Prions are a group of rather small, speedy petrels found in the southern oceans. They are about the size of a Common Nighthawk and fly like bats - fast with lots of twists and turns as they dart over the waves - which makes them very difficult to identify and photograph . The various species all appear very, very similar. The most distinctive features of each species are the shape and size of their bills (see here for comparison of some species skulls where you can appreciate the differences in bill size and shape), a feature which is is unfortunately something hard to discern when you are standing on a pitching deck 20 feet above the water and the birds are rocketing past. Therefore honest identification is often limited to "prion species." Richard White, an excellent birder from the Lindblad Expedition staff with more Antarctic birding experience than anyone I know, most often list the prions on our trips as "prion sp." unless he gets diagnostic views or photos.
Identification guides make statements like "even with perfect views, identification to species may not be possible" (Onley and Scofield) or "even given current knowledge and and experience, some forms are usually unidentifiable at sea" (Shirihai). Shirihai presents the most honest view on prion identification. After describing a set of characteristics to distinguish the species he writes "At sea it is often very difficult to appreciate these differences, because of their subtlety and due to these species fast flight, poor sea conditions/visibility, and/or various lighting effects." After describing the Slender-billed Prion, one of the more distinctive species, he states "Bear in mind it is never easy to appreciate any of these characteristics, even if alongside other species at close range, and even for experienced observers." He also lists "other prions" when describing identification confusion risks for all the prion species.
I feel that often people are deceived by the illusion presented in a number of identification guides that they can always identify birds to species when in fact for a number of very similar species it is often impossible to tell them apart. This pressure is probably most evident for birding guides, who are probably perceived by their clientele as not being a good guide or birder if they admit they are unable to make a specific identification, making the matter even worse and perpetuating the myth that these species are readily identifiable in the field.
Digital photography has certainly helped with identification of prions for me and hopefully this tool will help others be more critical of their identifications (or help admit they just can't tell one from another with the views they have). A trip report I read recently illustrates this point. The report stated something like "the abundant Slender-billed Prions we observed during the day were later determined to be Antarctic Prions after further analysis of the photos taken during the day"
There are eight species in the southern oceans, but thankfully for me trying to figure out which is which, only two, the Antarctic and the Slender-billed regularly occur in the Drake Passage.
Here are a few photos of what I have decided are most likely Antarctic Prions based on range (Drake Passage where the most likely candidates are Antarctic and Slender-billed), the tail configuration (dark central tail feathers with lighter out tail feathers, also narrowing the candidate species, helping confirm, but unfortunately not helping discern between the two already the most likely possibilities), bill shape and size (in the photos they look like they fit Antarctic better than Slender-billed because they appear larger and not as slender as they should for the latter but given my limited experience...), the contrast of the "m" markings on the back (more contrast suggesting Antarctic, the Slender-billed would have a much paler "M" mark), and abundance (Antarctic are the more common species in the area and are more prone to follow ships. This may be a self-fullfilling field mark but I will take it in this case).
I am still going through my prion photos to see if I can find one that looks more Slender-billed and Antarctic but as you can imagine, many (most) of the photos are going to be photos of a prion species, most likely Antarctic. Either way they it was certainly fun to watch these birds manouver through the fast changing thin zone between wind and waves.