The title is misleading, I admit, but it’s how the BBC describes research into which birds reject a cuckoo egg from their nest and why. Maybe, like me, you were expecting an update to Clifford Stoll’s famous book. Alas, it’s actually about real cuckoos and how they adapt to risk.

Cuckoos have target hosts. For example, a cuckoo that lays eggs in a redstart nest lays a blue egg. To the human eye, this is identical to the redstart egg.

However, the cuckoo that targets a dunnock nest lays a white egg with brown speckling, visibly different from the dunnock’s immaculately blue egg. Yet despite this obvious colour mismatch, dunnocks readily accept the foreign eggs, whereas redstarts are much more likely to eject the cuckoo’s egg.

The researchers give a couple theories for why a dunnock would put their own egg at risk (if a cuckoo egg hatches first the chick ejects the other eggs) instead of immediately rejecting the cuckoo’s egg.

Researchers think that naive hosts, like the dunnock, are still at early stages of the evolutionary arms race and; “they accept alien eggs, because they have not yet evolved defences against parasitism,” explains Ms Stoddard.

“Another’ hypothesis is that tolerating cuckoo eggs may be the most stable strategy for some hosts.”

So, for birds that do not often suffer cuckoo invasions, the overall “cost” of mistakenly ejecting their own eggs might be higher than the cost of tolerating the occasional parasite.

It sounds like they are either really dumb and unaware or…really smart and totally aware of the risks. That sure narrows it down.

I am now curious about the rate of a dunnock “mistakenly ejecting their own eggs”. If a dunnock is able to tell there is a difference and wants to eject the cuckoo egg (as the eggs are so different) then what causes a mistake? Clumsy footwork? I mean, if a cuckoo chick can tell the difference and eject the other eggs…