Here’s the original of my latest news story for Nature.
Some birds are predisposed to signal their intentions more clearly than others.
Some animals, like some people, are more aggressive than others - it's just the way they are. But new research suggests that, for birds at least, this personality is more subtle. Some are inclined to give out exaggerated signs of their aggressiveness, others to underplay it.
It's rather like the menacing biker who turns out to be a pussy-cat, or the wimpy geek who will break a bottle over your head. But the analogy with humans goes only so far, because many birds announce their aggression about mating and territory not by appearance but by song and gesture.
For example, the song sparrow indicates its intention to attack a dummy bird or a loudspeaker playing back its songs by either vocalizing distinctive ‘soft songs’ or by fluttering its wings (so-called wing waves), both of which are perceived as threatening .
Both aggressive signaling and the ensuing aggressive behaviour varies from one bird to another, in a way that correlates with other personality traits such as boldness . But these attributes also vary for a single individual at different times – they can have particularly grouchy or placid days. The degree of aggression implied by the precursory signals generally reflects the actual behaviour – it is what evolutionary biologists call an “honest signal”.
But not entirely honest. Earlier this year Michael Beecher and colleagues at the University of Washington in Seattle showed that there’s some variability in aggressive signaling that doesn’t match the behaviour: a bird might act stroppy but not follow through with an attack .
This variability could be just random, an imponderable quirk of bird-brains. But now Beecher and colleagues say it isn’t .
The researchers studied 69 labelled male song sparrows in their natural habitat during autumn and spring. They played the birds their own songs (which elicit aggression just as ‘stranger songs’ do) and watched how they responded – whether they displayed the aggression signals of soft songs and wing waves, and whether they followed through by attacking the loudspeakers or a dummy bird.
They found that, after allowing for variations that provide an honest signal of a bird’s fluctuations in aggressive mood, the remaining variability – if you like, the dishonest part of it – seems to be consistently displayed by particular birds.
Some have a predisposition – consistent from one season to the next – to give out false signals of how aggressive they intend to be, suggesting either too much or too little. Others are more consistently ‘honest’. Beecher and colleagues say that this behaviour too seems to be a robust characteristic of an individual bird’s personality, which the researchers call “communicativeness”.
“This is an important and novel paper”, says William Searcy, a behavioural ecologist at the University of Miami. “I think it’s highly likely that behaviours one can define in song sparrows can be identified in other birds, and other animals as well”, adds Jeremy Hyman of Western Carolina University, a specialist in bird behaviour.
Habitual ‘over-signallers’ may be good bluffers, who gamble on scaring away rivals that they won’t actually dare fight. ‘Under-signallers’, who attack without much warning, are harder to explain. “This behaviour is intriguing, and hasn't really been discussed in theory”, says Beecher. “There are benefits to signaling – a fight is avoided, potentially beneficial to both parties – so why not do it?”
One possibility is that under-signallers are genuine tough guys, so likely to win a bout that it’s not worth their while bothering with scare tactics. In this case the behaviour could be a beneficial adaptation. But another possibility is that some individuals just aren’t very good at getting the signaling codes right – it’s not an adaptation but a mistake.
“I don’t think there is enough evidence yet to know whether individual adaptive or error-based theories are more correct”, says Hyman. He adds that why personality traits exist at all is still a big question, but says “I think there’s enough evidence of links between personality and fitness to conclude that behavioural variation isn’t [adaptively] neutral.”
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