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Olfaction is an important sense that has contributed to reproductive isolation and speciation in many taxa. However, in birds, olfaction and its potential role in communication has historically been neglected. Thus, what role olfaction plays in avian speciation is mostly unknown. Here, we aim to guide future research by highlighting the likely potential for olfaction to contribute to reproductive isolation and speciation in birds. First, we detail the best-understood example found thus far that is consistent with reproductive isolation by olfaction in birds: black-capped and Carolina chickadees. These species show interspecific differences in preen oil chemistry and conspecific odour preferences within their natural hybrid zone. We then suggest a number of promising avenues for future research and the kinds of systems, approaches and evidence that would help to advance this severely understudied area. In sum, the growing body of research into the chemical ecology of birds suggests an important role of olfaction in many areas of avian life. Future research will be necessary to determine to what extent olfaction contributes to the formation and maintenance of species boundaries in birds. 

ABSTRACT Hybridization occurs when different species mate and produce offspring. Although hybridization can have negative consequences for cognitive performance, the mechanisms underlying those effects are still poorly understood. A fundamental physiological process found in all animals studied to date that could be disrupted in hybrids is sleep. Given that mechanisms that occur within the brain during sleep may help maintain optimal cognitive performance, here we outline the potential impacts of hybridization on sleep and cognition. We suggest that sleep loss caused by hybridization could lead to negative impacts for neural and molecular mechanisms (e.g. neurogenesis, synaptic plasticity, and brain gene expression) associated with cognition, which may help explain some of the cognitive deficiency recently observed in hybrid birds. However, we acknowledge that these mechanisms may instead be directly impacted by hybridization, which in turn could also disrupt sleep with similar negative consequences for cognition. Limitations in sleep processes apparent in hybrids might influence hybrid fitness and therefore act as a post‐zygotic isolating barrier. 

Thermoregulatory performance can be modified through changes in various subordinate traits, but the rate and magnitude of change in these traits is poorly understood. We investigated flexibility in traits that affect thermal balance between black-capped chickadees (Poecile atricapillus) acclimated for 6 weeks to cold (−5°C) or control (23°C) environments (n=7 per treatment). We made repeated measurements of basal and summit metabolic rates via flow-through respirometry and of body composition using quantitative magnetic resonance of live birds. At the end of the acclimation period, we measured thermal conductance of the combined feathers and skins. Cold-acclimated birds had a higher summit metabolic rate, reflecting a greater capacity for endogenous heat generation, and an increased lean mass. However, birds did not alter their thermal conductance. These results suggest that chickadees respond to cold stress by increasing their capacity for heat production rather than increasing heat retention, an energetically expensive strategy. 

Abstract Understanding how mating cues promote reproductive isolation upon secondary contact is important in describing the speciation process in animals. Divergent chemical cues have been shown to act in reproductive isolation across many animal taxa. However, such cues have been overlooked in avian speciation, particularly in passerines, in favor of more traditional signals such as song and plumage. Here, we aim to test the potential for odor to act as a mate choice cue, and therefore contribute to premating reproductive isolation between the black‐capped (Poecile atricapillus) and Carolina chickadee (P. carolinensis) in eastern Pennsylvania hybrid zone populations. Using gas chromatography–mass spectrometry, we document significant species differences in uropygial gland oil chemistry, especially in the ratio of ester to nonester compounds. We also show significant preferences for conspecific over heterospecific odor cues in wild chickadees using a Y‐maze design. Our results suggest that odor may be an overlooked but important mating cue in these chickadees, potentially promoting premating reproductive isolation. We further discuss several promising avenues for future research in songbird olfactory communication and speciation.