Mixed-species Feeding Flock
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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 A robber fly eating a hoverfly An insectivore is a carnivorous animal or plant which eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects. The first vertebrate insectivores we ...
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s of different
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
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 Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka, also known historically as Ceylon, is an island country in South Asia. It lies in the Indian Ocean, southwest of the Bay of Bengal, separated from the Indian subcontinent, ...
, for example, vocal mimicry by the
greater racket-tailed drongo The greater racket-tailed drongo (''Dicrurus paradiseus'') is a medium-sized Asian bird which is distinctive in having elongated outer tail feathers with webbing restricted to the tips. They are placed along with other drongos in the family Dicr ...
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.


Composition

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 Gleaning is the act of collecting leftover crops in the field after harvest. During harvest, there is food that is left or missed often because it does not meet store standards for uniformity. Sometimes, fields are left because they were not ec ...
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 Sociality is the degree to which individuals in an animal population tend to associate in social groups (gregariousness) and form cooperative societies. Sociality is a survival response to evolutionary pressures. For example, when a mother was ...
. Their role is to alert the other birds in the mixed-species flock to the arrival of potential predators.


Benefits

Ecologists generally assume that species in the same
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of Resource (biology), resources an ...
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 image:Hierarchical clustering diagram.png, 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme o ...
, 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.


Costs

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 Kleptoparasitism (originally spelt clepto-parasitism, meaning "parasitism by theft") is a form of feeding in which one animal deliberately takes food from another. The strategy is evolutionarily stable when stealing is less costly than direct f ...
.


In the Holarctic

In the North
Temperate Zone In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ra ...
, they are typically led by
Paridae The tits, chickadees, and titmice constitute the Paridae, a family of small passerine birds which occur mainly in the Northern Hemisphere and Africa. Many were formerly classified in the genus ''Parus''. Eurasian and African members of this f ...
(tits and chickadees), often joined by
nuthatch The nuthatches () constitute a genus, ''Sitta'', of small passerine birds belonging to the family Sittidae. Characterised by large heads, short tails, and powerful bills and feet, nuthatches advertise their territory using loud, simple songs. Mo ...
es, treecreepers,
woodpecker Woodpeckers are part of the bird family (biology), family Picidae, which also includes the piculets, wrynecks and sapsuckers. Members of this family are found worldwide, except for Australia, New Guinea, New Zealand, Madagascar and the extreme ...
s (such as the downy woodpecker and lesser spotted woodpecker),
kinglet A kinglet is a small bird in the family Regulidae. Species in this family were formerly classified with the Old World warblers. "Regulidae" is derived from the Latin word ''regulus'' for "petty king" or prince, and refers to the coloured crowns ...
s, and in North America
Parulidae The New World warblers or wood-warblers are a group of small, often colorful, passerine birds that make up the family Parulidae and are restricted to the New World. The family contains 120 species. They are not closely related to Old World warb ...
(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.


In the Neotropics

Insectivorous feeding flocks reach their fullest development in
tropical forest Tropical forests are forested ecoregions with tropical climates – that is, land areas approximately bounded by the Tropic of Cancer, tropics of Cancer and Tropic of Capricorn, Capricorn, but possibly affected by other factors such as prevailing ...
s, where they are a typical feature of bird life. In the
Neotropic The Neotropical realm is one of the eight biogeographic realms constituting Earth's land surface. Physically, it includes the tropical terrestrial ecoregions of the Americas and the entire South American temperate zone. Definition In biogeogra ...
s the leaders or "core" members may be
black-throated shrike-tanager The black-throated shrike-tanager (''Lanio aurantius'') is a species of bird in the family Thraupidae. It is found in Belize, Guatemala, Honduras, and Mexico. Its natural habitat is subtropical or tropical moist lowland forest A forest ...
s in southern
Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
, or three-striped warblers elsewhere in
Central America Central America is a subregion of North America. Its political boundaries are defined as bordering Mexico to the north, Colombia to the southeast, the Caribbean to the east, and the Pacific Ocean to the southwest. Central America is usually ...
. In
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
, 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 The New World warblers or wood-warblers are a group of small, often colorful, passerine birds that make up the family Parulidae and are restricted to the New World. The family contains 120 species. They are not closely related to Old World warb ...
(New World "warblers") like the golden-crowned warblers. In open
cerrado The Cerrado () is a vast ecoregion of Tropics, tropical savanna in central Brazil, being present in the states of Goiás, Mato Grosso do Sul, Mato Grosso, Tocantins, Maranhão, Piauí, Bahia, Minas Gerais, São Paulo (state), São Paulo, Paraná ...
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
predator Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...
s. 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 The Neotropical realm is one of the eight biogeographic realms constituting Earth's land surface. Physically, it includes the tropical terrestrial ecoregions of the Americas and the entire South American temperate zone. Definition In biogeogra ...
mixed flock feeding on swarming termites, it was observed that buff-throated 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. And while ''
Basileuterus ''Basileuterus'' is a genus of New World warblers, best represented in Central and South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller porti ...
'' species are initiators as well as core species, mixed flocks of '' Tangara'' species – in particular red-necked, brassy-breasted, and green-headed tanagers – often initiate formation of a larger and more diverse feeding flock, of which they are then only a less significant component.
Nine-primaried oscine The nine-primaried oscines is a group of bird families in the suborder Passeri (oscines) of the Passeriformes. The composition of the group has changed since the term was introduced but is now considered to consist of seven major families—Fringi ...
s make up much of almost every Neotropical mixed-species feeding flock. Namely, these birds are from
families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...
such as the cardinals, Parulidae (New World "warblers"), and in particular Passerellidae (American "sparrows") and
Thraupidae The tanagers (singular ) comprise the bird family (biology), family Thraupidae, in the order Passeriformes. The family has a Neotropical distribution and is the second-largest family of birds. It represents about 4% of all avian species and 12 ...
(tanagers). Other members of a Neotropic mixed feeding flock may come from most of the local families of smaller diurnal
insectivorous A robber fly eating a hoverfly An insectivore is a carnivorous animal or plant which eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects. The first vertebrate insectivores we ...
birds, and can also include woodpecker,
toucan Toucans (, ) are Neotropical birds in the family Ramphastidae. They are most closely related to the Semnornis, Toucan barbets. They are brightly marked and have large, often colorful Beak, bills. The family includes five genus, genera and over ...
s, and trogons. Most Furnariidae do not participate in mixed flocks, though there are exceptions such as '' Synallaxis'' spinetails and some species of the woodcreeper
subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end botanical subfamily names with "-oideae", and zo ...
– e.g. those mentioned above or the lesser woodcreeper – are common or even "core" members. Among the
tyrant flycatcher The tyrant flycatchers (Tyrannidae) comprise a family of passerine birds which is found virtually throughout North and South America. It is the world's largest family of birds, with more than 400 species, and is the most diverse avian family i ...
s there are also some species joining mixed flocks on a somewhat regular basis, including the sepia-capped flycatcher, eared pygmy tyrant, white-throated spadebill, and Oustalet's tyrannulet. 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., the red-eyed vireo) will even do so in their winter quarters. Of the three
subspecies In Taxonomy (biology), biological classification, subspecies (: subspecies) is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (Morphology (biology), morpholog ...
groups of the yellow-rumped warbler, only one ( Audubon's warbler) 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 is an opportunistic feeder that will appear at but keep its distance from any disturbance—be it a mixed feeding flock, an
army ant The name army ant (or legionary ant or ''marabunta'') is applied to over 200 ant species in different lineages. Because of their aggressive predatory foraging groups, known as "raids", a huge number of ants forage simultaneously over a limited ...
column or a group of
monkey Monkey is a common name that may refer to most mammals of the infraorder Simiiformes, also known as simians. Traditionally, all animals in the group now known as simians are counted as monkeys except the apes. Thus monkeys, in that sense, co ...
s – and pick off prey trying to flee. Gnateaters are notable for their absence from these flocks, while swifts and
swallows The swallows, martins, and saw-wings, or Hirundinidae are a family of passerine songbirds found around the world on all continents, including occasionally in Antarctica. Highly adapted to aerial feeding, they have a distinctive appearance. The t ...
rarely join them, but will if there is for example an
ant Ants are Eusociality, eusocial insects of the Family (biology), family Formicidae and, along with the related wasps and bees, belong to the Taxonomy (biology), order Hymenoptera. Ants evolved from Vespoidea, vespoid wasp ancestors in the Cre ...
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 may participate on more often. And though most
Tityridae Tityridae is family of suboscine passerine birds found in forest and woodland in the Neotropics. The 45 species in this family were formerly spread over the families Tyrannidae, Pipridae and Cotingidae (''see Taxonomy (biology), Taxonomy''). As y ...
rarely join mixed flocks, becards do so regularly. Tapaculos are rarely seen with mixed flocks, though the collared crescentchest, doubtfully assigned to that family, may be a regular member.
Icteridae Icterids () or New World blackbirds make up a family, the Icteridae (), of small to medium-sized, often colorful, New World passerine birds. The family contains 108 species and is divided into 30 genera. Most species have black as a predominant ...
(grackles and relatives) are also not too often seen to take part in these assemblages, though
caciques A cacique, sometimes spelled as cazique (; ; feminine form: ), was a tribal chieftain of the Taíno people, who were the Indigenous inhabitants of the Bahamas, the Greater Antilles, and the northern Lesser Antilles at the time of European cont ...
like the golden-winged or red-rumped cacique join mixed flocks on a somewhat more regular basis. Cuculiformes (
cuckoo Cuckoos are birds in the Cuculidae ( ) family, the sole taxon in the order Cuculiformes ( ). The cuckoo family includes the common or European cuckoo, roadrunners, koels, malkohas, couas, coucals, and anis. The coucals and anis are somet ...
s and allies) are usually absent from mixed feeding flocks, but some – for example, the squirrel cuckoo – can be encountered not infrequently. Some species appear to prefer when certain others are present: ''
Cyanolyca ''Cyanolyca'' is a genus of small jays found in humid highland forests in southern Mexico, Central America and the Andes in South America. All are largely blue and have a black mask. They also possess black bills and legs and are skulking birds. ...
'' jays like to flock with unicolored jays and the emerald toucanets
species complex In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...
. Many Icteridae associate only with related species, but the western subspecies of the yellow-backed oriole associates with jays and the band-backed wren. Other species participate to varying extents depending on location or altitude – presumably, the different
species composition Relative species abundance is a component of biodiversity and is a measure of how common or rare a species is relative to other species in a defined location or community.Hubbell, S. P. 2001. ''The unified neutral theory of biodiversity and biogeog ...
of mixed flocks at varying locations allows these irregular members more or less opportunity to get food. Such species include the grey-hooded flycatcher, or the plain antvireo and the red-crowned ant tanager which are often recorded in lowland flocks but rarely join them at least in some more
montane Montane ecosystems are found on the slopes of mountains. The alpine climate in these regions strongly affects the ecosystem because temperatures lapse rate, fall as elevation increases, causing the ecosystem to stratify. This stratification is ...
regions. A typical
Neotropic The Neotropical realm is one of the eight biogeographic realms constituting Earth's land surface. Physically, it includes the tropical terrestrial ecoregions of the Americas and the entire South American temperate zone. Definition In biogeogra ...
mixed feeding flock moves through the forest at about , 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.


In the Old World tropics

The flocks in the Old World are often much more loosely bonded than in the Neotropics, many being only casual associations lasting the time the flock of core species spends in the attendants' territory. The more stable flocks are observed in tropical Asia, and especially Sri Lanka. Flocks there 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 Sinharaja Forest Reserve is a forest reserve and a biodiversity hotspot in Sri Lanka. It is of international significance and has been designated a Biosphere Reserve and World Heritage Site by UNESCO. According to International Union for Conserv ...
in Sri Lanka, the typical daytime quiet of the jungle is broken by the noisy calls of the orange-billed babbler and
greater racket-tailed drongo The greater racket-tailed drongo (''Dicrurus paradiseus'') is a medium-sized Asian bird which is distinctive in having elongated outer tail feathers with webbing restricted to the tips. They are placed along with other drongos in the family Dicr ...
, joined by species such as the ashy-headed laughingthrush, Kashmir flycatcher, and velvet-fronted nuthatch. A mixed flock in the Cordillera Central of Luzon in the
Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...
was mainly composed of bar-bellied cuckooshrikes, Philippine fairy-bluebirds, and violaceous crows. Luzon hornbills were also recorded as present. With the crows only joining later and the large hornbills probably only opportunistic attendants rather than core species, it is likely that this flock was started by one of the former species – probably the bold and vocal cuckoo-shrikes rather than the more retiring fairy-bluebirds, which are known to seek out such opportunities to forage. African rainforests also hold mixed-species flocks, the core species including bulbuls and
sunbirds Sunbirds and spiderhunters make up the family (biology), family Nectariniidae of passerine birds. They are small, slender passerines from the Old World, usually with downward-curved bills. Many are brightly coloured, often with iridescent feathe ...
, and attendants being as diverse as the red-billed dwarf hornbill and the tit-hylia, the smallest bird of Africa. Drongos and paradise-flycatchers are sometimes described as the sentinels of the flock, but they are also known to steal prey from other flock members. Acanthizidae are typical core members in New Guinea and Australia; in Australia, fairy-wrens are also significant. The core species are joined by birds of other families such as minivets.


Notes


References

*


External links

* {{DEFAULTSORT:Mixed-Species Foraging Flock Zoology Bird behavior Ornithology Birds