Impact of feather molt on ectoparasites: looks can be deceiving

Animals possess a variety of well-documented defenses against ectoparasites, including morphological, behavioral, and immune responses. Another possible defense that has received relatively little attention is the shedding of the host's exterior. The conventional wisdom is that ectoparasite abundance is reduced when birds molt their feathers, mammals molt their hair, and reptiles shed their skin. We carried out an experimental test of this hypothesis for birds by manipulating molt in feral pigeons (Columba livia) infested with feather lice (Phthiraptera: Ischnocera). We used two standard methods, visual examination and body washing, to quantify the abundance of lice on the birds. The visual data indicated a significant effect of molt on lice. However, the more robust body washing method showed that molt had no effect on louse abundance. Two factors caused visual examination to underestimate the number of lice on molting birds. First, molt replaces worn feathers with new, lush plumage that obscures lice during visual examination. Second, we discovered that lice actively seek refuge inside the sheath that encases developing feathers, where the lice cannot be seen. The apparent reduction in louse abundance caused by these factors may account for the conventional wisdom that feather molt reduces ectoparasite abundance in birds. In light of our experimental results, we argue that it is necessary to reinterpret the conclusions of previous studies that were based on observational data. Additional experiments are needed to test whether shedding of the host's exterior reduces ectoparasites in other birds, mammals, and reptiles, similar to the impact of facultative leaf drop on herbivorous insects on trees.