What the pluck? The theft of mammal hair by birds is an overlooked but common behavior with fitness implications

Avian species across diverse lineages collect and incorporate mammalian hair into their nests (Tóth 2008). This widespread behavior can be adaptive, as hair, fur or wool insulates nests and so enhances nestling survival and recruitment in colder climates (Hilton et al. 2004, Mainwaring et al. 2014, Järvinen and Brommer 2020, Deeming et al. 2020; reviewed in Perez et al. 2020). How birds obtain mammal hair for their nests, however, is an open question. A common assumption is that birds gather mammal hair that has been shed into the environment or from carcasses (e.g. Tóth 2008), although anecdotal accounts exist in the scientific literature of birds plucking hair directly from live mammals. Here, we (H.S.P., Z.S.S. and J.D.B.) document and report our observation of a Tufted Titmouse (Baeolophus bicolor) exhibiting a behavior that we are terming “kleptotrichy” (from Greek “klepto-” – to steal + “trich-” – hair), after “kleptoptily” or feather theft described by Whitney (2007).