Why are seabird hybrids so rare?

Study reports hybrids between White-capped and Black-browed Albatross.

Hybridization in seabirds is rare. However, occasionally hybrids are reported, see for instance this recent case of giant petrels (genus Macronectes). In the ornithological journal Ibis, Richard Phillips, John Cooper and Theresa Burg report a similar case of hybridization in albatrosses (genus Thalassarche).

 

Three Chicks

In 2003 a male White-capped Albatross (T. steadi) was observed in a colony of Black-browed Albatrosses (T. melanophris) at Bird Island, South Georgia. This male paired with a female Black-browed Albatross, rearing three chicks (between 2008 and 2010). The researchers collected blood samples and sequenced the DNA of these putative hybrids. The genetic analyses revealed that two chicks were hybrids, whereas the third one was a pure Black-browed Albatross. The last chick was probably the result of extra-pair copulation, in which the female mated with another male (a behavior that is quite common in these species).

White-capped Albatross.JPG

A White-capped Albatross (from: http://nzbirdsonline.org.nz/)

 

Isolation Mechanisms

So, we can add another hybrid combination to our list of seabird hybrids. Although several hybrids have been reported, hybridization is still relatively rare in seabirds compared to terrestrial birds. Why is this? The authors propose several explanations.

First, exploratory behavior might influence the incidence of hybridization. Seabirds are very philopatric, they generally return to their place of birth for breeding. Breeding site vagrancy is quite rare and usually involves single individuals that explore new territories. The loyalty to their breeding grounds reduces the opportunities for hybridization.

But what about all these mixed colonies? Why are mixed pairings so rare despite the fact that some species breed side by side? There might be some morphological or behavioral differences that prevent the formation of mixed pairings. For example, variation in coloration of certain body parts can function as an isolation mechanism. Think of the distinctly colored feet of booby species (genus Sula). Alternatively, the species might breed at different times of the year (allochrony).

Clearly, there are several barriers to cross before the production of a hybrid chick. But it does happen, indicating that ‘life finds a way.’

black-browed albatross.jpg

A pair of Black-browed Albatrosses with a chick (from: http://ava7.com/)

 

References

Phillips, R.A., Cooper, J. & Burg, T.M. (2018) Breeding-site vagrancy and hybridization in albatrosses. Ibis

 

The paper has been added to the Procellariiformes page.

 

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