Its intermediate beak morphology might result in a broader range of prey species.
Scientific progress does not stand still. A few months ago, I published a review paper on hybrid bird species (check the blog post here) in which I assessed the evidence for seven putative hybrid species. In addition, I proposed a scheme to classify these cases into two types: type I where reproductive isolation is a direct consequence of hybridization and type II where reproductive isolation is the by‐product of other processes, such as geographical isolation. Recently, a study in the journal Molecular Biology and Evolution added another case to the list: the Salvin’s Prion (Pachyptila salvini). Let’s have a look at the evidence and see if we are dealing with a type I or type II hybrid species.
The prions (genera Pachyptila and Halobaena) are a group of small petrels. Their name comes from the Greek word for saw, referring to the serrated edges of their bills. Prions have a fringe of thin plate-like structures (so-called palate lamellae) in their beaks. In some species, such as the Broad-billed Prion (P. vittata), these lamellae are quite big and can be used for filter feeding, resulting in an exclusive diet of copepods. That is why they are sometimes called whale-birds. In other species, such as the Thin-billed Prion (P. belcheri), the lamellae are vestigial and not suitable for filter feeding. These species focus on other prey items, including small squids and fish.
Genetic and Morphological Evidence
An international team of researchers sequenced the DNA of six prion species. When they compared these species, they discovered that Salvin’s Prion was a genetic mixture of Antarctic Prion (P. desolata) and Broad-billed Prion. Further analyses indicated that an evolutionary model in which Salvin’s Prion was a hybrid between these two species was the most likely scenario.
The genetic analyses were complemented with morphological measurements. These showed that, in terms of beak morphology, Salvin’s Prion was intermediate between both parental species. This could potentially allow it to feed on a broad range of prey species by combining the filter feeding and non-filter feeding strategies described above.
There is clear genetic and morphological evidence for hybridization. But what about reproductive isolation? The authors suggest that Salvin’s Prion has an intermediate breeding schedule compared to the other two species. Antarctic Prions start laying eggs at the end of August, whereas Broad-billed Prion does this in December. Salvin’s Prion falls somewhere in the middle (early to mid-November). Because egg-laying correlates strongly with mating time, this pattern suggests that Salvin’s Prion is isolated from both parental species. However, this hypothesis remains to be tested.
This intermediate breeding schedule suggests a genetic basis of this behavior (similar to the intermediate beak morphology). If so, reproductive isolation is a direct consequence of hybridization. That would mean Salvin’s Prion is a type I hybrid species.
Masello, J. F., Quillfeldt, P., Sandoval-Castellanos, E., Alderman, R., Calderón, L., Cherel, Y., Cole, T.L., Cuthbert, R.J., Marin, M., Massaro, M., Navarro, J., Phillips, R.A., Ryan, P.G., Shepherd, L.D., Suazo, C.G., Weimerkirch, H. & Moodley, Y. (2019). Additive traits lead to feeding advantage and reproductive isolation, promoting homoploid hybrid speciation. Molecular Biology and Evolution. msz090.
Ottenburghs (2018) Exploring the hybrid speciation continuum in birds. Ecology and Evolution. 8(24): 13027-13034.
This paper has been added to the Procellariiformes page.