A mixed-species feeding flock, mixed-species foraging flock or mixed hunting party is a flock of birds of different species that join each other to search for food.
A proverb says, "Birds of a feather flock together," but birds of different taxa often occur together. They may do so at rich food sources (such as animal carcasses, termite swarms, garbage dumps, fruiting or flowering trees, schools of fish) or simply because they share habitat and tolerate each other, as many shorebirds, gulls, ducks, starlings, and icterids do.
They may also travel together. Many seed-eating Passeroidea (finches, sparrows, buntings, etc.) feed in mixed flocks, often with closely related species, in grassland or scrub. However, mixed feeding flocks are especially associated with forests.
How such flocks are initiated is not well known. Certainly, birds of different species gather at rich food sources and thus mixed flocks might spontaneously form. But in Sri Lanka for example, vocal mimicry by the Greater Racket-tailed Drongo (Dicrurus paradiseus) might have a key role in the initiation of mixed-species foraging flocks.
A typical Neotropic mixed feeding flock moves through the forest at about 0.3 kilometers per hour, with different species foraging in their preferred niches (on the ground, on trunks, in high or low foliage, and so on). Some species follow the flock all day, while others – such as the Long-billed Gnatwren (Ramphocaenus melanurus) – join it only as long as it crosses their own territories.
In the Holarctic
In the North Temperate Zone
, they are typically led by Paridae
(tits and chickadees), often joined by nuthatches
, such as the Downy Woodpecker "Picoides" pubescens
and Lesser Spotted Woodpecker "P." minor
), and in North America Parulidae
(New World "warblers") – all insect-eating
birds. This behavior is particularly common outside the breeding season.
The advantages of this behavior are not certain, but evidence suggests that it confers some safety from predators, especially for the less watchful birds such as Vireonidae (vireos) and woodpeckers, and also improves feeding efficiency, perhaps because arthropod prey that flee one bird may be caught by another.
In the Neotropics
Insectivorous feeding flocks reach their fullest development in tropical forests
, where they are a typical feature of bird life. In the Neotropics
the leaders or "core" members may be Black-throated Shrike-tanagers
) in southern Mexico
, Three-striped Warblers
) in Central America
, and Thamnophilidae
(antbirds) such as the Bluish-slate Antshrike
) in South America
. Core species
often have striking plumage and calls that attract other birds. But while such easy-to-locate birds species serve as a focal point for flock members, they do not necessarily initiate the flock: In one Neotropic
mixed flock feeding on swarming termites, it was observed that Red-rumped Warbling-finches
) were most conspicuous. As this species is not an aerial insectivore, it is unlikely to have actually initiated the flock rather than happening across it and joining in.
Other members of a Neotropic mixed feeding flock may come from most of the local families of small diurnal insectivorous birds, and can also include toucans and trogons. Conopophagidae (gnateaters) are notable for their absence from these flocks, while Apodidae (swifts) and Hirundinidae (swallows) rarely join them, but will if there is for example an ant or termite swarm.
However, even of commonly participating families not all species join mixed flocks. There are genera such as Vireo in which some species join mixed flocks and others do not. Of the three subspecies groups of the Yellow-rumped Warbler (Dendroica coronata), only one (Audubon's Warbler, D. c. auduboni group) typically does. Some species appear to prefer certain others: Cyanolyca jays prefer to flock with Unicolored Jays (Aphelocoma unicolor) and the Emerald Toucanets (Aulacorhynchus prasinus species complex). Many Icteridae (grackles and relatives) associate only with related species, but the western subspecies of the Yellow-backed Oriole (Icterus chrysater) associates with jays and the Band-backed Wren (Campylorhynchus zonatus).
In the rest of the tropics
Mixed-species flocks on other continents resemble those of the Neotropics. The core members in Africa
are often Paridae
(tits). In Asia
, they are often Dicruridae
(Old World babblers) and the Green Iora
are typical core members on New Guinea
) and Australia
); in Australia, fairy-wrens
) are also significant.
As in the Americas, the core species are joined by birds of other families such as Pycnonotidae (bulbuls), or minivets (Pericrocotus) of the Campephagidae.
In tropical Asia, where this phenomenon is arguably best developed, flocks may number several hundred birds spending the entire day together, and an observer in the rain forest may see virtually no birds except when encountering a flock. For example, as a flock approaches in the Sinharaja Forest Reserve in Sri Lanka, the typical daytime quiet of the jungle is broken by the noisy calls of Orange-billed Babblers (Turdoides rufescens) and a Greater Racket-tailed Drongo (Dicrurus paradiseus). As the birds pass, the observer can glimpse the quieter, more inconspicuous, members of the flock. If the flock crosses a track, its true numbers become clearer – instead of two Ashy-headed Laughingthrushes (Garrulax cinereifrons) there may be ten, and previously-missed small species such as the Kashmir Flycatcher (Ficedula subrubra) or the Velvet-fronted Nuthatch (Sitta frontalis) reveal themselves.
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