Could odor cues promote reproductive isolation?
To understand the origin of new bird species, ornithologists have mainly studied characters such as plumage color and song. If birds look or sound different, they probably represent distinct species. But what about less obvious features? What about smell? Recent work has shown that odors play an important role in avian ecology and evolution (see this paper for a nice review). In songbirds, olfactory cues are most likely produced by the uropygial gland. This organ is situated at the base of the tail and secretes complex mixtures of chemicals (generally called preen oil). Could these oils contribute to avian speciation? A recent paper in the journal Ecology and Evolution put this idea to the test.
Alex Van Huynh and Amber Rice focused on two chickadee species: the Black Capped Chickadee (Poelice atricapillus) and the Carolina Chickadee (P. carolinensis). These North American passerines interbreed along a hybrid zone, which has been studied extensively (you can check out the Paridae page for an overview). Because Black-capped Chickadee and Carolina Chickadee look very similar and can learn each others songs, these characters might not be reliable indicators in mate choice. Individuals can easily pick the ‘wrong’ species. Perhaps these birds use odor to chose their partner?
The researchers tested this hypothesis with an elegant experiment. They collected wild chickadees from the hybrid zone and placed them in a Y-shaped maze. Each arm of the maze contained the odor of one species (which was blown into the maze from a box containing a bird). Next, the researchers noted how much time the birds spend in the respective arms of the maze.
The results indicated that birds spend more time in the odor arm of their own species, suggesting that these birds can discriminate between species based on smell. Previous studies have reported similar results in other bird species, such Waxwings and Crimson Rosellas. But this is the first evidence of olfaction-based species discrimination in a natural hybrid zone.
The scent of hybrids
The results from this experiment are consistent with a possible role of odors in avian speciation. However, more work is needed to validate this idea. For example, what is the scent of hybrids? If hybrids smell different from ‘pure’ species, they might be less attractive and unable to find a partner. This selection against hybrids could increase genetic divergence between the species. In any case, this study shows that there is more to avian speciation than meets to eye (and ear).
Caro, S. P., Balthazart, J., & Bonadonna, F. (2015). The perfume of reproduction in birds: chemosignaling in avian social life. Hormones and behavior, 68, 25-42.
Mihailova, M., Berg, M. L., Buchanan, K. L., & Bennett, A. T. (2014). Odour-based discrimination of subspecies, species and sexes in an avian species complex, the crimson rosella. Animal Behaviour, 95, 155-164.
Van Huynh, A., & Rice, A. M. (2019). Conspecific olfactory preferences and interspecific divergence in odor cues in a chickadee hybrid zone. Ecology and Evolution.
Zhang, Y. H., Du, Y. F., & Zhang, J. X. (2013). Uropygial gland volatiles facilitate species recognition between two sympatric sibling bird species. Behavioral ecology, 24(6), 1271-1278.
This paper has been added to the Paridae page.