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 (Lanio aurantius) in southern Mexico, or Three-striped Warblers (Basileuterus tristriatus) elsewhere in Central America. In South America, core species may include antbirds (Thamnophilidae) such as Thamnomanes, antshrikes, Furnariidae (ovenbirds and woodcreepers) like the Buff-fronted Foliage-gleaner (Philydor rufum) or the Olivaceous Woodcreeper (Sittasomus griseicapillus), or Parulidae (New World "warblers") like the Golden-crowned Warblers (Basileuterus culicivorus). In open cerrado habitat, it may be White-rumped (Cypsnagra hirundinacea) or White-banded Tanagers (Neothraupis fasciata). 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.
But while such easy-to-locate bird 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 Buff-throated Warbling Finches (Poospiza lateralis) 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. And while Basileuterus species are initiators as well as core species, mixed flocks of Tangara species – in particular Red-necked (T. cyanocephala), Brassy-breasted (T. desmaresti) and Green-headed Tanager (T. seledon) – often initiate formation of a larger and more diverse feeding flock, of which they are then only a less significant component.
Nine-primaried oscines make up much of almost every Neotropical mixed-species feeding flock. Namely, these birds are from families such as the Cardinalidae (cardinals), Parulidae (New World "warblers"), and in particular Emberizidae (American "sparrows") and Thraupidae (tanagers). Other members of a Neotropic mixed feeding flock may come from most of the local families of smaller diurnal insectivorous birds, and can also include woodpeckers (Picidae), toucans (Ramphastidae) and trogons (Trogonidae). Most Furnariidae do not participate in mixed flocks very often, though there are exceptions such as Synallaxis spinetails and some species of the woodcreeper subfamily (Dendrocolaptinae) – e.g. those mentioned above or the Lesser Woodcreeper (Xiphorhynchus fuscus) – are common or even "core" members. Among the tyrant flycatchers (Tyrannidae) there are also some species joining mixed flocks on a somewhat regular basis, including the Sepia-capped Flycatcher (Leptopogon amaurocephalus), Eared Pygmy-tyrant (Myiornis auricularis), White-throated Spadebill (Platyrinchus mystaceus) and Oustalet's Tyrannulet (Phylloscartes oustaleti).
However, even of commonly-participating families not all species join mixed flocks. There are genera such as Vireo in which some species do not join mixed flocks, while others (e.g. Red-eyed Vireo, V. olivaceus) will even do so in their winter quarters. Of the three subspecies groups of the Yellow-rumped Warbler (Dendroica coronata), only one (Audubon's Warbler, D. c. auduboni group) typically does. And while the importance of certain Thraupidae in initiating and keeping together mixed flocks has been mentioned already, for example the Black-goggled Tanager (Trichothraupis melanops) is an opportunistic feeder that will appear at but keep its distance from any disturbance – be it a mixed feeding flock, an army ant column or a group of monkeys – and pick off prey trying to flee.
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. Cotingidae (cotingas) are mainly opportunistic associates which rarely join flocks for long if they do so at all; the same holds true for most Muscicapoidea (mockingbirds and relatives), though some thrushes (Turdidae) may participate on more often. And though most Tityridae rarely join mixed flocks, becards (Pachyramphus) do so regularly. Tapaculos (Rhinocryptidae) are rarely seen with mixed flocks, though the Collared Crescentchest (Melanopareia torquata), doubtfully assigned to that family, may be a regular member. Icteridae (grackles and relatives) are also not too often seen to take part in these assemblages, though caciques like the Golden-winged (Cacicus chrysopterus) or Red-rumped Cacique (C. haemorrhous) join mixed flocks on a somewhat more regular basis. Cuculiformes (cuckoos and allies) are usually absent from mixed feeding flocks, but some – e.g. the Squirrel Cuckoo (Piaya cayana) can be encountered not infrequently.
Some species appear to prefer when certain others are present: Cyanolyca jays like to flock with Unicolored Jays (Aphelocoma unicolor) and the Emerald Toucanets (Aulacorhynchus prasinus species complex). Many Icteridae 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).
Other species participate to varying extents depending on location or altitude – presumably, the different species composition of mixed flocks at varying locations allows these irregular members more or less opportunity to get food. Such species include the Grey-hooded Flycatcher (Mionectes rufiventris: Tyrannidae), or the Plain Antvireo (Dysithamnus mentalis: Thamnophilidae) and the Red-crowned Habia (Habia rubica: Cardinalidae) which are often recorded in lowland flocks but rarely join them at least in some more montane regions.
Read more about this topic: Mixed-species Foraging Flock