From “Cuckoo Tricks to Beat the Neighborhood Watch” (ScienceDaily, Aug. 2, 2012), we learn about the ways that nesting birds attempt to evade parasitic cuckoos:
Some female common cuckoos are grey and hawk-like, and previous research has shown that their resemblance to hawks reduces host bird attack. However, other females are bright rufous (brownish-red). The presence of alternate colour morphs in the same species is rare in birds, but frequent among the females of parasitic cuckoo species. The new research shows that this is another cuckoo trick: cuckoos combat reed warbler mobbing by coming in different guises.
Cuckoos are secretive. To widen their source of information about local cuckoo activity, reed warblers eavesdrop on the mobbing behaviour of their neighbours. In the study, the researchers manipulated local frequencies of the more common grey colour cuckoo and the less common (in the United Kingdom) rufous colour cuckoo by placing models of the birds at neighbouring nests. They then recorded how the experience of watching neighbours mob changed reed warbler responses back at their own nest.
They found that reed warblers increased their mobbing, but only to the cuckoo morph that their neighbours had mobbed. Therefore, as one cuckoo morph increases in frequency, local host populations will become alerted specifically to that morph. This means the alternate morph will be more likely to slip past host defences and lay undetected. This is the first time that ‘social learning’ has been documented in the evolution of mimicry as well as the evolution of different observable characteristics — such as colour — in the same species (called polymorphism).
Interesting example of the underrated complexity of bird behaviour, but calling it “evolution” does a disservice to the concept.
We really don’t know that the red/grey dimorphism in female cuckoos is much different in practice from the black/grey dimorphism in Eastern gray squirrels. It is easy to make up a just-so story after observing the effect of some such drift in the frequency of a given morph in the population. And scientists at the Smithsonian actually tried that very thing in the case of the squirrels, claiming to see “evolution in action” because they could think of a reason why one morph would have an advantage over the other.
The problem with claiming that it is a form of evolution is that there is no reason to believe that there will be any long term change as a result of one morph gaining a temporary advantage over the other. And “what the neighbours think” must be the most temporary of advantages; it perishes with them.
It is hard to know what to make of the notion that it is remarkable for flock birds to demonstrate “social learning” about an enemy species. That is precisely what being part of a flock enables.
The photo, by Per Harald Olsen, shows a warbler inadvertently feeding a cuckoo chick that made it through the “Neighbourhood Watch.” The problem seems to be that the bird is intelligent enough to have a neighbourhood watch but not intelligent enough to do anything further if that doesn’t work.
See also: Whether large bird and mammal brains arise from common descent or convergent evolution is uncertain