The seabird syndrome

Tony Gaston’s Seabirds: A Natural History (Yale UP, 2004) is a book-length exploration of an idea that he calls the “seabird syndrome.” Based on the idea that feeding ecology in the marine environment is what drove the evolution of all seabirds, Gaston’s treatment ties together almost every aspect of seabird life that one can imagine. Taken in this light, seabirds’ “low reproductive rates, long lives, deferred breeding, coloniality, and sexes that behave alike and look alike” can be explained by that one element of their lives: their feeding ecology. All of the above characteristics of seabirds “form part of a strongly correlated suite of adaptations that characterise birds that feed far out at sea” (22).

Since I don’t have much experience with this group of birds, I can only absorb information that looks like it will help me when I do get out to sea. Here’s something that seems to make a lot sense:

At first sight, the most salient characteristic of seabird wings is their length. The Wandering Albatross has the largest wingspan of all living birds and even small gulls seem to have huge wings. However, when we examine wing length in relation to mass, we find that, for a given weight, the wings of seabirds are not dramatically different from those of other birds, especially swallows, swifts, and birds of prey. The wings of seabirds appear particularly long for two reasons: the tail is usually rather short…and the wings tend to be narrow. (51)

That’s the kind of information I’m looking for in a book about an unfamiliar taxon (seabirds isn’t technically a taxonomic rank, but a convenient grouping). Gaston gives me information in a way that I can process it easily. Short tails and narrow wings provides a “long-winged jizz” that, when analyzed with respect to mass, is surprisingly similar to landbirds.

Another example of Gaston’s helpful presentation is in a discussion of how the shearwaters (fulmars, gadfly petrels, and shearwaters) hold their wings in flight. All the bird ID books mention that fulmars “look like gulls but flight pattern is different.” Well, Gaston tells us why the flight looks so different:

Initially, the large gulls appear somewhat convergent with the medium-sized tubenoses (fulmars, gadfly petrels and shearwaters). However, the way that the two groups use their wings is very different. No one can mistake the flight of a shearwater for that of a gull. Despite the difference in relative wing length, shearwaters look more like auks than like gulls, in flight. This is because they hold their wings very stiffly while flapping and gliding. Some albatrosses, large petrels, and shearwaters have a very broad patagium (skin membrane) between the humerus and ulna, supported at the trailing edge by a tendon that is held away from the ‘elbow joint’ by a small bone. The patagium thus formed is an important part of the gliding surface (Warham, 1996), making these large tubenoses somewhat convergent with Pterosaur dinosaurs (pterodactyls: having no feathers they depended entirely on the patagium, like modern bats). In addition, the tubenoses have a system by which the elbow joint locks in place, reducing the muscular tension required to keep the wing extended, but reducing its flexibility (Yudin, 1957). (55)

Gulls may be less efficient than petrels at long-distance flight, but they are much more versatile and manoeuvrable at slow speeds. (56)

How’s that for explaining the “stiff-winged” flight of the Northern Fulmar compared to the “deep, steady” wingbeats of many gulls?

Not everything Gaston writes about is entirely accurate, though. When he talks about how birds use the feet differently for take-off and for swimming, he draws an analogy between swimming humans and swimming birds that is a bit off:

The use of alternate strides at take-off, where the feet may function to raise the body off the water more than to create forward thrust, probably maintains greater continuity of thrust between wing-strokes than would be possible with feet together. However, the preference for using feet together underwater, rather than alternating, is harder to explain. It is consistent with the practice in frogs and marine mammals, but contrasts with the most efficient form of swimming in our own species (crawl beats breaststroke) and the technique preferred by Polar Bears. (61, my emphasis)

I’m not familiar enough with the underwater propulsion of birds to comment on relative efficiencies of alternate versus simultaneous recovery motions, but I can say that Gaston has never had to go through entire weeks of swim practice with an injured shoulder. Anyone who has, and remembers the hours upon hours of “kicking-only” practices will attest that underwater “dolphin” kicks, with feet together, are much more efficient than  the crawl kick, with alternating legs. There’s almost no comparison in speed and power.

Of course, Gaston is talking about the underwater kick motion of birds, which apparently recover to the side, rather than underneath (except for penguins, right?), and who can’t do the dolphin kick, presumably “because the rigid design of the body skeleton, in which the majority of the vertebrae are fused, prevents them from developing the sinuous body motion evident in seals and dolphins” (61). But even the breaststroke kick performed underwater is relatively more efficient for humans than the freestyle kick, when you throw in the arm motion that is occurring simultaneously. There simply isn’t an “underwater crawl” motion that would make sense, hydrodynamically.

Here are Gaston’s conclusions on the evolutionary adaptations that make the seabirds as a group distinct from landbirds:

The things that are most characteristic of seabirds, compared with other birds, are their well-developed salt glands, webbed feet and the rearward position of the legs. [… S]pecialisations for marine life do not necessarily preclude a return to land. On the other hand, landbirds are totally excluded from moving out to sea by the wetability of their plumage. It is the development of methods for staying warm and dry at sea which constitute the essential element in colonising the sea. (66)

To sum up: Seabirds have all evolved to handle one important ecological requirement: finding enough food in a large, unpredictable offshore environment that they can not only ensure their own survival, but that of their offspring as well. Different species have done this in a couple of different ways. Albatrosses, for example, have huge bodies and wingspans, and spend days on the wing foraging over vast distances to gather enough food to get back to the nest and feed their chick. Auks, on the other hand, have small bodies, remain fairly close to shore, and make repeated trips daily to feed their offspring. But what they have in common is their adaptation to the marine environment: the seabird syndrome.