When a predator attacks a songbird nest, the threatened birds often call on their neighbours for assistance. Birds such as pied flycatchers join together to mob the interloper and - hopefully - drive it away.
The existence of such cooperation between unrelated individuals has long puzzled biologists, because birds that do not bother joining mobs initiated by their neighbours could still gain help when they initiate a mob themselves. How can cooperation evolve in the face of such a strong incentive to freeload?
In humans, this conundrum is solved by the existence of so-called reciprocity: we remember who has helped us in the past and use this information to decide whom we should help in the future. But this tit-for-tat model was thought not to apply to birds, as it requires the ability to recognise individuals and remember information.
Now, a team of biologists has found evidence for reciprocity in pied flycatchers. Indrikis Krams and colleagues challenged flycatchers nesting in Latvian woodland with a stuffed tawny owl, which triggered mobbing. The team removed pairs of birds at random, so they could not come to the aid of their neighbours. When these birds were returned to their nests and shown the owl, they tried to initiate a mob. However, their slighted neighbours refused to join them, showing that the other birds remembered their apparent lack of cooperation and chose to ignore their calls for help.
Says Krams, "the fact that even small birds can play tit-for-tat shows terrific capabilities of their memories. This shows that animals may be involved in large and complex communication networks."
Source: Behavioral Ecology & Sociobiology, doi:10.1007/s00265-007-0484-1 Click here for full text article (subscription required)
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