Alison Davies's picture

How does a cuckoo know it's a cuckoo?

The Cuckoo – a puzzle that surfaces in my brain every year or so – How does a cuckoo get to meet another cuckoo, to mate and continue the species?

It is laid as a single egg in a host's nest. Its real mother clears off before it is hatched, and takes no interest at all. It has a sensitive spot on its back which triggers it to shove the other “rightful” fledglings out of the nest. (Presumably the single-egg-laying thing has been worked out as inadvisable by evolution – if two cuckoo eggs were laid in a nest I guess the two little parasites could shove each other out of the nest, i.e. at the same time, then there would be none. But that doesn't matter, there is one egg laid.)

.Anyway, how does the cuckoo, when it leaves the nest, find another cuckoo? How does it know it's a cuckoo and that it must mate with another cuckoo? It can't be visual, some sort of imprinting, because it has never seen another cuckoo, only seen the host parents. Plus there isn't a great population of cuckoos to bump into and recognise somehow innately, if visual plays any major part.

(Doubt it could be smell – they are pretty widely spaced out I would have thought.)
I had wondered if it was sound…and that could be it. I see on one website that the little cuckoo imitates the sounds of either the host parent or the host temporary fledglings, to attract the giving of food by the host parents. OK, it imitates the sounds of the host family, but the same website says that the eventual call that emerges is that of a cuckoo, so the true call is innately determined.

Could it therefore recognise the similar sound of another cuckoo, once it has started making its own call, i.e. something like its own voice? Possible, BUT – male and female cuckoos make different calls. The well-known cuckoo call is from the male, the female apparently makes a sort of bubbling call. So whichever sex of cuckoo is calling out would only recognise the sound of a same-sex cuckoo, IF it is recognising a similar call to home in on.

This leads me to think that even the recognition of the call of an opposite-sex cuckoo is also innate.

Other thing – the chances must be quite high of them mating with one of their brothers or sisters, given that the egg-laying female must operate in a particular area, visiting host nests. Wouldn't this likelihood tend to diminish the viability of the species, in a gene-pool way?




anonymous spotter's picture

A question of ego?

This is not just an issue for cuckoos. If you think about it, many other species emerge singly from eggs or similar stages with no parent or sibling in attendance.
You could ask the same question of pollen grains - how does a (say) primrose stigma recognise primrose pollen rather than (say) dog violet - or vice versa?
The answer is complicated, and I don't pretend to completely understand it: but here goes -
In animals, it is usually down to their primary sense. Some animals like birds use sight as their main sense, so the appearance is key. It's obviously quite a sophisticated mechanism, given the close resemblance of some species (chiff-chaffs and willow warblers, f'rinstance). Calls obviously play a part, too, in many birds, usually they are the "distant" notification.
Other species use scent, and produce characteristic odours that potential mates can recognise.
And, of course, it can go wrong: so you can end up with hybrids if the genetic make-up of each is compatible.
As for inbreeding, yes it is a real problem for species in "island communities", or those with very low natural population densities.
The adder (my favourite beast in many ways) is suffering badly from in-breeding as populations are separated by development and disturbance. It is not the only species in probably terminal decline because of this. That is why the best-maintained nature reserve is of little use if the species within cannot, long term, have their genes "stirred up" by input from more distant individuals. Reserves need to be part of a network that these species can move between.

Alison Davies's picture

How does a cuckoo know it's a cuckoo?

Roger, yes I would go for adders, well grass snakes and slow worms are exciting to me too, and toads. And lizards. Oh, and eels. Remember coming across an adder what, 45 years ago, lying half-coiled in the sun on a rock near the cliff edge of a beach in West Wales: absorbing the heat, I assumed, like lizards. Felt so privileged. Same holiday I saw two choughs up high on the cliff.

What would you reckon then, a propos the cuckoo, are you saying that perhaps call recognition is programmed in? Given that they aren't likely to just bump into another cuckoo and recognise it visually-programmed-wise?

anonymous spotter's picture

Programmed -

Yes, that's a good word. A lot of basic responses (reactions to predators, swimming, and so on) must be in the DNA - and in the brain pathways the genes produce. Quite how is another matter altogether!
The cuckoo probably recognises the call from a distance, and then switches to visual clues when it gets close. Otherwise it might try to mate with one of the mimics, like a starling...
Work with rare cranes in the US showed that they inherited lots of basic behaviour, but learnt other things from their parents.
As a result, when they matured, the human foster-parents (who had use "crane puppet" gloves to feed them and so on, to avoid imprinting) led them on their migration using a microlight aircraft! Most of the youngsters found their way back unguided the next year.