Jaegers and Skuas
At least two species of Jaegers (genus Stercorarius) and one species of Skua (formerly of the genus Cartharacta but now considered part of the Stercorarius clad) have been sighted in the Gloucester area during our whale watching season. The one Skua species that I have seen, the South Polar Skua (not shown), is a very rare offshore visitor that is nearly as large as our biggest Gulls but more robust over-all. They have a nearly all dark plumage with white "flashes" on the wrist of each wing. South Polar Skuas, as their name implies, breed in Antarctica and only visit this part of the North Atlantic in small numbers. I have seen this species twice on a 7 seas Whale Watch: once in August 1988 on Stellwagen Bank, and again in June 2003 on Jeffrey's Ledge.
The two Jaeger species that frequent our area are the Parasitic Jaeger and the Pomarine Jaeger. Both species are uncommon but regular migrants in the late summer and throughout the fall, with the Parasitic being slightly more common than Pomarine. Both of these birds have a "light phase" in which their plumage resembles that of Greater Shearwater, and a "dark phase" that looks much like the plumage of a Sooty Shearwater (or a very small South Polar Skua). Dark phase birds (which are less common than light phase birds) can be separated by overall size and shape (Pomarine is bigger, bulkier than Parasitic), and by the shape of the two central "retricies" or tail-feathers. In a Parasitic Jaeger the two central retricies are longer than the rest of the tail and are sharply pointed. In the Pomarine Jaeger the same two feathers are also longer than the rest of the tail but are twisted and have round tips
Light phase Jaegers can also be identified by the relative amount of a dark pigmentation which forms the breast band visible on the underside of the bird. Parasitic Jaegers have a weak, diffuse, usually broken breast band whereas the Pomarine has a heavy, dark, usually complete band.
Jaegers and Skuas are both "kleptoparasires". This means that they sometimes harass other birds into dropping a fish that they have worked hard to catch and then the Jaeger then steals it. The aptly-named Parasitic Jaeger seems to practice this feeding behavior more than other Jaeger species. During a kleptoparasitic attack (by either Jaeger species) the Jaeger will approach a bird with a fish in its bill (or an obviously full crop) with a fast, direct flight that in reminiscent of a hunting falcon. The victim will usually try and climb higher into the sky in order to escape, but Jaegers are very agile fliers and will pursue their target relentlessly. They then use their heavy, hooked bills and raucous call into intimidate the poor bird and cause it to abandon its catch. Jaegers can therefore often be located even from a great distance away simply by noticing one bird pursuing another bird higher and higher into the sky. Interestingly, Skuas use just the opposite approach. Skuas usually drop down from above and force their victim to the water's surface in order to persuade them into giving up their fish. This difference in behavior may be useful in separating dark phase Pomarine Jaegers from South Polar Skuas which can be similar in size and shape.
