Reciprocal cooperation in avian mobbing: playing nice pays.

Unrelated passerine birds often join together while mobbing, a widespread antipredator behavior during which birds harass a predator. Although previous analyses concluded that mobbing could not have evolved via reciprocity, Krams and colleagues' field experiments show that birds preferentially join mobs with neighbors that have aided them previously, suggesting that these birds utilize reciprocity-based strategies involving individual recognition and recollection of previous interactions with others. This implies a level of sophistication in bird communities greater than had previously been realized.