Whether Pink-footed Shearwater has a (very rare) dark morph, or whether occasional individuals are melanistic, are interesting semantic questions. But the bottom line for field observers is that apparently dark-plumaged Pink-footed Shearwaters are out there, and they could be confused with Flesh-footed Shearwater.
Although some have questioned this assumption (e.g., see pages 56-58 of the January 2012 issue of Birding magazine), their alternatives of an undescribed new species of large shearwater, or of hybrid Pink-footed x Flesh-footed Shearwaters, seem to overlook the principle of parsimony: the most likely = parsimonious explanation is that some Pink-footed Shearwaters are dark-bodied.
There’s even a sort-of continuum from white-bodied to dusky-bodied to dark-bodied Pink-foots. Moreover, to extend the Birding article’s “logic” one step further, should we say that no Flesh-footed Shearwater is safely identifiable in North America, and therefore cannot be counted on anyone’s list? What if the bird you saw was just a very dark, bright-billed Pink-footed? Sure, if you’ve seen a bunch of presumed Flesh-foots then some of them are good, but which ones?
However, the entire world of field birding is built on assumptions—I assume all the crows I see in California are American Crows, rather than vagrant Fish Crows or Northwestern Crows, but who knows?
I think it’s a reasonable assumption that apparently dark-bodied Pink-footed Shearwaters are, well, just that: they resemble Pink-footed in size and shape, not the slightly differently proportioned Flesh-footed Shearwater, and they have duller bills than typical of Flesh-footed. For now, I’m not quite prepared to kiss my assumptions goodbye until good evidence to the contrary appears.
Images of presumed dark Pink-foots are hard to find, so here are a few more for pelagic birders to mull over (images 01-05). A presumed Flesh-footed (image 06) we saw later in the day looked darker, more chocolate-brown overall, brighter-billed, and narrower-winged, but who knows…