When you think of a dove, what comes to mind? The crowned pigeon of New Guinea, largest dove in the world? The sparrow-sized ruddy ground dove? Or, perhaps one of the many colorful fruit doves scattered throughout the islands of Indonesia?
More than likely, it’s the domesticated rock pigeon, common resident of city streets and sidewalks.
At 348 species, this is one of the largest and most widespread groups of birds in the world. Although pigeons tend to be larger than doves, there is otherwise no clear distinction between the terms, and so, for the sake of this column, I’ll just refer to all of them as doves unless referring to a particular species.
The more common birds are, the more we take them for granted. Mourning doves are common, even abundant, therefore we tend to pay them little mind. But doves have some behaviors and characteristics that set them apart from other birds — characteristics that make them worthy of another look.
That said, the Eastern U.S. has only a handful of dove species. They are the mourning dove, white-winged dove, Eurasian collared dove, rock pigeon and common ground dove. Of these, the rock pigeon and Eurasian collared dove are introduced, or non-native species, the band-tailed pigeon and white-winged dove are rare visitors, and the common ground dove is nearly extinct in North Carolina, despite the name, anything but common. Thus, the mourning dove is the only common native dove in the state.
Among the characteristics that set doves apart from others is the absence of a preen gland. The gland which is present on most other birds is found at the base of the tail. It produces oil which the bird transfers from the gland by squeezing the gland with its bill or by rubbing its head against it, then rubbing against feathers. The oil functions to keep feathers in good condition.
The relatively few species, such as doves, that lack a preen gland possess powder down — continuously growing feathers that constantly disintegrate resulting in a powder that helps preserve and waterproof feathers.
Another unique trait is the way they drink. Most birds drink by dipping the bill in water, then tipping the head back and swallowing. Doves are among few that place the bill in water and suck as if with a straw. This may be an advantage because it allows the bird to take in water more quickly.
The bills of doves are rather weak, but still suitable for consuming a diet primarily of seeds and fruits which they can usually swallow whole. It’s perhaps because of the relative weakness of the bird’s bill that these birds tend to build flimsy nests, some so slight that the eggs can be seen from below.
While most songbirds produce three to five eggs per clutch and many waterfowl and gamebirds often lay a dozen or more, most doves produce only two, and a few species produce only one. That brings us to the most unique feature of doves—the methods of feeding their nestlings.
The young of many birds such as ducks and grouse feed themselves soon after hatching and leaving the nest. They learn what to eat by mimicking their parents. Others, such as robins, are fed by the parent inserting food into the nestling’s waiting gullet. Still others, such as herons, regurgitate recently captured prey directly into the nestling’s throat or on the floor of the nest for nestlings to pick up. Puffins, for example, nest in cave-like burrows where there is little light, and yet within a day or two after hatching, they pick up fish after the parents deliver them, their very first meals, and swallow them whole.
But doves feed their nestlings in a different way.
For many birds, the crop, which is a part of the digestive system just after the esophagus, is largely a food storage container. Seed-eating birds consume food which are stored in the crop until it passes into the stomach for digestion.
The crop works the same way for doves. But when their eggs begin to hatch, the crop undergoes a transformation. The lining of the crop begins to slough off. The resulting substance is called crop-milk and is produced and fed to the nestlings by both male and female. It’s regurgitated into the throats of the newly hatched squabs and is their sole source of food for the next several days.
As the nestling grows, more and more of the food eaten by the parents is incorporated in the crop-milk until the young bird leaves the nest, at which time it’s able to eat the same food as its parents.
While doves are the birds best-known to employ crop-milk, flamingos and penguins, birds with otherwise radically different lifestyles, also share this interesting adaptation.
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