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).

Winter wildlife extravaganza

in Juneau’s Auke Bay

During late November and early December, 2015, Auke Bay harbor put on a wildlife spectacular, drawing photographers, reporters, and just plain gawkers (such as me). Hordes of young-of-the-year herring, mixed with a few capelin and sand lance, milled around the docks and boats. The banquet of small fish also drew many predators, who put on a good show for observers.

Why are there so many young herring in the harbor this year? There are possibly several reasons, suggests Michelle Ridgway (Oceanus Alaska). It may have been a good spawning season in spring. The sunny spring, plus an El Niño, warmed the harbor waters, even at considerable depths, and all the spring run-off from the soggy land brought in nutrients. Those conditions produced a fine bloom of phytoplankton, which led to good body condition and burgeoning populations of zooplankton. For example, Ridgway has noticed extended reproduction of little shrimp-like crustaceans called mysids, extra-large fat globules in copepods, an abundance of amphipods not far from the surface, and an unusual influx of ‘sea butterflies’ (molluscs that fly through the water). That made excellent foraging for baby herring. In addition, young herring may seek protection from the massive maws of humpback whales by moving into shallower bays and harbors, with docks and boats, where the whales are less likely to forage intensively.

Herring and other so-called forage fishes often form densely packed balls, especially when predators are lurking about. When a predator dives through the ball of fish, the survivors scatter in all directions, but not far and only briefly, before returning to the tight cluster. Researches have called this behavior ‘the geometry of the selfish herd’: each fish trying to put as many other little fish as possible between itself and predators. The result is a tight ball of nervous, jittery fish.

The baby herring in the harbor had every reason to be jittery. The millions of small fish were being attacked on all sides by throngs of predators. They may have eluded most of the whales in the confines of the harbor, but other predators took advantage of the great aggregation.

A gang of Steller’s sea lions cruised rapidly back and forth, diving continually, probably after pollock that were gorging on the herring. The pollock drove the little fish toward the surface. The sea gulls knew this, of course, and hung about, just waiting for the fleeing fish to get close to the surface where the gulls could nab them. Indeed, the fish were caught ‘between the devil and the deep’ (the gulls and the aquatic predators respectively).

Several harbor seals were there, some with well-grown pups. They did their share of fish-driving too, but usually not near the sea lions. I watched one seal surface-swimming slowly along, on its back, in a most relaxed fashion. It may have been looking down into the depths, for eventually its head went down, followed by the plump body, into a mob of fish.

Scattered Pacific loons and little clusters of common mergansers foraged away from the biggest crowds of predators. Marbled murrelets in snazzy winter plumage could be observed at close range; they were much less skittish than in the breeding season.

The most amazing sight was the huge flock of common murres—many hundreds of them. They rafted up just beyond the last float and split off occasional smaller bunches that moved in among the inner floats. They, and everybody else except the gulls, avoided the sea lions that charged to and fro. I had never seen so many murres before, except at the St Lazaria nesting colony on the outer coast. The murres talked to each other constantly, except when they were diving.

murres-in-Auke-Bay-Jos
Photo by Jos Bakker

That huge concentration of murres was arguably the most unusual happening in the harbor. All the other predators visit the area rather regularly to feast on small fish that spend the winter there. Although murres nest on the outer coast, they tend to move closer to shore in winter, congregating where prey is abundant. But we don’t customarily see the murres in such numbers in Auke Bay harbor. Furthermore, over near Glacier Bay, good observers reported uncountably huge numbers of murres moving about.

The throngs of murres may be a sign of bad news, however, according to John Moran (NOAA). A mysterious oceanic anomaly in the Gulf of Alaska called the Warm Blob (because water temperatures are as much as five degrees (F) above average) created nutrient-poor conditions that greatly reduced productivity and thus decreased the abundance of fishes that feed on plankton, or at least caused them to move to deeper waters where diving birds can’t get them. The Blob developed in 2013 and its effects have contributed to reduced nesting success and great mortality of some marine birds in the Gulf. The poor food supply may have been one factor that drove the murres we’ve seen in Auke Bay out of their usual foraging areas in search of better feeding conditions.

All those baby herring in the harbor seem to offer a ready banquet but, in fact, those little fellows have very little fat because they put their energy into growing as fast as they can. On a gram-for-gram basis, they are much less rewarding than capelin or adult herring, for example, and even less than krill, according to data of Moran and colleagues. So a murre or any other predator would need a lot of them in order to survive—and that’s certainly what was available in the harbor.

Humpback whales were reported to pass by the harbor upon occasion, but there are other spots where they might find better foraging. Seymour Canal is a good place for foraging on krill, for example, and adult herring (far more nutritious than the young ones) from all over Lynn Canal winter in deep, dense schools northward of Tee Harbor. When a whale dives deep through such a school of herring, some of the fish try to escape up shallower water, but there the sea lions can get them. Sea lions themselves may attract the attention of transient killer whales; a few years ago, Moran watched killer whales take down five sea lions (plus two probable kills) in five days. But in Auke Bay harbor, the foraging sea lions were quite safe from the killers.

Eagles were notably scarce in Auke Bay harbor during this extravaganza, although they are known to feed on murres (we saw the evidence in Berners Bay one spring). Perhaps the eagles sought out the adult herring to the north.

A little squad of goldeneye ducks quietly kept to themselves along a rocky shore of the harbor. Seemingly uninterested in the shimmering mass of herring, they may have been looking for molluscs.

Thanks to John Moran of the NOAA lab and Michelle Ridgway of Oceanus Alaska for extensive discussion, not all of which could be packed into this essay.