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.

While it is currently unknown how mixed-species foraging flocks originate, researchers have proposed a few mechanisms for their initiation. Many believe that nuclear species play a vital role in mixed-species flock initiation. Additionally, the forest structure is hypothesized to play a vital role in these flocks' formation. 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 packs of foraging golden-crowned warblers might play the same role.

Mixed-species foraging flocks tend to form around a "nuclear" species. Researchers believe nuclear species both stimulate the formation of a mixed-species flock and maintain the cohesion between bird species. They tend to have a disproportionately large influence on the flock. Nuclear species have a few universal qualities. Typically, they are both generalists that employ a gleaning foraging strategy and intraspecifically social birds. "Associate" or "attendant" species are birds that trail the flock only after it has entered their territory. Researchers have shown that these species tend to have a higher fitness following mixed-species foraging flocks. The third class of birds found in mixed-species flocks have been termed "sentinel" species. Unlike nuclear species, sentinels are fly-catching birds that are rarely gregarious. Their role is to alert the other birds in the mixed-species flock to the arrival of potential predators.

Ecologists generally assume that species in the same ecological niche compete for resources. The formation of mixed-species flocks demonstrates a possible exception to this universal ecological assumption. Instead of competing with one another for limited resources, some bird species who share the same food source can co-exist in mixed-species flocks. In fact, the more similar body size, taxonomy, and foraging style two bird species are, the more likely they are to be found cooperating in mixed-species flocks. Researchers have proposed two primary evolutionary mechanisms to explain the formation of mixed-species flocks. The first mechanistic explanation is that these different bird species cooperate to gain access to more food. Studies have shown that birds in mixed-species flocks are more likely to spot potential food sources, avoid already exploited locations, and drive insects out of hiding. The second mechanistic explanation is that birds join mixed-species flocks to avoid predation. A bird reduces its risk of being eaten when it is surrounded by other birds who can be potential food for the predator instead. Other studies have hypothesized that multi-species flocks form because large groups reduce a predator's ability to single out one prey, while others have hypothesized that multi-species flocks are more likely to spot predators.

Mixed-species feeding flocks are not purely beneficial for their member species. Some bird species suffer a higher cost when joining mixed-species flocks. Studies have shown that some bird species will leave their standard optimal feeding area to travel to a worse foraging location in order to follow the path of a mixed-species flock. Birds may also be forced to change their foraging strategy in order to conform with the flock. Another third proposed cost of mixed-species flocks is an increased 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.

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 vireos and woodpeckers, and also improves feeding efficiency, perhaps because arthropod prey that flee one bird may be caught by another.

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, or three-striped warblers elsewhere in Central America. In South America, core species may include antbirds such as Thamnomanes, antshrikes, Furnariidae (ovenbirds and woodcreepers) like the buff-fronted foliage-gleaner or the olivaceous woodcreeper, or Parulidae (New World "warblers") like the golden-crowned warblers. In open cerrado habitat, it may be white-rumped or white-banded tanagers. Core species often have striking plumage and calls that attract other birds; they are often also known to be very active sentinels, providing warning of would-be predators.