Olfactory camouflage and communication in birds

Abstract

Smell is a sensory modality that is rarely considered in birds, but evidence is mounting that olfaction is an important aspect of avian behaviour and ecology. The uropygial gland produces an odoriferous secretion (preen oil) that can differ seasonally and between the sexes. These differences are hypothesized to function in olfactory camouflage, i.e. minimizing detection by nest predators (olfactory crypsis hypothesis), and/or intraspecific olfactory communication, particularly during breeding (sex semiochemical hypothesis). However, evidence for seasonal and sex differences in preen oil is mixed, with some studies finding differences and others not, and direct evidence for the putative function(s) of seasonal variation and sex differences in preen oil remains limited. We conducted a systematic review of the evidence for such changes in preen oil chemical composition, finding seasonal differences in 95% of species (57/60 species in 35 studies) and sex differences in 47% of species (28/59 species in 46 studies). We then conducted phylogenetic comparative analyses using data from 59 bird species to evaluate evidence for both the olfactory crypsis and sex semiochemical hypotheses. Seasonal differences were more likely in the incubating than non‐incubating sex in ground‐nesting species, but were equally likely regardless of incubation strategy in non‐ground‐nesting species. This result supports the olfactory crypsis hypothesis, if ground nesters are more vulnerable to olfactorily searching predators than non‐ground nesters. Sex differences were more likely in species with uniparental than biparental incubation and during breeding than non‐breeding, consistent with both the olfactory crypsis and sex semiochemical hypotheses. At present, the data do not allow us to disentangle these two hypotheses, but we provide recommendations that will enable researchers to do so.

Birds differentially prioritize visual and olfactory foraging cues depending on habitat of origin and sex

“…Olfactory function is linked to breeding activity in male European starlings ( Sturnus vulgaris ) [ 57 ] and testosterone level in male blue tits [ 22 ], suggesting probable seasonal variation in sensitivity. In addition, seasonal changes and sex differences in preen oil composition have been reported for many bird species, indicating increased importance of olfaction during breeding [ 58 ]. Therefore, attention towards olfactory stimuli in birds might increase at the start of breeding season during bud burst in spring, when plants start producing HIPVs [ 22 ].…”

Feather function and the evolution of birds

“…The diversity and distribution of such bacterial mutualisms may provide a fruitful avenue for future research into avian defences against ectoparasites (Barbosa et al ., 2002). In addition to antibiotic compounds, feathers may provide defence against ectoparasites by exuding or containing odour‐emitting compounds (Clayton et al ., 2010), but the functions of feather odours remain poorly studied (Grieves et al ., 2022; see Section II.4. i ).…”

How woodcocks produce the most brilliant white plumage patches among the birds

“…The use of contrasting plumage patches or complex feather structures to convey information is widespread in birds (reviewed in [1,2]). Unlike in diurnal birds, visual signals in nocturnal and crepuscular species are understudied, and communication was, until recently, considered to follow chemical and acoustic channels [3][4][5]. However, in dim light environments, plumage characteristics have emerged that maximize reflectance of available light [6,7].…”