Many birds make long journeys over the sea. They’re not all sea birds, for instance about two-thirds of the migratory song birds of North America cross the Gulf of Mexico twice each year. North-south that’s about 850 km (530 miles). Tiny warblers and hummingbirds regularly venturing further from the shore than many of the birds we think of as seabirds.
So what is a seabird?
Before I launch into a list of characteristics let’s think about the difference between a fish and a scuba diver. One lives in the sea, the other makes special preparations and chooses their conditions for a brief visit. So it is with migratory birds, they fuel up first by laying down fat. They do their best to choose periods when the weather is favorable. Then they make their journey. They don’t eat or drink en route. If they get it wrong they die.
On the other hand true seabirds can feed and drink at sea, withstand the weather and stay out there for long periods, sometimes years. They don’t quite fit the fish analogy because they can’t lay their eggs on the water so they need suitable breeding sites on land.
If you head to the sea and stand on a cliff you see birds like gulls, terns, cormorants and gannets. These are birds of inshore waters, the neritic zone. They can drink salt water and secrete excess salt from a gland adjacent to their nose. They generally don’t venture far from shore, they may wander widely during the non-breeding season but feed locally when they have chicks to rear. Several families of birds fit the bill. Depending on which cliff you choose you may see penguins, auks, gannets, boobies, pelicans or frigatebirds.
Out beyond the continental shelf is the pelagic zone. Out here one family dominates, the Procellariiformes, procella being Latin for storm. In English the tubenoses or albatrosses and petrels. They wander long distances even when feeding young which means just one chick every year or two. They produce an oil in their stomach which is stored in the proventriculus. It is composed of wax esters and triglycerides and is solid at room temperature. At 9.6 kcal/gram it is a very efficient way of storing energy with the advantage over fat that it can be regurgitated. It can be fed to their chick or sprayed over predators. The smell is said to be very offensive and very persistent.
Long lives, delayed reproductive maturity followed by low birthrate, stable population size are all features of a life strategy designated as K. Plague locusts and mice have short lives, many off spring, population boom and bust. These and many others are R stategists. Procellariforms are very K. If their population is depressed because of an increased mortality rate soon extinction beckons.
The White-chinned Petrel, Procellaria aequinoctialis, is a fairly typical tubenose that was numerous off Portland the other day. They breed on many of the subantarctic islands. The species is classified as Vulnerable by the IUCN, the main threatening processes are long-line fishing and habitat degradation on the breeding islands.