Birds strengthen social bonds when they sense trouble

Maori warriors use the haka to bond before battle. Now it seems that birds also demonstrate bonding behaviour when they think there might be trouble with the neighbours. Green woodhoopoes strengthen bonds when they are on the edge of their territory, where conflicts are more likely.

8 July 2010, by Adele Rackley

Maori warriors use the haka to bond before battle. Now it appears that birds also demonstrate bonding behaviour when they think they might have trouble with the neighbours.

Green woodhoopoe

The green woodhoopoe, Phoeniculus purpureus.

Research has shown that sub-Saharan green woodhoopoes strengthen their social bonds when they are on the edge of their territory, where conflicts with neighbouring groups are more likely.

Scientists knew that social birds become closer immediately after conflict with other groups, but until now little was known about how the risk of future conflict influenced animal behaviour.

Green woodhoopoes are ideal subjects for investigating this issue. They live in small groups in permanent territories, conflict between groups is frequent and mostly happens within 100 metres of territorial boundaries, and allopreening – when one bird preens another – is an important part of group behaviour.

So Dr Andy Radford of the University of Bristol studied them to see if their behaviour changed in the face of possible territorial conflict.

Over seven months he watched twelve groups of the birds in the rocky and thickly forested river valleys of the Eastern Cape Province, South Africa. The groups typically consisted of a dominant breeding pair and up to six subordinate 'helpers'. The subordinates are usually the previous season's young, who help the parents bring up the next brood.

Green woodhoopoe

The green woodhoopoe preening.

Radford recorded the length of any periods of self-preening and allopreening, noted which individuals in the group were involved, and where in the territory the birds were when the preening took place.

His results, published in Biology Letters, show that both the frequency of allopreening within the group, and the amount of time the birds spent doing it, increased when the group was at the edge of its territory, where there was increased potential for conflict with neighbouring groups.

There was no change in the time the birds spent self-preening, which is a sign of stress in some species, or in preening each others' heads, which is done for hygienic reasons (because the birds can't preen their own). But allopreening of other areas of the body, which is mainly a social function, increased significantly.

'It's possible that territorial border areas are simply more stressful for the birds,' says Radford. 'But this isn't a likely explanation, because self-preening activity doesn't change, and not all members of the group change their behaviour in the same way,' he adds.

Radford found the most marked increase was in the amount of preening given by the dominant birds to the helpers in the group.

This 'affiliative' behaviour is likely to reassure subordinates and increase closeness within the group, so ensuring the birds all stick together if battle ensues. It looks like dominant birds might be exchanging preening now for participation in possible conflicts later.

Although dominant birds are likely to benefit in other ways from maintaining social bonds, more allopreening means they have less time for important things like feeding. So there has to be a good reason for them to spend more time on allopreening. The likelihood of conflict is such a reason.

Surprisingly, on occasions when this behaviour was observed there had been no visual or vocal evidence of other woodhoopoe groups for at least an hour. This suggests that, rather than bonding in response to an immediate threat, the birds' behaviour was in anticipation of a possible future threat.

'This looks like anticipatory behaviour,' says Radford. 'They know conflict has happened in that place in the past.'

These results are important for understanding group dynamics and social evolution, which has been studied relatively rarely in birds until now. But another exciting outcome of Radford's work is that 'the birds' behaviour brings to mind the possibility of social contracts and even planning for the future' - traits usually considered the preserve of us primates.

'Our understanding of birds' abilities has changed considerably over the past ten years,' says Radford. 'It would be wrong to say this behaviour is firm evidence for forwarding planning in birds, but it is very exciting to have seen this link between potential intergroup conflict and current intragroup behaviour in the wild.'