Birds underwater

a variety of avian submersion strategies

Many kinds of bird regularly forage for prey underwater. These birds have a variety of ways of doing so and adaptations to match. Life in the water is very different from life in the air.

The first hurdle to overcome is simply getting there. Some species start from the water surface. A few are able to just sink below the surface by decreasing their buoyancy: small grebes and anhingas do this by compressing the plumage (thus pushing air out) and exhaling. Others tuck their heads and kick with their webbed or lobed feet (e.g., mergansers, goldeneyes, buffleheads, most cormorants, loons, and some grebes) or flip their wings (murres, long-tailed ducks, dippers). Those that surface-dive a lot (e.g., loons) typically have legs set well back on the body, making them awkward on land. 

Another way to get underwater is from above the surface.Dippers often dive into a stream from a rock or low-hanging branches not far from the water surface. Kingfishers may plungefrom several meters above the surface, folding the wings closer to the sides. Brown pelicans can dive from a height of twenty meters, extending the neck and angling the wings back, making a more streamlined shape. The grand champion divers may be seabirds called gannets and boobies; they can start a dive from almost a hundred meters up, turning the body into a sleek dart, with the neck well-extended and the wings held back close to the body. The dives can reach a speed of sixty mph; to protect the bird from the resulting high impact, the skull is reinforced and subcutaneous air sacs on the chest and sides cushion the jolt.(https://vimeo.com/319325491. By Bob Armstrong, in Loreto, Mexico.)

Belted kingfisher diving and rising with fish. Photo by Bob Armstrong

Most of these dives are quite shallow, but some species are adapted for deeper ventures, with heavier, stronger bones than other birds, to resist water pressure and decrease floatation. Gannets are quite deep divers, sometimes going on down to twenty meters. Loons may dive as deep as seventy-five meters and some of the murres and their relatives go down over a hundred meters; the common murre is said to be the deepest diver (sometimes down to 180 meters) in Alaska. Penguins often launch from ice-ledges; small ones make fairly shallow dives, but the emperor penguin can dive down more than five hundred meters!

The second hurdle to underwater foraging is locomotion in a medium that is denser than air. Most aquatic birds have webbed feet, often set far back on the body for good propulsion and steering; grebes have broadly lobed toes instead. But fancy feet are not always sufficient—some of these birds use their wings to swim in pursuit of prey. Gannets and cormorants can wing-it underwater; murres and puffins have narrow, stiff wings adapted to underwater ‘flight’ (without forsaking aerial flight); penguins swim with their flipper-like wings (and cannot fly) and steer with their webbed feet (some of them are very fast swimmers, clocked at over twenty mph).

Murre underwater. Photo by Bob Armstrong

Kingfishers and dippers don’t have webbed feet, so they have their own ways of moving in water. Kingfishers seem to rise buoyantly to the surface after a dive, wing-fluttering as they lift back into the air. Dippers have strong toes for clinging to rocks and walking even in fast currents, and they swim with their wings for short distances in pursuit of prey; they are the only songbird known to do so and do not have the same adaptions of bones and wings as other, more aquatic, birds do.

Plumages of birds that forage underwater are generally dense and well-waterproofed with oils from the preen gland. Penguinplumage has unusually many tiny filaments that hold air bubbles; when the bird swims, the bubbles are released, which decreases the density of water around the body, allowing faster swimming. Birds that decrease buoyancy by compressing the feathers might get a little of this effect, but penguin plumage can hold more bubbles and release them more gradually.

Diving birds hold their breath underwater, storing oxygen in their lungs. But they can also store extra oxygen in their muscles, in a compound called myoglobin–which, like hemoglobin, is a specialized protein with iron-containing compounds that hold oxygen. Species that engage in long dives and underwater pursuits have more myoglobin than those that spend shorter times without access to air. Emperor penguins can stay underwater for twelve minutes or more (for comparison, humans can normally manage to hold breath for less than two minutes).

Bird stories

nest-builders, scat-shifters, and spring singers

One day in late April, two friends and I scrambled up a steep stream-side slope to a perch on a cliff below a waterfall. We hoped to locate a nest of American dippers, which have nested in this spot for many years. Although a dipper sat near the pool below the falls, it eventually just flew up over the falls, and we were no wiser about a possible nest location.

However, as we surveyed the pool and falls, another bird was busy, attending to a clump of moss on a spruce branch above us. A male Pacific wren (formerly known as the winter wren) zipped back and forth, carrying twiglets to that mossy clump, which was obviously intended to become a nest. Male wrens commonly build more than one nest, which are inspected by females during the courtship process. When a female selects one of these male-built nests, she adds a little threshold to the entrance, claiming that nest as her own. If a male builds several good nests, he may attract two or even three females who will raise his chicks.

arnie-dipper1
American dipper. Photo by Arnie Hanger

This nest- building male became disturbed at our presence, fidgeting about while peering at us and then finding an elevated perch from which he sang loudly, as if to make sure we knew we were not welcome (songs are how songbirds advertise ownership of their territories). Birds really do not like to be observed when nest-building—egg- and chick-predators such as Steller’s jays are always on the watch for tasty morsels, and the busy activity of a bird carrying nesting material gives away a prospective nest for the jay or other predator to raid.

We go the male’s message and backed away a little. Although he was still nervous, he resumed carrying small twigs and fibers to the growing nest in the ball of moss. Suddenly, the entire bottom of the nest ball fell out! Apparently, the scrawny twigs of the spruce branch weren’t sufficiently substantial to support the structure or all the in-and-out visits of the builder. The wren vanished into the forest.

A couple of weeks later, I returned to this site. Now the tattered remains of the wren’s nest were tipped catty-wompus, barely clinging to the frail spruce twigs. The wren had clearly abandoned this effort and decided to build elsewhere. I could hear him singing, a little deeper into the woods.

On this visit, however, I did see the dippers in action. They were not building in their traditional site in the cliff beside the falls, but under a mid-stream log, instead. These dippers were a bit late in getting started; dippers on some other streams were already incubating clutches of eggs, the incubating females sometimes fed by the male.

In the middle of May, Gold Ridge still had lots of snow, attracting brave or foolhardy skiers up the trail. Ravens were soaring and cavorting, as usual, over the end of the ridge, sometimes peeling away from the group to roll and tumble acrobatically or to chase a passing eagle.

Two ravens perched on a rock outcrop. Both birds picked up something lumpy and white and moved behind another outcrop just uphill. They came back to the first outcrop without the white lumps and picked up two more. They flew downhill a little way, and there they deposited these objects, carefully placing them in nooks and crevices of the rock. Then they flew away.

This time I could see where the white lumps were placed, so of course I went to look. The lumps were the old scats of a wolf and perhaps a bear, all dry and winter-whitened. What in the world did these ravens want with these old scats? Were they playing some kind of game?

Robins and fox sparrows were singing all over the shrubby slopes above the tram. Above the cross, snow still covered much of the ground, and ptarmigan had left the digested remains of their dinners in the places where they had burrowed under the snow in winter. A flock of pipits flew in and began to forage for insects and perhaps a few seeds in the snow-free patches. Pipits look more slender than sparrows and they typically walk and run instead of hop. They sometimes nest high in the alpine tundra on the ridges.

On my home pond, the mallard battles are over. As many as four males hang out amicably, eating seeds that drop from the hanging feeder and sleeping next to each other. No need to fight now; all the females are incubating clutches of fertilized eggs. This is a big contrast to early-season relationships, when each male fiercely defended his female from the attentions of other males. That doesn’t always work, by the way—the interlopers are sometimes successful. Meanwhile, up on Gold Creek, a pair of harlequin ducks was consorting and foraging. She will nest up there somewhere, and when the clutch of eggs is complete, she will incubate them and he will go back out to sea to lollygag with his chums on some rocky point. That’s the way of it, with ducks!

How young birds learn

out of the nest and into the stream

Someone saw a photo of a fledgling dipper in a local stream and asked me how young dippers learn to do all the things an adult must do—fly, swim, dive, forage, and (by next spring) sing the proper song and engage in courtship and chick care. The simple answer to this question is that I don’t know—and I don’t think anyone else has a precise answer either, because the development of behavior in this species has not been studied well.

But we can make some semi-educated guesses about some of it. Rather than dealing with the vast literature on learning by birds in general, perhaps it is useful to focus on this local charismatic species as a way of introducing the subject of learning in birds (even if there are still a lot of unknowns for this particular species). It seems likely that much of the education of juvenile dippers occurs by some observational learning (i.e., watching parents) mixed with a lot of trial and error. In other words, a lot like human toddlers.

Just before leaving the crowded nest, young dippers start to flap their puny wings, but there is not much room for wing action. When they jump out of the nest, they flap, and flap some more, building up strength as they flutter from rock to rock. Their flight feathers develop more, and with increased strength and better feathers, they flap more effectively. Parents may encourage them by holding a juicy prey item at a little distance, telling the chick to come and get it. From there on, it is probably a matter of trying variations in coordination and seeing what works, and then a good deal of practice.

Swimming skill may develop gradually. It may be innate (genetically programmed) to paddle the feet when the fledglings land on water, but then they need to practice and see what moves work best. They need to learn to use the currents and eddies to best advantage. Diving is another matter altogether. Do they see their parents do it and try to imitate? Or do they start diving spontaneously, even if parents are not demonstrating?? When do they begin diving? Who knows?

All the aquatic activity requires the ability to shed water from the feathers so the bird does not get waterlogged, and that, in turn, requires dressing the feathers with oil from the big gland at the base of the tail. So it probably takes several days before the chicks figure out how to preen their feathers properly, so that swimming and especially diving can be done without getting soaked. When the feathers get soaked, all that nice insulation that keep the bird warm in the cold water becomes useless. Furthermore, a soggy cloak of feathers doesn’t hold much air, so a wet bird is less buoyant when surface swimming. Preening is thus an important activity, but when it becomes fully functional is a mystery at present.

american-dipper-juv-and-adult-by-bob-armstrong
Photo by Bob Armstrong

Young dippers have some experience with bugs and fish that are brought to them by their parents. They may recognize some of these prey items when they venture out of the nest and start peering into the water. But if they are like baby barnyard chickens, they probably poke and peck at some inappropriate and inedible items too, before they hone their prey-recognition skills.

After two or three months, juvenile dippers are fairly proficient at flying, swimming, diving, and foraging.

They need to be! The short days of winter and the (usually) cold weather mean that food-finding must be really effective, to get the energy needed to keep warm. And come spring, the new adult must be ready to hold a territory, find a mate, and care for chicks of its own.

How do dippers learn their proper song??I don’t think that song-learning has been studied in this species, and it is hard to make decent generalizations, because song-learning varies hugely among species. For instance, in some species, the song is entirely innate (e.g., brood parasites, whose chicks are reared by foster parents of other species), but this is less likely to be true of dippers, who (like most birds) raise their own young. In many songbirds, there is a critical, limited period for learning the proper song from their parents and neighbors; beyond a certain age, no further song-learning occurs. Still others go on learning new songs their whole lives, adding new notes and patterns to the repertoire and deleting others (e.g., mockingbirds). For dippers, it is likely that songs are learned, and polished with practice, but the specific details of song development are not known. However, even very young fledglings start to sing a soft sub-song, especially when their siblings are near, so the process begins early.