Tropical seabirds observed over oiled Gulf waters, extending reach of spill

brown_booby credit NOAA

Photo courtesy of NOAA

After abandoning the rest of his life to hurriedly catch the research vessel Walton Smith on its voyage from Gulfport, Mississippi, bound for Miami, Florida, biologist David S. Lee settled into a familiar routine of the sea watch. Given how far north the ship remained during the afternoon of June 7, he was not really expecting much in the way of interesting bird life. The ocean, however, has an uncanny way of delivering surprises to a persistent researcher, and Dave is very persistent. 

At 1 p.m., the ocean delivered Dave his first surprise. The Walton Smith was smack in the middle of the spill zone when two adult brown boobies flew in and, as is often their habit, tried to land on the ship for a short rest. Dave’s sighting was, however, a very long way from this seabird’s nearest breeding colonies. A large gannet-like bird, brown boobies are very uncommon far out at sea anywhere, especially in these northern reaches of the Gulf of Mexico. They spend most of their time in coastal zones within sight of land. Plunging like pelicans to catch fish below the ocean surface, they would be highly vulnerable to any oil concentrated on surface waters. 

Later in the afternoon, Dave found more surprises. Just a few miles southeast of the gushing oil well, four adult sooty terns put in an appearance, soaring high over the Gulf waters. Because their nearest breeding colony was more than 350 miles to the south, at the Dry Tortugas, Florida, their presence during this season in this part of the Gulf was equally perplexing. 

Sooty terns live on the wing. Known for their ability to stay airborne for years at a time between fledging and their first breeding attempt, sooty terns never come to land during the non-breeding season. They essentially never rest on the water, either, although they are known to occasionally piggy-back on resting sea turtles and flotsam. Unlike most terns, sooty terns feed either by “dipping” (brushing the water surface with their bill and head) or by snatching small fish in midair as they are escaping predatory fish swimming up from below. Sooty terns are fond of following schools of tuna, and nabbing the smaller fry driven to the surface by the larger fish patrolling below. Although less vulnerable to oil than the plunge-diving brown booby, their use of waters so near the spill was nevertheless troubling. 

When Dave reached Miami, he called me to relay his discoveries. After minutes of brainstorming, we grasped some significant implications: seabirds like sooty terns and boobies, with their bright white breasts and bellies, would be sensitive indicators for showing color or shading contrasts after encountering any oil in Gulf waters. In fact, many seabirds possess a stark counter-shading plumage pattern, dark above and very light below. It is thought that this counter-shading makes seabirds less visible to their prey near the ocean surface, as the lighter colors gives less contrast against the sky and allows these aerial predators to catch fish, squid, and other meals by surprise. 

Masked Boobies credit NOAA

Photo courtesy of NOAA

Realizing now that these tropical seabirds were capable of using virtually any portion of the Gulf of Mexico, and therefore running into trouble with oil, what was the best means to find out whether in fact they were being harmed by the spill? Searches by ship would continue, of course, but it was also obvious that biologists ought to check any seabirds that might return to land. If biologists were to visit  just three major seabird colony areas that stand guard over each ocean exit for the Gulf of Mexico every few weeks, then any birds discolored by oil might be better detected. These major colonies were located at Cay Sal Bank, in the Bahamas; the Dry Tortugas, in the Florida Keys; and the various atolls and small islands dotting Mexico’s Campeche Bank north of the Yucatan. In fact, and despite the remote locations of these breeding islands, Dave and I suspect that it would be far easier to detect any oiled birds on land at colonies than far out at sea from a boat. By looking at breeding islands, hundreds or thousands of potentially oiled birds could be examined. 

 Whatever findings lay ahead, on land or sea, seabird biologists dealing with the Gulf oil spill now had their first concrete evidence – pelagic seabirds invisible from land were directly in harm’s way. There was no doubt about that. Even seabirds that bred far away from the spill zone were, for whatever reason, drawn to the northern Gulf of Mexico where harm from the oil is most likely.