Gleaning (birds)
Gleaning is a feeding strategy by birds and bats in which they catch invertebrate prey, mainly arthropods, by plucking them from foliage or the ground, from crevices such as rock faces and under the eaves of houses, or even, as in the case of ticks and lice, from living animals.
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.
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.
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.
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.
[7] Many hummingbirds take small insects from flowers while probing for nectar, and some species glean actively among bark and leaves.
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.
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.
[10] Clusters of dead leaves also often harbor invertebrate prey, and the Bewick's wren[11] and worm-eating warbler[2] of North America have long bills well-suited for probing them, as do certain Asian babblers, such as the rusty-cheeked scimitar-babbler.
[3] Another kind of rocky habitat is found along mountain streams, where birds such as the Louisiana waterthrush[2] of North America and the forktails[5] of Asia pick over stream-side rocks and exposed roots for aquatic insects and other moisture-loving prey.
Foraging for invertebrate prey on the ground often involves gleaning the leaf litter of the forest floor, sometimes flicking, flipping, or scratching through dead leaves.
Ground-foraging birds can be very hard for humans to observe, as they often occupy densely vegetated habitat, as in the case of the Bornean wren-babbler, which specializes in gleaning leaf litter in gullies in the forest of Southeast Asia.
[5] A feeding technique that is somewhere between gleaning and hawking is where a bird flies from a perch and takes prey off foliage; this is called "sally-gleaning".
These small birds choose a perch within their lush forest and plantation habitats in the Greater Antilles, from which they scan the undersides of leaves above them.
[16] Similarly, on the island of Borneo, the Bornean ground-cuckoo will follow wild pigs and sun bears as they turn up soil while foraging in the forest.
Second, while a bird focuses on examining an area for prey items, it must necessarily divert its attention from scanning its surroundings for predators.
