Losing the last feather: feather loss as an antipredator adaptation in birds

Birds often lose feathers during predation attempts, and this ability has evolved as a means of escape. Because predators are more likely to grab feathers on the rump and the back than on the ventral side of an escaping bird, we predicted that the former feathers would have evolved to be relatively loosely attached as an antipredator strategy in species that frequently die from predation. We estimated the force required to remove feathers from the rump, back, and breast by pulling feathers with a spring balance from a range of European bird species in an attempt to investigate ecological factors associated with ease of feather loss during predation attempts. The force required to loosen a feather from the rump was less than that required to loosen a feather from back, which in turn was less than that required to loosen a feather from the breast. The relative force needed to loosen rump feathers compared with feathers from the back and the breast was smaller for prey species preferred by the most common predator of small passerine birds, the sparrowhawk Accipiter nisus. Likewise, the relative force was also smaller in species with a high frequency of complete tail loss among free-living birds, which we used as an index of the frequency of failed predation attempts. The relative force required to remove feathers from the rump was smaller in species with a high frequency of fear screams, another measure of the relative importance of predation as a cause of death. Feather loss required particularly little force among solitarily breeding bird species that suffer the highest degree of predation. Antipredator defense in terms of force required to remove feathers from the rump was larger in species with a strong antiparasite defense in terms of T-cell-mediated immune response. These findings are consistent with the hypothesis that different defenses are antagonistic and that they are traded off against each other.