Physical characteristics

Kingfishers are a uniform and distinctive group of birds, all immediately recognizable as members of the Alcedinidae. They are small to medium-sized birds with a large head, long pointed bill, compact body, short neck, small legs, and weak feet. Kingfishers' feet have three front toes that are fused at their bases. Most have a fast direct flight on rounded wings and a short tail. The greatest differences are in overall form; the shape of the bill, from narrow and dagger-like to broad and shovel-like; or the development of long central tail feathers. Species range in mass from the 0.3-0.4 oz (9-11 g) African dwarf kingfisher (Myioceyx lecontei) to the 6.7-16 oz (190-465 g) laughing kookaburra (Dacelo novaeguineae). The sexes of

A pink-breasted paradise-kingfisher (Tanysiptera nympha) in flight. (Photo by C.H. Greenewalt/VIREO. Reproduced by permission.)

A pink-breasted paradise-kingfisher (Tanysiptera nympha) in flight. (Photo by C.H. Greenewalt/VIREO. Reproduced by permission.)

most species are similar in size. In a few species, one or the other sex is slightly larger, but only in the two largest species of kookaburra are the females markedly larger, while in some paradise kingfishers the males have longer streamers to the central tail feathers.

The sexes of most species are also similar in plumage, bill, and foot color, while juveniles are generally similar to adults except that the bill is often a dull black. Most species have at least some iridescent blue or green in the plumage, offset by large areas of black, white, or brown. The bill and feet are black or brown in many species, but in others one or other appendage may be bright yellow, orange, or red. The iris is dark in most species, with only three exceptions. In many of the cerylines and halcyonines, the sexes are distinguishable through differences in the color of the breast bands or back, but only in two species of the alcedinines is there obvious sexual dichromatism.

The bill shape is generally suggestive of feeding habits, being laterally flattened and dagger-like in species that regularly dive into water after slippery aquatic prey, but dorsoventrally flattened and more scoop-like in species that catch small animals on the ground, and especially wide in those forest species that dig in soil or leaf litter for their prey. One species has a hook and another has serrations at the tip of the bill, but both are of unknown function.

The eyes of kingfishers are also specialized for sighting prey. Ganglion cells that connect the light-sensitive cone cells on the retina are especially dense across a horizontal streak, at each end of which is a depression or fovea packed with cone cells and, by its shape, especially sensitive to movement across its surface. The outer or temporal fovea includes the area of binocular vision, while the inner or nasal fovea covers monocular vision and is also especially densely packed with ganglion cells. The angles of the streak and the well-connected nasal fovea coincide with what would be predicted for birds that search below them for prey and are especially sensitive to movement in their peripheral monocular vision. The birds' ability to turn the head through a wide angle allows fixation of the object with the binocular vision of the temporal fovea. The cone cells of kingfishers are also especially rich in the droplets of red oil that signal excellent color vision. One species has already tested positive for vision near the ultraviolet range.

Species that dive into water in pursuit of prey also have to cope with the problem of the different refractive indices of air and water, and the effect that this has on the apparent location of an object due to bending of light rays at the surface. Tests have shown that pied kingfishers are able to compensate for this, mainly by increasing the dive angle and speed as the depth of prey below the water increases. This species, the most specialized piscivore of all kingfishers, also has a bony plate on the prefrontal area that slides across and screens the eyes as the head strikes the water.

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