the underwater hunter

Quite by accident, while I was out in the Grass Lake marshes looking for the ospreys that should have been near their platform nest (the one usurped by a Canada Goose), a lone Common Loon popped up right in front of me.

Common Loon-

The morning had the perfect kind of filtered sunlight to bring out all the “color” in this black and white bird. For example, I’ve never noticed before how just the right angle of light turns the usually dark feathers below its white necklace into a turquoise band.  The loon’s head feathers are actually faintly iridescent.

During the breeding season, both male and female loons have brilliant red eyes, which might well be an indicator of their readiness to mate.  But loons are fiercely territorial and protective of their chicks, driving off other loons or intruders (like Canada Geese) far larger than they are, and it has been suggested that their red eyes are a threat advertisement of their presence.

Common Loon-viewing fish underwater

The loon frequently dipped its head below the water while paddling around.  It looks like a snorkeler checking out underwater life.

Common Loon-

Several times, following a bit of snorkeling, the loon would quickly dive below the surface, never making a sound or a splash.  I never saw it bring up a fish to swallow, but loons often swallow their smaller prey while underwater.

The Common Loon, or Great Northern Diver, as it is known throughout its range in Eurasia, has some unusual anatomical adaptations for its underwater life:

  • solid, instead of the hollow bones that characterize most birds, decrease the bird’s buoyancy in water and allow it to sink quickly during a dive
  • legs are placed so far to the rear of its body, loons can’t stand up on land, but must push themselves forward on their belly
  • rather short wings decrease drag as the bird propels itself through the water, but the trade-off is that reduced lift provided by the wings requires a long space for taking off into the air

But how does a bird that depends on its eyes for hunting underwater see both in air and in water?  Some have suggested that the birds’ third eyelid (nictatating membrane) which usually has a protective function, is more transparent in loons and other diving birds, and acts like a pair of goggles to preserve an air space between the pupil and the water.  Check out the video below for a good illustration of the underwater swimming activity of these unusual birds.

Common Loon-

Cormorant capers

While taking a walk around Lake Temescal in the Berkeley, CA hills the other day, I noticed there were a few Cormorants in among the rest of the aquatic inhabitants of the lake.

waterfowl temescal park

Double-crested Cormorants (bottom center) among Ring-billed Gulls, Mallard ducks, Coots, Pigeons, and a stray female Wood Duck at Temescal Lake.

The Cormorants must have just finished their morning meal because most of them were intent on spreading their wings to dry off.

double-crested cormorant

Any structure will do as a drying platform — and their big webbed toes provide stability on this tippy buoy. Note that Cormorants do not obey the “restricted area” warnings…

double-crested cormorants

Cormorant feathers are “wettable”, unlike those of ducks and many other aquatic birds.  This reduces their buoyancy as they dive for food.  But the disadvantage of lacking the water-repellent oil on their feathers is that they have to dry off after a feeding bout in order to regain aerodynamic efficiency.

double-crested cormorants

After I got too close to them on the beach, this pair swam off together, still holding their heads up at the same angle as when they stood on beach sand.

That strange head positioning, like a person tilting their head to look out through the bottom of their bifocal lens got me to wondering why they maintain their heads at that angle.  Is there something peculiar about Cormorant vision related to their prowess as underwater fish catchers?

Some researchers speculated that Cormorants might possess superior underwater vision because they are so successful in capturing prey in 70-80% of their attempts. But, in fact, their visual acuity is no better than ours (without a dive mask) underwater, so how do they do it?

double-crested-cormorant

And how do they manage to catch anything in water like this?

Cormorant visual acuity is definitely poorer under conditions of low light, high turbidity, and in open water lacking any sort of underwater structures.  In fact, they do far better in habitats where they can chase fish out of their hiding places.  And they will strike at anything that moves — fish or other, moving objects — by lunging at the intended prey with a rapid neck extension, much like herons and egrets striking from above down into the water.  So they are apparently cued by movement, not definitive shape of a prey item.

double-crested-cormorant-National Geographic-by Sandy Scott

Cormorants can rotate their eyes independently forward, up, down, etc.  But when they need to see what it at the end of their bill, the eyes turn inward, and with head slightly up, they can see straight ahead with binocular vision.

Once they have snatched something in one of their diving forays, they bring it up to the surface to inspect it, and powerful extra-ocular muscles move the eyes to focus on the tip of their bill to see what they have caught.  If it’s edible, down the gullet it goes; if not, they spit it out.

Diving for food

Brown Pelicans soar high above the ocean scoping out the site of their next dive, then fold their wings and drop headfirst like an arrow toward the surface.

This bird is circling to get into position for its dive, probably 100 feet above the school of fish it is following.

This bird is circling to get into position for its dive, probably 100 feet above the school of fish it is following.  Does greater height help them see the fish schools more clearly?

Brown Pelican

Brown Pelican diving

Brown Pelican diving

An Olympic 10 for little entry splash!

An Olympic 10 for little entry splash!

How do they keep from bashing their heads on the rocks?

How do they keep from bashing their heads on the rocks?

Brown Pelicans are one of the two pelican species that dive for their food — the other species hunt much closer to the water surface. Hitting the water surface continually from great height would seem to be injurious, but these pelicans buffer the impact with air sacs (part of their respiratory system) located between their neck and body that absorb the shock.

Brown Pelican