Here is your assignment for today: Make a list of birds that often sit on the ocean surface. These would include sea ducks, gulls, shearwaters, phalaropes, jaegers, gannets and members of the puffin family.
Notice anything in common?
Most have white bellies. I always figured it wasn’t just coincidence that puffins, razorbills, murres and dovekies all have white bellies. The black guillemot is the only local member of the puffin family that doesn’t. All the adult gulls are white underneath. Loons and mergansers are white underneath.
Cory’s, Manx and great shearwaters are white underneath. Evolution must be up to something.
The answer probably is obvious. When an undersea predator looks up, the sky is white. A white-bellied bird is harder to notice. Evolution favors white.
Dr. James Sulikowski is a marine biologist at the University of New England in Biddeford. His friends call him Dr. Shark. If seabirds have evolved white bellies to avoid being eaten, there must be something out there that would eat them, right? Dr. Sulikowski told me tiger sharks are notorious for eating birds.
These sharks prefer warmer water, so they are rare in the Gulf of Maine. But blue sharks sometimes sill attack seabirds from below, and they are common in our waters.
Our seabirds have little to fear from seals. East Coast seals are fish eaters, and their fondness for sushi keeps birds off the menu. But sea lions in the Pacific will take an occasional bird. Penguins are the main prey of leopard seals in the Antarctic. Because many of the birds we see in the Gulf of Maine are “from away,” natural selection favors white bellies wherever they grow up.
In fact, sharks and fish exhibit these same shading traits. Most fish are darker on top and lighter on the bottom. Thus, fish near the surface are harder to see against the sky, and fish below the predator are harder to see against the sandy bottom or inky depths.
So the question becomes, why do some birds not have white bellies? I’m just guessing here. The black guillemot is the only East Coast puffin relative that is dark underneath. It’s the only one that nests along the mainland and the only one that doesn’t wander out to sea after breeding.
Furthermore, it turns mostly white in winter.
Common eider hens are completely brown. This may put them at a slight disadvantage on the ocean, but it gives them much better camouflage when sitting on their nests, where eagles are a graver threat.
Wilson’s storm-petrels are all black, except for a white rump patch. I don’t know — maybe they’re just too small to justify the effort of catching one? Most shearwaters are white underneath, but sooty shearwaters are dark all over. Elsewhere in the world, the flesh-footed shearwater is relatively dark underneath. We have now gone beyond my ability to speculate why.
Tis the season to take a closer look at this phenomenon. The Gulf of Maine is a feeding ground for whales and birds. A whale watching trip usually presents an opportunity to see both. Underwater ledges push food toward the surface whenever the tide is running strongly over them. Sea life congregates in these spots, and we are blessed to have several of them just off the coast of Acadia National Park. Bar Harbor Whale Watch takes its fast catamarans out there several times a day. Robertson’s Sea Tours in Milbridge makes the same run in small boats. Though smaller and slower, they also are uncrowded, carrying a maximum of six passengers. Dang, you can get close to the birds and whales! You’re just above the waterline yourself.
Venturing out to view ocean species is called pelagic birding. “Pelagic” comes from the Greek word for open ocean. Many of the birds in the Gulf of Maine flew a long way to get here. Wilson’s storm-petrels nest on islands off the southern coast of Argentina. Great shearwaters nest in huge colonies on four islands in the south Atlantic. Sooty shearwaters nest on the Falkland Islands, Tierra del Fuego and islands near New Zealand. Cory’s shearwaters nest in the Mediterranean. The fact that they all converge here is a pretty impressive indication of how special the Gulf of Maine is.
Maine Audubon’s annual pelagic trip is scheduled for Saturday, Sept. 12. For more information, visit maineaudubon.org. We’ll commandeer a Bar Harbor Whale Watch catamaran and spend six hours at sea, chasing down everything that flies.
They’ll all have white bellies.
Bob Duchesne serves as a Maine Audubon trustee and vice president of its Penobscot Valley Chapter. Bob developed the Maine Birding Trail, with information at mainebirdingtrail.com. Bob can be reached at duchesne@midmaine.com.