Chemical aposematism: the potential for non‐host odours in avian defence

Abstract

Many birds display plumage coloration and patterns that act as visual aposematic and mimetic signals for predators. Avian aposematism can also be communicated via chemical cues, where so‐called non‐host odours (NHO) emitted from the plumage, the uropygial (preen) gland or other chemical sources denote the status of birds as dangerous or unprofitable hosts for ectoparasitic and haematophagous arthropods. Avian NHO are aposematic signals that in natural concentrations repel nuisance arthropods or inhibit their attraction to hosts, as opposed to being generally unpalatable or toxic to predators. Birds that emit NHO preempt encounters with nuisance arthropods and the pathogens they transmit. Nuisance arthropods that respond to NHO avoid birds that exhibit disabling behavioural, anatomical, physiological and/or chemical defences. As with distinctive plumage patterns that evolved as visual warnings to predators, birds are selected to advertise their non‐host status to nuisance arthropods via distinctive semiochemicals. Short‐chain (C6–C10) aldehydes emitted from the plumage of some birds may act as NHO against a variety of nuisance arthropods. In some birds, the size of the uropygial gland (and presumably its secretory output) is negatively correlated with the degree of ectoparasite infestation or the incidence of malarial infection, thus implicating uropygial secretions in defence against dipteran vectors of haemosporidian pathogens. A field experiment demonstrates that volatiles emanating from the uropygial gland of the Eurasian Hoopoe Upupa epops deter some biting flies. Birds' risky habit of plucking hair from live mammals and incorporating the stolen hair tufts into their nests is hypothesized to serve as a mechanism of appropriating NHO from mammalian skin secretions for nest defence against arthropods. Feathers plucked from heterospecifics and shed snake skins that are incorporated into nests also may emit NHO. Behavioural assays measuring the repellence or attraction‐inhibition of arthropods are needed to elucidate avian NHO. Experimental efforts to elucidate the underlying defences that support avian NHO entail challenges that include the enactment of defences that are variously effective against different arthropods, and the emission of chemical signals that are variously ‘understood’, i.e. avoided, by different arthropods.