Gleaning (birds) - Techniques and Adaptations

Techniques and Adaptations

Foliage gleaning, the strategy of gleaning over the leaves and branches of trees and shrubs, can involve a variety of styles and maneuvers. Some birds, such as the Common Chiffchaff of Eurasia and the Wilson's Warbler of North America, feed actively and appear energetic. Some will even hover in the air near a twig while gleaning from it; this behavior is called "hover-gleaning". Other birds are more methodical in their approach to gleaning, even seeming lethargic as they perch upon and deliberately pick over foliage. This behavior is characteristic of the Bay-breasted Warbler and many vireos. Another tactic is to hang upside-down from the tips of branches to glean the undersides of leaves. Tits such as the familiar Black-capped Chickadee are often observed feeding in this manner. Some birds, like the Ruby-crowned Kinglet and Red-eyed Vireo of North America use a combination of these tactics.

Gleaning birds are typically small with compact bodies and have small, sharply pointed bills. These features are even seen in gleaning birds that are not closely related. For example, in flycatchers of the family Tyrannidae, in which some member species are more adapted for hawking insects on the wing and others for gleaning, the gleaners have bills that resemble those of tits and warblers, unlike their larger-billed relatives. Also, some members of the woodpecker family, particularly piculets such as the Rufous Piculet of Southeast Asia, are similarly adapted for gleaning, with small, compact bodies and sharp bills, rather than the long, supportive tails and wedge-shaped bills more typical of woodpeckers. Birds such as the aforementioned piculet are specialized for gleaning the bark of trees, as are nuthatches, woodcreepers, and treecreepers. Most bark-gleaners work their way up tree trunks or along branches, though nuthatches are well known as the birds that can go the opposite direction, facing down and working their way down the trunk, as well. This requires strong legs and feet on the part of the nuthatch and piculet, while birds that face upwards tend to have stiff tail feathers to prop them up.

Birds often specialize in a particular niche, such as a particular stratum of forest or type of vegetation. In South and Southeast Asia, for example, the Mountain Tailorbird is often found gleaning in thickets and stands of bamboo, Abbott's Babbler gleans lower-storey foliage in lowland forest, the Rufous-chested Flycatcher and Brown Fulvetta are birds of the mid-storey forest, the Yellow-breasted Warbler gleans in the mid- to upper-storey, and the Greater Green Leafbird specializes in the upper-storey forest. The Javan White-eye is a bird of coastal scrub and mangroves, while the related Black-capped White-eye is restricted to montane forest.

Further specialization within a habitat is associated with behaviors and morphological adaptations (physical traits of size and shape). Tiny birds are lightweight enough to hang onto the ends of twigs and pluck small prey; the Goldcrest of Europe and its counterpart the Golden-crowned Kinglet of North America exhibit this feeding style. The related Common Firecrest is very similar in size and shape, but slightly bulkier, and has less of a tendency to glean along twigs and more of a habit of flying from perch to perch. Having a very small bill seems to be good for taking tiny prey from the surfaces of leaves, and small-billed birds such as the Blue Tit forage in broad-leafed woodlands. The Long-billed Gnatwren and Speckled Spinetail of Central and South America, and the Ashy Tailorbird and Striped Tit-babbler of South Asia, show a preference for gleaning in tangles of vines. The Ash-browed Spinetail of South America specializes in gleaning among epiphytes on moss-covered tree branches. Many hummingbirds take small insects from flowers while probing for nectar, and some species glean actively among bark and leaves. The Puerto Rican Emerald is one such hummingbird. Found only on the island of Puerto Rico, the female subsists on insects and spiders, while the male has a typical hummingbird diet of nectar. Hummingbirds and other gleaners are also sometimes attracted to the sap wells created by sapsuckers. Sapsuckers, which are in the woodpecker family, drill small holes in living tree branches to get the sap flowing. The sap and the insects it attracts are then consumed, and Rufous Hummingbirds have been observed to follow the movements of sapsuckers and take advantage of this food source. Clusters of dead leaves also often harbor invertebrate prey, and the Bewick's Wren and Worm-eating Warbler of North America have long bills well-suited for probing them, as do certain Asian babblers, such as the Rusty-cheeked Scimitar-babbler. In Central and South America, foliage-gleaners such as the Red-faced Spinetail and Buff-throated Foliage-gleaner are also examples of birds that glean clusters of dead leaves.

Crevice-gleaning is a niche particular to dry and rocky habitats. Adaptations for crevice-gleaning are similar to that of bark-gleaning. Just as the Bewick's Wren, as mentioned in the preceding paragraph, has a long bill suited for poking around in the small places of woods and gardens, another North American wren, the Canyon Wren, has an even longer bill, which allows it to probe crevices in rocky cliffs. It also has skeletal adaptations to aid it in reaching deep into small spaces. These same traits are useful for gleaning the sides of buildings, as well. Another kind of rocky habitat is found along mountain streams, where birds such as the Louisiana Waterthrush of North America and the forktails of Asia pick over stream-side rocks and exposed roots for aquatic insects and other moisture-loving prey.