DOLPHIN FLEET BIRDWATCHING NOTES: 1 – 15 JULY, 2008
John C. Conlon
Summer is here. So are the summer crowds. So are summer pelagic birds; and in substantial numbers. Sooty and greater shearwaters are plentiful, while manx shearwaters are few and far between so far this year. Cory’s shearwaters are unusually abundant. Wilson’s storm petrels are regularly seen and there is even still an occasional immature northern gannet. Occasional wind and heavy fog days have made for some difficult bird watching which is highly dependent on vision. Seabirds are also highly dependent on vision. Their eyes are proportionately larger than many vertebrates and they have a higher range of light sensitivity than humans. I often wonder how these days affect the birds’ ability to forage.
A couple particular sights are worth mentioning first. On the sixth of July we spotted a flock of 15 eider ducks along the southern edge of Stellwagen Bank. Several were young males and the remainder were females and immatures. Common eiders are around Cape Cod in large numbers during the winter as this is a well known wintering ground. They tend to be much less common during summer as most have migrated toward the northeastern and northern Canadian coasts for the summer breeding season.
On that same July day we also had great looks at two immature pomarine jaegers. Pomarines are generally seen later in the summer and through the autumn. They are the largest and heaviest jaeger that we would expect here. I often tell people to watch for behavior to help determine species, or at least eliminate possibilities. In this case the two pomarines were on the trail of sooty shearwaters that were in flight. Jaegers often perform spectacular aerial pursuits of other seabirds in order to steal carried or swallowed food from their targeted bird. This technique is known as kleptoparasitism. The targeted bird species will vary somewhat from one type of jaeger to another and pomarines are not shy about chasing down larger seabirds including larger gulls and shearwaters.
One should look into mixed feeding flocks of gulls and terns to see kleptoparasitism in action. The windy evening of July thirteenth, however, gave us close looks of an adult parasitic jaeger in pursuit of a common tern. They were above the bow of our boat. Forked tail feathers were splayed wide on both birds. The parasitic jaeger’s falcon–like wing beats allowing it to keep close harassing proximity to the tern that historically would have been referred to as a sea swallow. Notice the resemblance in their shape and proportion to swallows. Jaegers and terns are two of the most agile birds that we see over the water.
My late June notes talked about greater and sooty shearwaters, but except for mentioning an occasional Cory’s shearwater on the boat I did not think I’d be writing about Cory’s shearwaters in any detail. Things have changed! Cory’s shearwaters are the largest shearwater that we would find here. They have an approximately 18 inch length, an approximately 44 inch wingspan, and weigh in at 1.8 to 2 pounds. They are broader in overall proportion than greater shearwaters: broader in the breast and shoulders as well as the head. The wings are longer, broader, and clear white on the underside with a black perimeter and tips. The latter makes Cory’s easy to distinguish from greaters with their diagonal lines across their wing’s underside. So does the heavier yellow with dark-tipped bill. Again, as with all pelagic birding, look more for contrast and less for particular colors. The Cory’s are overall lighter and more evenly brown than the greaters and have no noticeable black crown. Their wing beats are slower and more fluid than the greaters.
Cory’s shearwaters, also known as Mediterranean, shearwaters are primarily an Atlantic species though they occasionally wander into the western Indian Ocean off southern Africa. They can be found in the western North Atlantic as far north as the Gulf of Maine from July through October as they make a loop across the ocean. From here they aim south along the eastern seaboard and diagonally back across the open Atlantic toward southwestern Africa. Their migration then takes them up the west coast of Africa toward their nesting islands of the Cape Verdes and Canarys off northwestern Africa, Madeira and the Azores, while over a thousand per hour will pass through the Straits of Gibraltar toward Mediterranean islands.
Locally, Cory’s shearwaters tend to stay farther offshore and farther south of Cape Cod though an occasional sighting is not unusual. Off of Martha’s Vineyard and Nantucket would be better places to see Cory’s during the summer months. Occasionally (every three to five or more years) though we get an irruption in their distribution and the Cory’s flood into our range in large numbers. An irruption is the occasional non-weather-related, non–migrational movement of a population into atypical and temporary ranges. Irruptions are believed to be related to changes in prey distribution. Birds that irrupt into new areas often have little diet variation. So, a spike or crash in prey numbers can have a dramatic effect on a predator’s distribution.
Irruptions, as is the case with much of pelagic bird ecology, are poorly understood and difficult to predict. But this irruption has worked well for us as we now see flocks of 40 to 60 or more Cory’s shearwaters outside the Race Point sand bars. As we move offshore they join mixed flocks of gulls, terns, and greater and sooty shearwaters. These flocks hover above bubble feeding humpback whales. As humpbacks feed they push the sand lance that are their preferred prey toward the surface. The avian feeding flocks hover above to take advantage of the humpback whales’ efforts and gain an easier meal. Cory’s shearwaters rarely dive from the surface. Instead they skim along the water’s surface or dive from 10 to 20 feet in the air to get the fish. Sometimes we see Cory’s by hundreds gliding above their mirrored reflection on the water. As they soar upward we see the clear white underside reflecting the setting sun’s pastel hues while brilliant yellow gleams from their beaks.
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