divers and burrowers

I’ve been thinking about kingfishers lately, although I don’t know what brought them to mind. I certainly haven’t seen any recently—their freshwater fishing spots are mostly iced-over, so they are out along the saltwater shores where I seldom go. Our species is formally called the Belted Kingfisher; males have a blue belt and females have both a blue and a rusty-orange belt across the belly.

With kingfishers on my mind, I found I was recalling a nest I once found. Several years ago, while we were deeply involved in a study of American dippers, I was watching a pair of dippers that had a nest in an old broken-down wooden dam. I sat on a brushy cliff above the nest site, watching the dippers go about their business of raising a family. The dippers paid me no heed, coming and going up- and downstream unperturbed.

Not so the kingfishers, who—I soon discovered—had a nest across the creek. Kingfishers nest in burrows dug one or two yards deep (but sometime more than four yards deep!) in earthen banks, and this one was beautifully inaccessible to almost any predator. The dirt bank was vertical, and the nest entrance was well below the top of the bank and several yards above the stream. No way was I a threat, nor was anything else. But every time they arrived and discovered me on my perch, those parent birds sat in trees just upstream and glared at me for long minutes. Eventually, if I stayed very still indeed, they would go to the nest.

I don’t know if they raised their chicks successfully, because my dippers finished raising their young ones before the long chick-rearing period of the kingfishers was done. However, it is likely that they were successful, because they nested in this same bank for several years. The steep dirt bank eroded a bit almost every fall and winter, but the birds just dug a little deeper, extending the burrow, and carried on.

At the end of a nesting burrow, kingfishers excavate a nesting chamber, big enough for five or six eggs and an incubating adult and, later, for those chicks to grow to full size. The chicks stay in their chamber for about four weeks. The parents, but especially the male, feed them diligently, first on partly digested fish and then on small but whole fish. These parents typically had to fly at least half a mile downstream to find good fishing, because small fish are not abundant in the upper reaches of this creek. Long foraging flights are not unusual for these birds.

Unlike songbirds, kingfisher parents do no nest sanitation. Instead, the chicks squirt the walls of the chamber with their liquid excrement and then pound the walls above the wet spot, putting a thin layer of dirt over it. I have to wonder how that habit got started!

Most of us have seen kingfishers perched on a branch over the water or even hovering over the water surface, looking, looking, looking. And if they spot something good, then down they plunge. Most dives are shallow, not much below the surface of the water. Usually they capture fish, but they can also eat frogs, large insects, crayfish and so on.

An interesting thing happens as the diver enters the water. Think about this: if you reach down into water to grab something, you often find that the object of your reach isn’t quite where you thought it was, and you miss it by several inches. That’s because water bends the light rays more than air does and your eyes don’t compensate for the change of direction. This problem occurs when light crosses a surface between two media at an angle.

Photo by Kathy Hocker

Kingfishers face the same problem. Sighting the prey from the air only gives them an approximate locations to aim for. But their eyes (and those of other birds that pursue active prey) have a feature we don’t have. In the back of a human eye, on the retina, is a little pit (the fovea) where there is a high density of visual cells, so this is an area of very acute vision in one medium (usually air). However, these birds have not one but two foveas, separated by a little distance, and their vision shifts smoothly from one to the other as they enter the water. One fovea works primarily for lateral, monocular vision, with the line of sight directed mostly to the side, but the second one is located where the line of sight converges with that of the other eye, producing very sharp binocular vision over a wide field of view. So when the diver enters the water, the second fovea is engaged, and that gives the diving bird high visual acuity at the all-important last moment before the prey is grabbed in the bill.

I have not found much published information on the success of kingfishers in capturing prey. A study of the Amazon kingfisher (which lives in Latin America) showed that capture success ranged from eighteen to sixty-two percent of attempts, depending on circumstances. A study of our Belted Kingfisher in California found that over half of attempts were successful if the bird was hunting from a perch but only twenty percent if the bird was hovering.