Mixed species flock

A mixed-species feeding flock, also termed a mixed-species foraging flock, mixed hunting party or informally bird wave, is a flock of usually insectivorous birds of different species that join each other and move together while foraging. These are different from feeding aggregations, which are congregations of several species of bird at areas of high food availability.

A mixed-species foraging flock typically has "nuclear" species that appear to be central to its formation and movement. Species that trail them are termed "attendants". Attendants tend to join the foraging flock only when the flock enters their territory.

How such flocks are initiated is under investigation. In Sri Lanka, for example, vocal mimicry by the greater racket-tailed drongo might have a key role in the initiation of mixed-species foraging flocks, while in parts of the American tropics noisy packs of foraging golden-crowned warblers might play the same role. Forest structure is also believed to be an important factor deciding the propensity to form flocks. In tropical forests, birds that glean food from foliage were the most abundant species in mixed-species flocks.

A typical Neotropic mixed feeding flock moves through the forest at about 0.3 kilometers per hour (0.19 mph), with different species foraging in their preferred niches (on the ground, on trunks, in high or low foliage, etc.). Some species follow the flock all day, while others – such as the long-billed gnatwren – join it only as long as it crosses their own territories.

Several evolutionary mechanisms have been proposed to explain the formation of mixed-species flocks. These are usually described in terms of the costs and benefits to individuals. The key benefits that have been suggested are a reduction in predation risk through increased vigilance, that is, more eyes that can spot predators and raise an alarm and increased foraging efficiency. Costs could include the risk of kleptoparasitism.

In the North Temperate Zone, they are typically led by Paridae (tits and chickadees), often joined by nuthatches,treecreepers, woodpeckers (such as the downy woodpecker and lesser spotted woodpecker),kinglets, and in North America Parulidae (New World "warblers") – all insect-eating birds. This behavior is particularly common outside the breeding season.

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