Bell’s vireos, Vireo bellii pusillus, are an endangered subspecies of the Bell's vireo. Members of the subspecies are limited in their range, breeding only in southwestern California and northern Mexico. Least Bell's vireos migrate and spend the winter in southern Baja California. (Alderfer, 2006; Arlott, 2011; BirdLife International, 2017; Kus, 2002; Kus, et al., 2010; Lebbin, et al., 2010; Sandberg, et al., 2000; "The land manager's guide to the birds of the South", 1992; Wells, 2007), are a migratory species native to the southwestern and central parts of the United States and into northern Mexico. Their breeding range covers the midwestern part of the U.S. It extends from south central North Dakota, southern Wisconsin, southern Michigan, and western Ohio going south to Texas. Bell’s vireos' range continues into northern Mexico and then west across southern Arizona, New Mexico, and Nevada to California. Their fall migratory patterns usually begin around August to September. During the winter, the birds will migrate into the western part of Mexico and into Central America along the Pacific Coast. Least Bell's vireos,
In the breeding season, Bell's vireos can be found in riparian habitats with diverse vegetation and in dense early successional habitats. Shrubs, trees, and brushy fields are also suitable locations for them. Plant communities that attract them are willow-cottonwood forests, oak (Quercus), woodlands, shrubby thickets, and dry washes. During the migration period, Bell's vireos make use of coastal scrub, woodland, and riparian habitats. Winter habitats are very similar to breeding habitats, but they will aim to distribute away from water ways during their winter period. Bell's vireos are substantially absent in elevations above 1300 meters in the United States, and 1900 meters in Mexico. (Alderfer, 2006; Barlow, 1962; BirdLife International, 2017; "Ecology and conservation of the endangered least Bell's vireo", 1989; Kus, et al., 2008; Lebbin, et al., 2010; Pitelka and Koestner, 1942; "The land manager's guide to the birds of the South", 1992; Wells, 2007)
Bell’s vireos are song birds that are sexually monomorphic in plumage coloration throughout all seasons. Plumage color will vary by region. They are colored with a dull ash gray to green on their heads and upper parts of the body. Their underside is purely white, including under their wing coverts; on their breast sometimes a slight faint tint of brownish gray is evident. The sides under their wings are tinted with yellow. Bell’s vireos have distinguishing white spectacles and dark lores. Adults reach total lengths of 115-125 mm and weigh around 7-10 g. Their wingspan averages about 18 centimeters. Size and weight are identical for both females and males.
Hatchlings are naked with pink skin, lacking any visible down. They weigh approximately 1 g at hatching. Feathers begin appearing quickly around day 6 to 7. The plumage of juvenile birds will resemble that of the adults, with the exception that their underparts are whiter with distinct wing bars. (Alderfer, 2006; Arlott, 2011; Barlow, 1962; BirdLife International, 2017; "Ecology and conservation of the endangered least Bell's vireo", 1989; Kus, et al., 2010)
Bell’s vireos generally form a monogamous mating pair. Adults may switch mates between the years and infrequently between nesting attempts. The precise formation of pairs is unknown.
Courtship behaviors are violent during the early stage, with males attacking the females. Older males are found to be less aggressive. The females will lure the males into rapid looping chases; these chases will commence as nest-building begins. The males will occasionally intentionally collide with the female during flight. Barlow (1962) described several displays and postures performed during courtship. The first displays are greeting ceremonies; the pairs are 3-13 cm apart from another with the feathers on one sleeked and on the other fluffed. Both will flick their wings and tails rapidly and a “chee” is emitted during this performance. Another is “pouncing”; where the male hits the female with his breast or beak. If a female fails to accept this socialization posture she may not be sexually mature or ready for a mate. A third display is the “leap-flutter,” where the male fans his tail before the female and then suddenly leaps 20-25 cm and flutters for a few seconds before sitting back down to his original perch. A fourth display is the pre-copulatory display, a performance that may last around three minutes. This performance includes a swing of the body and a song, received by the female. The final two displays are the copulation and post-copulation display. During copulation the male positions himself dorsally on the females back and flutters during the few seconds of cloacal contact. The post-copulation is performed with the male approaching the female from the front.
Courtship is highly associated with nest-building. Both sexes contribute to the building of the nest. The males will interrupt nest-building to court the female. The females will become more responsive to the males as nest-building progresses. Copulation generally occurs during the third day of nest-building to the first day of egg laying. Bell’s vireos examine and select nest sites together with the female leading the search. They construct their nest using grass, straw-like stems, leaves, paper, and even strips of bark that are tied together using spider silk. Their nests are structured like a bag or cup, with an opening at the top of the nest. Soft matter such as feathers or moss is used to cushion the lining of the nest. The outside diameter averages 71 mm and the inside roughly 45 mm. Nests are only used once per breading season. (Baicich and Harrison, 1992; Barlow, 1962; BirdLife International, 2017; Budnik, et al., 2000; Howell, et al., 2010; Kus, et al., 2010; Pitelka and Koestner, 1942; Reed, 1965)
Bell’s vireos begin their breeding season in early April at the southern end of the range and in late May towards the northern end of the range. The breeding season usually lasts until early July. Females will lay 3 to 4 eggs per season; occasionally 2 and rarely 5 will be laid. Bell’s vireos are capable of laying up to two clutches per year, but due to nest failure and predation, the second attempt is usually unsuccessful.
The females will lay 1 egg per day. Their oval eggs are white with speckled brown, black, or reddish brown. On average, they measure 17 by 13 mm and weigh around 1-2 g. Both sexes incubate the egg and the incubation period is 14 days. Bell’s vireos chicks usually hatch on day 14 or 15 of incubation. About 24 hours prior to hatching, the eggs may have cracks or appear unnatural looking. Two hours before the shell is halved, holes may appear. Hatching can occur at any time in the day. The adults will help with the hatching process.
The chicks are born altricial; they will weigh around 1.8 g when born. Nestlings grow rapidly for the next 10 days. They are able to lift their heads at 12 hrs old. Both sexes brood and feed the nestlings. Adults will partially crush or regurgitate food to the newly-hatched young. Once the nestlings' eyes open, the adults will feed them whole insects. The nestling period lasts 9 to 12 days. Premature fledging may occur due to human interaction and nest interference. The nestlings will fledge at day 10 to 12. By day 10, the fledglings are able to hop, flutter their wings, and are completely feathered. After 25 days fledglings become vocal and sociable. The adults will continue to feed the young until they are around 20-25 days old, and are fully capable of foraging and feeding themselves. At day 40 the fledglings are able to produce songs and calls. Bell’s vireos are independent after 20-40 days. Age of sexual or reproductive maturity for both males and females are reached by the following breeding season (about 1 year after hatching). (Baicich and Harrison, 1992; Barlow, 1962; Budnik, et al., 2000; Howell, et al., 2010; Kus, et al., 2010; Pitelka and Koestner, 1942; Reed, 1965; "The land manager's guide to the birds of the South", 1992; Wells, 2007)
Both male and female Bell’s vireos are included and invested in raising their young. They will share the duties of nest-building, incubation, brooding, feeding, and protection of young. Both sexes incubate the eggs, although females incubate more often during the day. When the male is ready to incubate he will sing while approaching the nest, signaling to the female he is ready to trade positions. The female will respond with several calls and then will fly off. Males and females will both assist in the hatching process and will later remove the egg shells from the nest. The exchange of brooding duties mirrors that described during the exchange of incubation. Both adults will feed the young by regurgitation of food. Once the nestlings' eyes open, whole insects will be delivered. Parents will continue to care for young until they are around 30 days or they are independent. (Baicich and Harrison, 1992; Barlow, 1962; BirdLife International, 2017; Budnik, et al., 2000; Kus, et al., 2010; Pitelka and Koestner, 1942; Reed, 1965; "The land manager's guide to the birds of the South", 1992; Wells, 2007)
Bell's vireos have an expected average lifespan of 3-4 years. Mortality can be a result of predation, parasitism, lack of suitable habitat, and human interaction. Although their expected lifespan is low at only 3-4 years, they can live 7 years. Survivorship of juveniles during their first year is considerably lower than that of adults.
The longest known lifespan in the wild is reported at 9.1 years. Bell's vireos are monitored by being captured, banded, and released; this technique is used to observe and track their lifespan/longevity, migratory patterns, and breeding successes. These birds are not typically kept in captivity. (Barlow, 1962; BirdLife International, 2017; Kus, et al., 2010; Wells, 2007)
Bell's vireos are a gregarious species. They socialize primarily within their species only. Daily they can be observed bobbing their longs tails, hopping between branches, and chasing insects. During a typical flight, numerous rapid and shallow wing beats follow a fixed-wing glide. They undulate slightly during flight. Looping flights have been observed during territorial disputes and courtship. Bell’s vireos forage in a routine pattern, never straying too far from their territory. They hop between branches during foraging and are rarely seen on the ground. They bathe themselves by dipping into water or rubbing against wet leaves.
Monogamous pairs interact throughout the breeding season, performing visual and auditory displays before and around copulation and working together to build the nest. Both sexes care for the young during incubation and up to 30 days after hatching.
Bell’s vireos are a nocturnal migrant. Their migratory behavior is poorly known; males may sing full songs during migration. Places of vocalization are not chosen by favoritism or pattern; rather they are chosen at random. Males will begin singing before the sun rises and will sing continually throughout the day, without a noticeable change until the evening. Their daily time budget, roosting and sleeping is unknown. (Alderfer, 2006; Barlow, 1962; "Ecology and conservation of the endangered least Bell's vireo", 1989; Joos, et al., 2014; Kus, et al., 2010; "Wisconsin Bell's vireo species guidance", 2013)
Bell’s vireos males will select a specific territory and defend it. Male and females will spend most of their time in pair-formation, courtship, copulation, nesting, raising their young, and foraging in their territory. Territorial conflicts involve physical contact and high-intensity singing. Females rarely assist in defense of their territory. Their territory is primarily maintained by their songs. Size is not a determining factor in territory size.
As nest-building begins, the stabilization of territorial boundaries becomes more identified. Their territory size ranges between 0.105 ha to 1.214 ha (average 0.506 ha). Males will follow a very predictable path; they will only travel about 46 m from the nest. (Barlow, 1962; "Ecology and conservation of the endangered least Bell's vireo", 1989; Joos, et al., 2014; Kus, et al., 2010; "The land manager's guide to the birds of the South", 1992; "Wisconsin Bell's vireo species guidance", 2013)
Bell’s vireos have a distinctive song. Their song is usually alternated by birds and consists of sharp or slurred notes given in couplets. The song ends with either ascending or descending notes. The song is usually a two part series, alternating between two sounds: "cheedle-ee" and "cheedle-ew." They sing regularly but singing intensifies during the breeding season. When undisturbed, they sing less than when they identify an intruder or other disturbance. Their heads vibrate while they sing. Bell’s vireos do not sing during flight. Bell's vireos communicate and perceive information primarily by sight and sound; from what is known, their tactile senses are expressed between mating pairs and parents and their young. Besides their primary song, Bell’s vireos vocalize in five other ways: the courtship song, the distress call, the alarm note, the zip note, and the generalized call note. Each of these vocalizations are different and selective to specific circumstances or stresses. The courtship song is a higher pitched version of the primary song; this song is strictly used in relation to nest-building, egg laying, and other engagement between mating pairs. The distress call is a high pitched vocalization that is uttered as a dolorous note. The alarm note is a three-note call that is only vocalized by the males. This note is given during the beginning of mating pair-formation and nest-building. This call is usually given from an area distant, but still in full view of the offending individual or article. The zip note is a special note that is used when an intruder initially approaches the nest. This note is not as loud as the normal "chee" that is used; rather it sounds like zip-zip-zip. The generalized call is the distinctive "chee" note that may be heard in association under numerous circumstances. These circumstances may include feeding time, courtship, and anxiety. Bell’s vireo also uses geomagnetic directional information during migration periods. (Alderfer, 2006; Arlott, 2011; Kus, et al., 2010; Sandberg, et al., 2000)
Bell’s vireo prey on a multitude of insects. They consume insects such as true bugs (Hemiptera), beetles (Coleoptera), grasshoppers (Orthoptera), bees, wasps, and ants (Hymenoptera), leafhoppers (Homoptera), stinkbugs (Pentatomidae), flies (Diptera) and moths and caterpillars in both the adult and larval stage (Lepidoptera). Their more common insect of choice are larger insects like grasshoppers. During the breeding season they will also consume small spiders (Araneae). Their diet primarily consist of insects and spiders (99.3%), but some will consume vegetables and fruits (0.7%).
Multiple studies have examined the stomach contents of Bell’s vireos during the breeding season. Although the dominant insect group varied among the studies, they consistently consumed caterpillars, beetles, spiders, grasshoppers, and true bugs. The percents of insects in the stomach varied greatly due to area availability and dispersion. Observation of drinking water is slim; water intake is assumed to be obtained through the diet. They consume their prey using several different foraging techniques. These methods include: gleaning – picking up their prey from a substrate such as leaf or bark; hawking – an aerial pursuit and capture of prey in flight; and hovering – removal of prey from a vegetated landscape surface. Bell’s vireos forage in areas of high and low vegetation layers, so as long as foliage is dense, there foraging bouts often take place in a riparian zone. During the breeding season they may forage in pairs. (Barlow, 1964; Kus, et al., 2010; Terres, 1959; "The land manager's guide to the birds of the South", 1992)
Bell’s vireos are susceptible to predation from many different species. Predators for the Bell's vireos include species of other birds, mammals, reptiles, insects, and snakes. Confirmed bird predators include: western scrub-jay (Aphelocoma californica), and Gila woodpecker (Melanerpes uropygialis). Established mammal predators include: Virginia opossum (Didelphis virginiana), domestic cats (Felis catus), and humans (Homo sapiens). Confirmed snake predators include: gopher snake (Pituophis catenifer), western rat snake (Pantherophis obsoletus), red-sided garter snake (Thamnophis sirtalis infernalis), and California kingsnake (Lampropeltis californiae). Argentine ants (Linepithema humile) and the southern alligator lizard (Elgaria multicarinata) are also known predators.
Bell's vireos respond to predation differently depending on the type of predator. They may protect their nests from intruders through aggression, chiding or flying around the nest, and alarm calls. The effectiveness of their protective behaviors is not clearly known. ("Ecology and conservation of the endangered least Bell's vireo", 1989; Kus, et al., 2010; Kus, et al., 2008; Pitelka and Koestner, 1942; Sharp and Kus, 2004; Wells, 2007)
Bell's vireos may cross territories and migratory patterns with many of their congeneric species. Vireos may challenge other species with calls or may tolerate the foraging and interaction with other subspecies.
Brown-headed cowbirds (Molothrus ater) play a vital role in the numbers of the Bell's vireos. When female Bell's vireos leave nests unattended, brown-headed cowbirds deposit their eggs into the nests. Female vireos have been observed chasing off these brood parasites. Early in the nesting season, prior to the egg laying of the vireos, the presence of a cowbird egg will lead to the abandonment of the nest. Eggs laid by the cowbird later in the season after the vireos have laid their eggs will not cause abandonment. If eggs have been removed by the cowbird then vireos will abandon the nest. Brown-headed cowbird parasitism has led to increased nest failures and abandonment.
Northern fowl mites (Ornithonyssus sylviarum) infest incubating adults and young in the nest. The bites from the parasite lead to small, scab-like marks on the birds' tarsi. The bites can occasionally lead to death of the Bell's vireos.
Bell's vireos feed on a variety of insects. Their consumption of insects is not influential enough to affect the overpopulation of insects. They are prey to numerous snakes and mammals. (Alderfer, 2006; Barlow, 1962; BirdLife International, 2017; "Ecology and conservation of the endangered least Bell's vireo", 1989; Kosciuch and Sandercock, 2007; Kus, et al., 2010; Kus, et al., 2008; Sharp and Kus, 2004; Wells, 2007)
Bell's vireos' economic importance for humans is primarily that of research of and education for the species. Birdwatchers and researchers spend time studying the vireo, particularly the subspecies that is federally listed, least Bell's vireo (Vireo bellii pusillus). (BirdLife International, 2017; "Ecology and conservation of the endangered least Bell's vireo", 1989; Kus, et al., 2010)
Bell's vireos negatively impact humans primarily from the money spent on research and conservation. As this includes an endangered subspecies (Vireo bellii pusillus), the money spent and land saved for conservation efforts has had an impact on human economics. The conservation efforts could be at the expense of commercial and residential developments, as well as federal and state funds being allocated to protect the vireos. (BirdLife International, 2017; "Ecology and conservation of the endangered least Bell's vireo", 1989; Kus, et al., 2010)
Bell's vireos' overall population has decreased substantially. The reduction in population size has listed them as "Near Threatened" with a decreasing population trend on the IUCN Red List. Like all native migratory species in the U.S., they have been listed as "Protected" under the U.S. Migratory Bird Treaty Act. Their subspecies least Bell's vireos (Vireo bellii pusillus) are federally endangered under the U.S. Federal List. They have no special status under CITES or the State of Michigan's Rare Animal List.
The continual decline of mature individuals is thought to be influenced by numerous variables. Threats include residential and commercial development, wood harvesting, parasitism, agriculture and aquaculture, natural system modifications (dams and water management), and pollution/pesticides and other contaminants. Least Bell's vireos' endangerment and population declines are strongly affected brown-headed cowbird (Molothrus ater) nest parasitism. Reports of eggs containing high levels of strontium could pose problems with calcium metabolism. Degradation of habitat through developments, wood harvesting, agriculture, and aquaculture have influenced the breeding and habitat range and has increased the abundance of cowbird parasitism. In southern California wildfires pose a threat to the vireos. Human interference has also let to premature fledging of the young.
Conservation efforts to manage the vireos have not been fully successful to restoring the populations to a stable level but have had significant impacts on population levels. The primary means of management is through cowbird control and habitat restoration. Removal of cowbird eggs has shown to increase nest success and population productivity. Entrapment of cowbirds and nest manipulation have shown positive results in protection of the vireos. Nest manipulation is a more efficient and cost-effective technique. Conservation sites have been identified and habitat restoration actions have begun. Creation of new habitats through plantings of riparian species has shown to attract vireos. Removal of invasive non-native vegetation and re-establishment of native vegetation have also shown to be effective in population production. Reintroduction into areas within their historical range has been considered but actions have not been put into place. Refined education and awareness of the Bell's vireo is need substantially as a conservation action. (Alderfer, 2006; Barlow, 1962; BirdLife International, 2017; "Ecology and conservation of the endangered least Bell's vireo", 1989; Howell, et al., 2010; Kosciuch and Sandercock, 2007; Kus, 2002; Kus, et al., 2010; Kus, et al., 2008; Lebbin, et al., 2010; Sharp and Kus, 2004; Wells, 2007)
Lauren Burroughs (author), Radford University, Layne DiBuono (editor), Radford University, Lindsey Lee (editor), Radford University, Kioshi Lettsome (editor), Radford University, Karen Powers (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
uses sound to communicate
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
Referring to an animal that lives in trees; tree-climbing.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
An animal that eats mainly insects or spiders.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
(as perception channel keyword). This animal has a special ability to detect the Earth's magnetic fields.
parental care is carried out by males
makes seasonal movements between breeding and wintering grounds
Having one mate at a time.
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
uses touch to communicate
Living on the ground.
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
movements of a hard surface that are produced by animals as signals to others
uses sight to communicate
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