Tuesday 21 May
Bell's vireo (Vireo bellii)
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Bell's vireo fact file
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Bell's vireo description
A shy and secretive bird, Bell's vireo (Vireo bellii) is more often heard than seen (4) (5). While the plumage of this small songbird varies with location, it is generally described as being rather drab in colour (2) (3).
There is little difference between the male and female Bell’s vireo, with the plumage on the upperparts ranging form dull grey to green, and the underparts from white to yellow (2). The markings of Bell’s vireo are indistinct, with two dull, whitish bars on the wings, and a faint white ring around each eye (2) (3) (4) (5). The wings are short and rounded, and the bill is short, straight and blunt. The legs range from dark bluish-grey to black (2).
The juvenile Bell’s vireo can be distinguished from the adult by the more distinct wingbars and whiter underparts (2).
Described as a rapid series of harsh, scolding notes (4) (5), the song of Bell’s vireo is an unmistakable “cheedle cheedle chee? Cheedle cheedle chew!” (2). Typically, the first phrase of the song ends on an ascending note, while the second phrase ends on a descending note (3).
There are currently four recognised subspecies of Bell’s vireo. The nominate subspecies, Vireo bellii bellii, tends to be slightly brighter and more colourful than its eastern counterparts, Vireo bellii medius, Arizona Bell’s vireo (Vireo bellii arizonae) and least Bell’s vireo (Vireo bellii pusillis) (2).Top
Bell's vireo biology
An energetic forager, Bell’s vireo searches for its invertebrate prey while hopping between branches, often flicking or bobbing its tail as it goes (2) (4). Bell’s vireo often flicks its wings while eating, and will take larger, hard-bodied prey to a branch and hammer it with its bill before consuming it (2) (4). A pair may forage together during the breeding season, spiralling up the vegetation, gleaning insects as they go. During the breeding season, the prey of Bell’s vireo includes beetles, grasshoppers, moths, caterpillars and small spiders. Prey is also sometimes captured in flight (2).
Bell’s vireo typically begins nesting in April, with nesting usually continuing through to July (2) (7). The male will establish and defend a territory, using high intensity singing to warn off rivals, although physical conflict may also occur. Singing begins before sunrise and continues persistently throughout the day (2).
Courtship and pair-bonding behaviour in Bell’s vireo involves aggression directed toward the female, mid-air chases, posturing and calling. Bell’s vireo pairs tend to be monogamous, although birds may switch mates from year to year, and occasionally between breeding attempts within the same year. Both adults share the nest building and incubation duties, although the female may incubate the eggs more often during the day, and always at night. The nest is an open, basket-like cup structure made of grass, straw-like stems and other plant material. It is often suspended from the forks of low branches, about one metre above the ground, and is decorated with spider egg cases (2).
Typically, 3 to 4 eggs are laid, and the chicks hatch after around 14 days. Bell’s vireo hatchlings are naked and pink, and take around 10 to 12 days to develop feathers and fledge. If the pair does not manage to successfully rear the young to fledging, they may re-nest, with pairs making up to seven nesting attempts per season (2). Bell’s vireo may be vulnerable to nest predation by a number of species, including ants, snakes, birds and mammals, such as rats (2).Top
Bell's vireo range
Bell’s vireo is native to North and Central America (2) (6), breeding in Mexico, Baja California and south western California, east through Nevada, Arizona and New Mexico, and into Texas. The northern limit of its breeding range includes North Dakota and Minnesota (2).Top
Bell's vireo habitat
In its breeding range, Bell’s vireo shows a preference for the dense, low, shrubby vegetation typical of early successional habitat (2) (3) (6). It can be found in riparian areas, brushy fields, young forests and brushland composed of mesquite (Prosopisspp.). The presence of surface water is also important to Bell’s vireo, especially in arid regions (2).
While Bell’s vireo inhabits areas with a similar vegetation structure in its wintering range, it can often be found in more arid areas, away from water. In general, this species can be found wintering in thornscrub vegetation, riparian forest, tropical deciduous forest and arid tropical scrub (2).Top
Bell's vireo status
Bell's vireo is classified as Near Threatened (NT) on the IUCN Red List (1).Top
Bell's vireo threats
The population of Bell’s vireo has undergone a large decline in the last 40 years, with habitat loss affecting this species throughout its range (2) (6) (7) (8). Changes in land use, particularly in riparian areas, are strongly associated with the abundance of Bell’s vireo in the habitat. Overgrazing and wildfires also reduce the amount of available habitat for Bell’s vireo (2), as does the encroachment of non-native plant species (8) (9).
The impact of habitat loss has further been exacerbated by an increase in brood parasitism by the brown-headed cowbird (Molothrus ater), which lowers the reproductive success of Bell’s vireo. The brown-headed cowbird appears to increase in number in patchy habitats where development or agricultural practices occur on land adjacent to the breeding grounds of Bell’s vireo (7).Top
Bell's vireo conservation
Two subspecies of Bell’s vireo, the least bell’s vireo and the Arizona Bell’s vireo, are listed as ‘endangered’ under the California Endangered Species Act (2) (7) (10), which aims to protect threatened species in California (10). Least Bell’s vireo is also listed as federally endangered, and is therefore protected throughout its range (2) (7). The federal listing also provides least Bell’s vireo with a species recovery plan, which aims to see this species downlisted from ‘endangered’ to ‘threatened’ (9).
The control of the brown-headed cowbird population has proved to be an effective conservation measure throughout the range of Bell’s vireo, greatly increasing the reproductive success of this species (2) (6). Habitat restoration and non-native plant removal are also likely to be beneficial to Bell’s vireo (2) (9).Top
Find out more
Find out more about Bell’s vireo:
BirdLife International - Bell's vireo:
The Cornell Lab of Ornithology: All About Birds - Bell's vireo
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- Brood parasite
- An animal that lays its eggs in the nests of members of its own or other species; the host then raises the young as its own.
- A plant that sheds its leaves at the end of the growing season.
- The catching of prey by plucking from, or within foliage.
- The act of incubating eggs, that is, keeping them warm so that development is possible.
- Animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, cnidarians (jellyfish, corals, sea anemones) and echinoderms.
- Having only one mate during a breeding season, or throughout the breeding life of a pair.
- Nominate subspecies
- When a species is divided into subspecies, the originally described population is classified as the nominate subspecies. Indicated by the repetition of the species name; for example, Cyclura nubila nubila is the nominate subspecies of the Cayman Islands ground iguana, Cyclura nubila.
- Relating to the banks of rivers and streams.
- A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
- The progressive sequence of changes in vegetation types and animal life within a community that, if allowed to continue, results in the formation of a ‘climax community’ (a mature, stable community in equilibrium with the environment).
- An area occupied and defended by an animal, a pair of animals or a group.
IUCN Red List (February, 2012)
Kus, B., Hopp, S.L., Johnson, R. and Brown, B.T. (2010) Bell's vireo (Vireo bellii). In: Poole, A. (Ed.) The Birds of North America Online. Cornell Lab of Ornithology, Ithaca. Available at:
The Cornell Lab of Ornithology: All About Birds - Bell's vireo (February, 2012)
- Alderfer, J. (Ed.) (2006) National Geographic Field Guide to Birds: Arizona and New Mexico. National Geographic, Monterey.
- Kaufman, K. (2005) Field Guide to Birds of North America. Hillstar Editions L.C., New York.
BirdLife International - Bell's vireo (February, 2012)
Desert Renewable Energy Conservation Plan - Preliminary Conservation Strategy, Bell’s vireo (February, 2012)
U.S. Department of the Interior Bureau of Land Management - Least Bell's vireo (February, 2012)
U.S. Fish & Wildlife Service: Species Profile - Least Bell's vireo (February, 2012)
California Endangered Species Act (CESA ) (February, 2012)
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