Mixed-species foraging flock

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Black-headed gulls, bar-tailed godwits and sanderlings foraging on a beach

A mixed-species feeding flock, also termed a mixed-species foraging flock, mixed hunting party or informally bird wave, is a

insectivorous birds of different species that join each other and move together while foraging.[1]
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.[2] Additionally, the forest structure is hypothesized to play a vital role in these flocks' formation.[3] 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,[4] while in parts of the American tropics packs of foraging golden-crowned warblers might play the same role.[5]

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.

generalists[7] that employ a gleaning foraging strategy[8] and intraspecifically social birds.[9] "Associate" or "attendant" species are birds that trail the flock only after it has entered their territory.[10] Researchers have shown that these species tend to have a higher fitness following mixed-species foraging flocks.[11] 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.[12] Their role is to alert the other birds in the mixed-species flock to the arrival of potential predators.[13]

Benefits

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.[14] 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,[15] avoid already exploited locations,[16] and drive insects out of hiding.[17] 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.[18] Other studies have hypothesized that multi-species flocks form because large groups reduce a predator's ability to single out one prey,[19] while others have hypothesized that multi-species flocks are more likely to spot predators.[20]

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.[21] Birds may also be forced to change their foraging strategy in order to conform with the flock.[22] Another third proposed cost of mixed-species flocks is an increased risk of kleptoparasitism.[23]

In the Holarctic

In the North

Parulidae (New World "warblers")[27] – all insect-eating birds. This behavior is particularly common outside the breeding season.[24]

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.[24]

In the Neotropics

Insectivorous feeding flocks reach their fullest development in

predators.[28][29]

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

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.[5]

insectivorous birds, and can also include woodpecker, toucans, 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 – e.g. those mentioned above or the lesser woodcreeper – are common or even "core" members. Among the tyrant flycatchers 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.[5][28][30]

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.[5] Of the three subspecies 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 column or a group of monkeys – and pick off prey trying to flee.[5]

Cuculiformes (cuckoos and allies) are usually absent from mixed feeding flocks, but some – for example, the squirrel cuckoo – can be encountered not infrequently.[5]

Some species appear to prefer when certain others are present:

species complex. Many Icteridae associate only with related species, but the western subspecies of the yellow-backed oriole associates with jays and the band-backed wren.[34]

Other species participate to varying extents depending on location or altitude – presumably, the different

montane regions.[5]

A typical

long-billed gnatwren[31] – join it only as long as it crosses their own territories.[29]

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 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, joined by species such as the ashy-headed laughingthrush, Kashmir flycatcher, and velvet-fronted nuthatch.

A mixed flock in the

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.[35]

African rainforests also hold mixed-species flocks, the core species including

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.[36]

Notes

  1. PMID 8433996
    .
  2. ^
    OCLC 432686637
    .
  3. S2CID 86835417. Closed access icon
  4. S2CID 53148848
    .
  5. ^
    ISSN 0034-7108
    .
  6. PMID 31798886
    .
  7. JSTOR 3677336
    .
  8. S2CID 9687855
    .
  9. doi:10.1111/j.1600-048X.2009.04828.x
    .
  10. ^ Faaborg, John; Chaplin, Susan B. (2011). Ornithology: an Ecological Approach. Prentice-Hall.
  11. S2CID 42672383
    .
  12. S2CID 40812217
    .
  13. ISSN 1938-4254
    .
  14. S2CID 8402443
    .
  15. PMID 28230159
    .
  16. ISSN 0030-1299
    .
  17. doi:10.1111/j.1474-919X.1943.tb03857.x
    .
  18. S2CID 4365789
    .
  19. doi:10.1111/j.1469-7998.1974.tb04385.x
    .
  20. PMID 4734745
    .
  21. JSTOR 3565809
    .
  22. JSTOR 1369572
    .
  23. JSTOR
    4513268.
  24. ^ a b c Ehrlich, Paul R.; Dobkin, David S.; Wheye, Darryl. "Mixed-Species Flocking".
  25. ^ Perrins 2003, "Nuthatches".
  26. ^ Perrins 2003, "Woodpeckers".
  27. ISBN 978-1-55407-046-6
    .
  28. ^
    PMID 11188872
    .
  29. ^ a b Perrins 2003, "Antbirds".
  30. ^ a b c Olson, Storrs L.; Alvarenga, Herculano M.F. (September 2006). "An extraordinary feeding assemblage of birds at a termite swarm in the Serra da Mantiqueira, São Paulo, Brazil" (PDF). Revista Brasileira de Ornitologia. 14 (3): 297–299. Archived from the original (PDF) on 2017-02-02. Retrieved 2016-10-01.
  31. ^ a b Perrins 2003, "Gnateaters".
  32. ^ a b Perrins 2003, "Swallows".
  33. ^ Perrins 2003, "Swifts".
  34. ISBN 978-0-19-854012-0
    .
  35. ^ Nuytemans, H. (January 1998). "Notes on Philippine birds: interesting records from northern Luzon and Batan Island" (PDF). Forktail. 14: 39–42.
  36. ^ Perrins 2003, "Cuckoo-shrikes".

References

External links
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