Red List. If you are forced to find a new host species, how quickly will you be able to adapt with matching eggs?

Outbreak of muttering from the Marsh Warbler Cuckoo gens faction

That's right ladies. You may have noticed the scarcity of Marsh Warbler nests available for parasitism. And the Meadow Pipit Cuckoo and Dunnock Cuckoo gentes may not be far behind you. You ladies of the Reed Warbler Cuckoo gens have little to worry about at the present time, but who knows what the future may hold in these times of uncertainty. You must all be prepared.

It used to be believed that you flew from nest to nest, with your egg in your beak, visually comparing yours with those already in the nests, and finally dropped it into the nest of a victim whose eggs you judged most closely resembled the one you had just laid. But that's not how it works at all, is it?

Chuckles of derision

No. I agree. Quite ridiculous.

But it's not difficult to see how this misunderstanding first arose. You have been observed, clinging to the edges of nests holding an egg in your bill. The observers have, evidently, not observed closely enough or long enough to realise that you had removed this egg from the nest just prior to laying your own.

It takes generations of evolution to perfect egg mimicry. The more discriminating the host, the better the imitation has to be in order for the trick to work. The colour and size of egg is passed from mother to daughter, and the female cuckoo must always try to lay her eggs in the nests of the species that raised her. If no such nests are available, then she has to resort to another species, that is only too likely to spot the alien egg and either eject it from the nest or abandon the nest and start again. A very sad state of affairs!

But even that situation isn't hopeless. You have to start somewhere. The Dunnock Cuckoo gens has, in all likelihood, had to make such a new start in relatively recent times. The eggs of the Dunnock Cuckoo gens are easily distinguished from the eggs of the host. The reason the Dunnock is gulled...

Groan

...Oh dear. My terrible jokes. I do apologise ladies. Anyway. Where was I. Oh yes! The reason the Dunnock is so easily tricked, is, very probably, because it has only recently fallen victim to parasitization by Cuckoos. It is naive! Sooner or later, it may realize that all is not as it should be with its broods. In fact, if the Dunnock is to survive, it is inevitable that it must fight back, and start rejecting eggs that are obviously not its own! At that point, natural selection will start to alter the appearance of your eggs, because only the eggs that are not rejected by the Dunnocks will supply the next generation of Dunnock Cuckoos. You are in an arms race, ladies!

The news is not all bad though. You are adaptable. And your hosts - even the most discriminating - are not infallible. Even with the best matches for colour and pattern, your eggs are generally a little bigger⁵ than the hosts', but they still fail to spot the difference. That might suggest the next possible escalation in the arms race for some of you. If a host species begins to discern the slight size difference, a selection pressure will be exerted to reduce the size of your eggs even further.

The pressure to down-size has had one very advantageous side-effect, as I'm sure you realise. The cost to such a large bird, of producing such tiny eggs⁶, is small - and very much lower than the cost of egg production for most other species. The shells are thicker than the hosts' eggs too. The advantages of a thick shell, we can guess. It's less likely to get damaged as you drop it from the inevitable height of the top of the nest, that you cannot fit your great big bottom into...

Mutter mutter

...I'm sorry ladies. That was rude of me. Not at all funny. What I mean is, the nests are tiny and you are too large to get in and lay your egg as the host would, with no distance at all between cloaca and nest floor. And, in addition, it will not be easy for the host to peck a hole in the egg, as they sometimes attempt to do if they suspect a changeling.

I don't need to remind you, do I, that you must always wait for the host to start laying, before you leave your egg? They always know, if an egg appears in the nest before they've started laying themselves, and they will abandon that nest and start again. Wait until there are three to five eggs. Eat one or two (use your judgement) and then deposit your own. When they return, things should look just as they did before you left your little treasure.

The general rule is, a host will always abandon a single egg, but they will never abandon a single chick. So you must never remove all the host's eggs. The eggception...

Groan

Sorry. Couldn't resist.

The exception to the rule is this. If you find a suitable host that has already completed its clutch and started brooding, you are too late. The brood will start to hatch before the young Cuckoo is ready. There is only one solution to this problem. You must eat every egg, thereby forcing the host to start again. Timing is everything.

Other than that, the rule stands. As you may recall, from your own childhood, the newly hatched Cuckoo can be trusted to force any other nestlings or eggs from the nest, unaided. Once the young parasite has hatched, visual mimicry can be abandoned. Cuckoo chicks look nothing like the hosts' own young. At the hatching stage, the chick takes control of the hosts' nervous system. That will be the subject of our next lecture.

And that concludes this afternoon's talk, ladies.

Remember we have a guest speaker coming to discuss migration tomorrow morning. Half-past dawn chorus ladies. Don't be late!