One of the most iconic birds of summertime in the Gulf Stream is the Black-capped Petrel (Pterodroma hasitata), and with two days aboard the Stormy Petrel II off the coast of Hatteras with Brian Patteson, we had ample time to photograph and study them. On August 12th, we saw 56 Black-capped Petrels and on August 13th, we had an incredible 126! Many birds were distant and only seen naked eye or through binoculars, but with close to 200 individuals over the two trips we were also treated to quite a few Black-caps that came in very close to the boat. We left an almost constant slick of fish oil and chum behind the boat, and Wilson’s Storm-Petrels followed us around wherever we went. Every few minutes, a Black-capped Petrel would intersect the slick and typically hit the brakes, arcing up, and changing course to follow the slick up until sometimes within 25m of the boat before flaring off and heading out over the ocean. With so many cooperative birds, everyone on board really had great opportunities to study the petrels well.
These long-winged Pterodroma are fairly straightforward to identify with their striking black-and-white underside and dark gray upper-side, highlighted by a white collar and white rump – a trait only shown by this species and easy to pick out on even the most distant bird.
A closer look at the bird’s back shows a black bar along both wings creating a M-shaped pattern, similar to the thick black bars on the petrel’s underwings. From there, though, the Black-capped Petrel shows a bit of variation in plumages. Some bird’s are particularly ‘black-faced’ (also called ‘dark-faced’) while others are much more ‘light-faced’. The black-faced birds show obvious black caps, extending down over the bird’s eyes and sometimes on their cheeks. Black-faced individuals also typically show a broad, dark collar on either side of the neck. The light-faced individuals have a much more reduced cap, which is sometimes more dark gray than black. Most light-faced birds have very faint collars, or none at all. The extent of the black bar on the under-wings of Black-capped Petrels seems to vary quite a bit within both black-faced and light-faced types, and intermediate birds are also seen occasionally. Its not know if these intermediate birds represent a different population or are simply variations (age/sex).
During our two trips, we did notice that most of the individuals we saw that were molting their flight feathers were typically black-faced birds. Could these plumage differences indicate distinct subspecies or even represent multiple species altogether? Currently two subspecies of Black-capped Petrels are recognized,
“the nominate hasitata of Hispaniola, Guadeloupe, and Dominica; and caribbaea, formerly of Jamaica, but which now probably is extinct (e.g. Carte 1866, Jouanin and Mougin 1979, AOU 1998). More recently many authors consider carribaea to be a separate species, Jamaican Petrel Pterodroma caribbaea. However, note that Howell and Patteson (2008) have provided evidence highlighting a closer examination of nominate hasitata, in that it may contain several cryptic taxa. If molt patterns indicate different species, then geographic variation within nominate hasitata may not be particularly likely. However, if molt patterns are not reflective of species limits, geographic variation as represented by white-, black-, and intermediate-faced plumages may have some relevance.”
– Neotropical Birds Online
Additional work published 2013 (Manly et. al.) used mitocondrial DNA from museum specimens to look at population structure. Their analysis revealed that genetic differences also support two distinct populations. They found divergence between the two color morphs is approximately five times greater than the variation within old and new world bird species, and within the range observed within polytypic species and between sister species. Even with this strong evidence supporting two populations and potentially two distinct species, the authors could not make this determination. There are some huge unknowns which currently prevent this decision from being made. At this time we lack sufficient population and behavioral data from breeding sites. The remote nature of these sites and the fact that some are still unknown, provide a major hurdle here and also prevent fresh DNA samples from being collected and analyzed.
Hopefully more work is done with Black-capped Petrels, not only to try and figure out if more than one species is actually hiding under our noses but also to try and come up with conservation plans for these distinct populations. In the meantime, here are a whole bunch of photos of Black-capped Petrels showing the plumage variation we were able to see earlier this month.
‘Black-faced’ Black-capped Petrel
‘Light-faced’ Black-capped Petrel
And a few intermediate type birds, or individuals that we are having trouble sorting out to either black-faced or light-faced….
Farnsworth, Andrew. 2010. Black-capped Petrel (Pterodroma hasitata), Neotropical Birds Online (T. S. Schulenberg, Editor). Ithaca: Cornell Lab of Ornithology; retrieved from Neotropical Birds Online: http://neotropical.birds.cornell.edu/portal/species/overview?p_p_spp=21830
Howell, S.N.G. & Patterson, J.B. 2008. Variation in the Black-capped Petrel – one species or more?
Alula 14(2): 70-83.
Howell, S.N.G, in collaboration with J. Brian Patterson, Kate Sutherland, and Debra L. Shearwater. Petrels, Albatrosses, and Storm-Petrels of North America, A Photographic Guide. Princeton University Press. 2012.
Manly, B., B.S. Arbogast, D.S. Lee, and M. Van Tuinen. 2013. Mitochondrial DNA analysis reveals substantial population structure within the endangered Black-capped Petrel (Pterodroma hasitata). Waterbirds 36: 228-233.