Sympatric populations that breed in different seasons are genetically differentiated.
There is more diversity than we can see. Some species are morphologically indistinguishable but they differ in other aspects, such genetics, song or behavior. These so-called cryptic species are difficult to detect because humans are visual species. More detailed investigations are needed to uncover this hidden diversity of cryptic species. In 2001, for example, Darren Irwin and his colleagues showed that the morphologically similar taxa humei and inornatus (which were classified as the same species: Yellow-browed Warbler, Phylloscopus inornatus) were in fact different species. They are genetically distinct and sing different songs. A recent study in the journal Molecular Phylogenetics and Evolution checked whether there are also cryptic species among the storm-petrels (genus Hydrobates).
Storm-petrels are small seabirds with a wide distribution. They can be found on several tropical and sub-tropical islands in the Pacific and Atlantic Oceans. On some islands, there are populations with distinct breeding seasons. Previous work already showed that populations on the Azores are genetically differentiated. Moreover, birds from these different breeding populations produced different vocalizations, leading ornithologists to recognize them as distinct species.
Do similar processes occur on other islands? To answer this question, Rebecca Taylor and her colleagues collected blood samples from 754 storm-petrels, covering numerous breeding areas (see map below).
The genetic analyses uncovered seven clusters, indicating that the different archipelagos are genetically distinct. Moreover, within three clusters, the researchers could even differentiate between hot and cold season breeders. These results suggest that the storm-petrels represent multiple cryptic species.
Interestingly, the level of genetic differentiation between the hot and cold season breeders varies from island to island. On the Azores, the populations are clearly distinct and can be seen as different species (as mentioned above). On the Selvagem island, the cool season breeders still clustered with the hot season breeders. This suggests that these breeding populations are still differentiating and thus represent an earlier stage in the speciation process.
The existence of multiple cryptic species begs the question: what is driving the genetic divergence between these populations? The differences between islands can be explained by philopatry, the tendency of birds to breed in the same location. Occasionally, an individual might switch between islands but in general birds stay loyal to their breeding grounds. In addition, differences in ocean regimes (e.g., surface temperature, ocean currents) can act as physical barriers between islands.
The genetic differentiation on islands is mainly due to allochrony: different populations breed at different times of the year. What processes lead to allochrony is still an open question. Birds might breed at different times because of competition for nest sites or food. Or perhaps predators can force individuals to start nesting at another time. More research is needed to sort this out.
Also check out this great blog post at the British Ornithologists’ Union (BOU): “Just what is a species?“, written by Rebecca Taylor.
Bolton, M., Smith, A. L., Gomez-Diaz, E., Friesen, V. L., Medeiros, R., Bried, J., Roscales, J. L. & Furness, R. W. (2008). Monteiro’s Storm‐petrel Oceanodroma monteiroi: a new species from the Azores. Ibis, 150(4):717-727.
Irwin, D. E., Alström, P. , Olsson, U., & Benowitz‐Fredericks, Z. M. (2001). Cryptic species in the genus Phylloscopus (Old World leaf warblers). Ibis, 143(2):233-247.
Friesen, V. L., Smith, A. L., Gomez-Diaz, E., Bolton, M., Furness, R. W., González-Solís, J., & Monteiro, L. R. (2007). Sympatric speciation by allochrony in a seabird. Proceedings of the National Academy of Sciences, 104(47):18589-18594.
Taylor, R. S., Bolton, M., Beard, A., Birt, T., Deane-Coe, P., Raine, A. F., Gonzalez-Solis, J., Lougheed, S. C. & Friesen, V. L. (2019). Cryptic species and independent origins of allochronic populations within a seabird species complex (Hydrobates spp.). Molecular Phylogenetics and Evolution, 139:106552.
This paper has been added to the Procellariiformes page.