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Caves, hummingbirds, and rarities
Answers to readers' questions about nesting, hovering, and birding
Contributed by Julie Craves
Published: June 24, 2011
Do any birds live in caves? — Chuck Finley, Miami, Florida
A few species utilize caves during at least a part of their annual cycles. Here in North America, the Cave Swallow nests in caves in Mexico, the Caribbean, Texas, and New Mexico. (See “The Cave Swallows of Carlsbad Cavern,” October 2010, page 22.)
Perhaps the most remarkable is the Oilbird (Steatornis caripensis), a large South American bird similar to nightjars. (It’s in the same order but its own family.) The unusual bird roosts in caves during the day, and it departs at night to feed on ripe fruit, making it the world’s only nocturnal fruit-eating bird species. (See “Amazing Birds,” October 2007, page 56.)
Like bats, Oilbirds have the ability to use echolocation to avoid collisions in the darkness of caves. The sounds used for echolocation are described as rapid clicks or clucks, and they join a weird vocal repertoire of screams, shrieks, snores, and snarls. The birds also have excellent night vision, and it is thought they may also use their sense of smell to locate fragrant, ripe fruit.
They have strong, hooked bills, which they use to pluck fruit from trees, most often by hovering or diving. Each fruit is swallowed whole, and the seed is later regurgitated or defecated. Therefore, Oilbirds perform an important ecological function by spreading seeds throughout the tropical forest.
Oilbirds are gregarious and typically nest in colonies inside caves. Nests are made of regurgitated or rotten fruit, mud, or droppings. Young Oilbirds are fed partially digested and, later, whole fruit. The birds got their name because one of their most common food items is the fruit of the oil palm.
Other birds often found in caves are Asian swiftlets. The Edible-nest Swiftlet of southeast Asia, for example, nests in caves and on cliffs. Bird’s-nest soup, a delicacy in Chinese cuisine, is made from its nest.
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This spring, I saw a hummingbird hovering near a spider’s web. I know hummingbirds eat small insects. Could this one have been trying to eat the spider or maybe a prey item caught in the web? — Herman Collins, Phoenix, Arizona
It’s possible that the hummingbird was foraging, but it’s more likely that it was gathering a portion of the spider’s web for use in building a nest.
Nearly all hummingbirds use silk in their nests. It’s strong and flexible and is used to strengthen the overall nest structure; to secure the nest to a tree limb; or to anchor lichen, bits of leaves, or other materials to the nest to provide camouflage. In addition to spider’s webs, the birds may obtain silk from spider egg sacs or the cocoons or silken day shelters of caterpillars.
Other bird species that use spider or caterpillar silk in nest construction include flycatchers, gnatcatchers, kinglets, vireos, warblers, American Goldfinch and other finches, and many tropical birds.
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A fairly rare bird showed up near us some time ago, in what seemed to me to be a pretty unremarkable woodlot. Within a few days, several other uncommon species were located at the same spot. Someone called this a “picnic-table effect,” but I don’t know what that meant. — Carlton Lundy, Ocean City, New Jersey
The full name of the phenomenon is the Patagonia Picnic Table Effect. It refers to a situation in which a rare bird attracts many observers, who in turn discover more rare birds at the same spot. I believe there is a corollary occurrence in which excellent birders often get spectacular birds in their yards. (I saw my life Harris’s Sparrow and Mountain Bluebird in the same yard in rural southwest Ontario.) What it boils down to is that hotspots are often where the birders are, and rare birds are probably more common than we think.
The name comes from a small roadside rest stop in Patagonia, Arizona. Many years ago, it hosted a pair of Rose-throated Becards, rare birds in the United States. They attracted scores of birders, and, as you may have deduced, the steady stream of visitors found a number of other unusual species. The rest stop is nothing special. I’ve been there myself and can’t recall if a picnic table is actually present. The becards were there, though!
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