Great tits are more likely to join defensive mobs with birds in nearby nests that are 'familiar neighbours' rather than new arrivals, Oxford University research has found.

Many small birds will defend their nests by joint mobbing, where individuals gang up to harass a potential predator. Scientists studying great tit populations in Wytham Woods, Oxfordshire, wondered whether this sort of defensive behaviour might be behind observations showing that birds successfully raised more chicks when they were alongside familiar neighbours -- those that had occupied the nest box next door for several breeding seasons.

After tagging the birds with paint to show which nest boxes they came from the researchers simulated the approach of a predator by rustling leaves and scraping a pole against individual trees and nest boxes. They then observed the mobbing behaviour of the great tits from this and nearby boxes, calculated the distances between boxes, and compared this with information on how long nearby birds had been neighbours.

'We found that nesting great tits join their neighbours' mob if they are familiar with them from the previous year but that birds that weren't familiar were less likely to join, and young birds that haven't bred before didn't join their neighbours at all,' said Ada Grabowska-Zhang of Oxford University's Department of Zoology who led the research.

But whilst the study showed, for the first time, a link between familiarity and nest defence, the behaviour may not be evidence of altruism and a 'love thy neighbour' approach:

'It could be that they join because their own nest might also be at risk, or they may be playing 'tit-for-tat' and joining the mob because their familiar neighbours have joined theirs before,' said Ms Grabowska-Zhang, 'more work is needed to find out what is driving this remarkable behaviour.'