Pooecetes gramineusvesper sparrow

Geographic Range

Vesper sparrows (Pooecetes gramineus) reside throughout the nearctic region. During the summer breeding season, the southern limit of their range extends to northern regions of the eastern United States, including Pennsylvania, Ohio, Indiana, the northern half of Missouri, and the Appalachian region of Virginia. Farther west in the United States they are found in the majority of Nevada, northern Arizona and New Mexico, western Colorado, northward into northern Nebraska. The northern limit of their breeding range includes southern Canada, extending northwest into Alberta and Saskatchewan.

In the winter, during non-breeding season, the northern boundary of their range includes the southern United States, including South Carolina, Georgia, Alabama, Mississippi, Louisiana, and Texas. The northern boundary includes the southernmost parts of California, Arizona, New Mexico, and Arkansas. The southern boundary of their winter range extends southwards into Oaxaca, in Mexico.

Vesper sparrows migrate through Kansas and Kentucky, and parts of Oklahoma, Missouri, Tennessee, West Virginia, Virginia, and North Carolina.

Vesper sparrows have four subspecies, divided geographically. In summer, individuals of Pooecetes gramineus gramineus are found in northeastern North America, reaching northern North Carolina. In winter, they are found in the southern portion of the breeding range, generally staying east of the Texas-New Mexico border. This subspecies is also found occasionally in Mexico and Bermuda. The summer range of P. g. confinis includes northwestern North America, and the winter range includes the southern and southeastern United States, as well parts of northern Mexico. Members of P. g. affinis are found in the western United States and winter southward, in Baja California, Mexico. Finally, members of P. g. altus range from southern Utah to northern Arizona and share a wintering range with individuals of P. g. confinis. (BirdLife International, 2016; Jones and Cornely, 2002; Sibley, 2014)

Habitat

Vesper sparrows are a generalist species and nest in early successional areas, such as croplands, meadows, and pastures, within an elevation range of 0 to 3000 m above sea level. However, they are found more commonly in dry areas with patchy, shrub-like vegetation. Vesper sparrows establish territories by vocalizing from high perches. In non-breeding season, their habitats consist of dry shrublands and grasslands. Vesper sparrows generally avoid dense vegetation. (Best and Rodenhouse, 1984; Jones and Cornely, 2002; Macías-Duarte and Panjabi, 2013; Wiens, 1969)

  • Range elevation
    0 to 3000 m
    0.00 to 9842.52 ft

Physical Description

Vesper sparrows are streaky-brown birds with cream-colored stomachs, white outer tail feathers, white eye rings, white malar stripes, a notched tail, dark brown to reddish-brown eyes, dusky pink feet, and conical pinkish bills. Males are slightly larger than females, with a body mass ranging from 20.9 to 28 g (average 26.5 g), compared to females with a weight range of 19.5 to 28.3 g (average 24.9 g). Vesper sparrows have a recorded average length of 15.9 cm (range 13.02 to 16.1 cm), with a wingspan averaging 25.4 cm. Males, females, and juveniles have similar plumage that, in spring and summer, become grayer with more prominent streaks. Juveniles lack the rufous lesser coverts that adults have.

Different subspecies of vesper sparrows have slightly different appearances. Individuals of the subspecies P. g. gramineus are considered to have a medium size and tail length compared to the species average, slightly stout bills, pale gray upper feathers, and whitish bellies. Members of P. g. confinis are relatively large in comparison to other subspecies, with longer tails and more slender bills. Their upper feathers are a dusky gray-brown and their bellies are cream colored. Individuals of P. g. affinis are relatively small and short compared to the overall species average, with slender bills, brown upper feathers, and buff bellies. Individuals of P. g. altus are the same size as P. g. confinis, but have dark to grayish-brown upper feathers and whitish bellies.

Vesper sparrows hatch with closed eyes and pink skin, with patches of gray where feathers eventually grow. At 3 to 5 days, their eyes open and, at six days, feather sheaths start to rupture. By day 9, these birds are covered in their juvenile feathers, which continue to grow until about 31 days after birth. Vesper sparrow juveniles get their adult feathers after the definitive prebasic molt, which occurs between July and November. (Dawson and Evans, 1960; Jones and Cornely, 2002; Sibley, 2014)

  • Sexual Dimorphism
  • sexes alike
  • male larger
  • Range mass
    20.9 to 28 g
    0.74 to 0.99 oz
  • Average mass
    26 g
    0.92 oz
  • Range length
    13.02 to 16.1 cm
    5.13 to 6.34 in
  • Average length
    15.9 cm
    6.26 in
  • Average wingspan
    25.4 cm
    10.00 in

Reproduction

Male vesper sparrows attract females through vocalizations, most often from raised perches. Additionally, during courtship, males move around females with their wing and tail feathers raised and spread. During this display, they will also occasionally take flight to vocalize. Vesper sparrows are often seasonally monogamous, but cases of polygyny have been reported. Research has found that vesper sparrow males that arrive in mid-April, about two weeks into breeding season, were less likely to successfully find a mate. Additionally, males that established home ranges containing more shrub cover were more likely to be successful in finding a mate. (Best and Rodenhouse, 1984; Jones and Cornely, 2002)

Since they are migratory birds, vesper sparrows reach their summer breeding ranges as early as April and stay until late August. Vesper sparrows usually have two to three broods per season, with 2 to 5 eggs each, although more have been reported. They may have 4 to 15 eggs per season. Throughout the season, clutch size tends to decrease from an average of 4 eggs per nest in May through June to 3 eggs per nest in July. Both parents feed nestlings after incubating the eggs for 11 to 14 days (average 13). At hatching, nestlings are an average of two grams at birth, and are ready to leave the nest after 7 to 14 days – on average 9.6 days. However, juveniles generally remain dependent on their parents for 20 to 29 days after hatching. Juveniles, both females and males, become sexually mature by next mating season.

Nest success is relatively low, with Ludlow et al (2014) and Rodenhouse and Best (1983) reporting an average of only 29% of nests being successful. Due to the many causes of nest failure on croplands, annual productivity for each territory was only 2.8 fledglings. (Dawson and Evans, 1960; Jones and Cornely, 2002; Ludlow, et al., 2014; Rodenhouse and Best, 1983)

  • Breeding interval
    Vesper sparrows usually breed once or twice every year during the breeding season, sometimes more often.
  • Breeding season
    Breeding occurs from April to August.
  • Range eggs per season
    4 to 15
  • Average eggs per season
    7.5
  • Range time to hatching
    11 to 14 days
  • Average time to hatching
    13 days
  • Range fledging age
    7 to 14 days
  • Average fledging age
    9.6 days
  • Range time to independence
    20 to 29 days
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (male)
    1 years

There is parental investment from both male and female vesper sparrows. Females build nests and incubate eggs most often. Males incubate on occasion and may take responsibility for one brood while females start another. Finally both parents feed nestlings and protect the nest. Research suggests that in general, birds may be more aggressive in defending nestlings as they reach fledging age, especially later in the season when they may not be able to have another brood. There is no research on post fledgling dependence. (Dawson and Evans, 1960; Jones and Cornely, 2002; Montgomerie and Weatherhead, 1988)

  • Parental Investment
  • altricial
  • male parental care
  • female parental care
  • pre-hatching/birth
    • provisioning
      • male
      • female
    • protecting
      • male
      • female
  • pre-weaning/fledging
    • provisioning
      • male
      • female
    • protecting
      • male
      • female
  • pre-independence
    • provisioning
      • male
      • female
    • protecting
      • male
      • female

Lifespan/Longevity

The oldest recorded vesper sparrow caught in the wild was 7 years and one month old. No information is known about the expected lifespan of vesper sparrows in the wild. Vesper sparrows are not kept in captivity. (Klimkiewicz and Futcher, 1989)

  • Range lifespan
    Status: wild
    7.08 (high) years

Behavior

Vesper sparrows use song to establish and defend territories, and for mate selection. During mating and courtship, males and females communicate using visual and auditory cues. Males moves toward females, or in front of her, with their wings and tails raised and spread. During courtship, males will also occasionally take flight and sing. In birds of the order Passeriformes, including vesper sparrows, males may feed females, a behavior known as courtship feeding that may also occur outside of courtship.

Vesper sparrows forage for food by walking or flying to different sites within their territory, traveling further away as the season progresses. During the winter, Gryzbowski (1983) reported that vesper sparrows were found to have formed small groups in southern Texas where food was relatively abundant. Additionally, flock size increased when closer to cover, and individual vesper sparrows generally foraged alone when greater than four meters away from cover.

Reed (1984) noted during his field studies that if a vesper sparrow flew into the territory of another when it was being flushed out from hiding, it was chased out by the resident sparrow. (Gryzbowski, 1983; Pulliam and Mills, 1977; Reed, 1984; Rodenhouse and Best, 1983)

Home Range

In central Iowa, Rodenhouse and Best (1983) found that male vesper sparrows migrated in and established their territories beginning in mid-to-late April, with females arriving a few days afterword. On cropland, almost all territories were found along fencerows and spread out up to 80 meters into the fields on both sides of the fence, with the exception of only one sparrow not including a fence line in its home territory. Rodenhouse and Best (1983) found that the home range ranged from 1.8 to 3.2 ha, with an average of 2.3 ha. In 2009, Macías-Duarte and Panjabi found that in the Chihuahuan Desert of northern Mexico, home range was an average of 108.46 ha, whereas in 2010, home range size was an average of 30.91 ha (2013). On grazed pasture, Wiens found the average home range size to be 0.9 ha (1969). In Montana, vesper sparrow territories ranged from 0.25 ha to 5.09 ha, with an average of 1.65 ha (Reed 1984). Home ranges with food that is either widely dispersed and/or scarce are generally larger than ranges with an abundance of food. Furthermore, the density of vesper sparrows was lower in croplands than uncultivated areas. Territory size has never been quantified in literature but based on general bird behavior is most likely just the area surrounding the nest. (Macías-Duarte and Panjabi, 2013; Reed, 1984; Rodenhouse and Best, 1983; Wiens, 1969)

Communication and Perception

Like most other passerines (Passeriformes), vesper sparrows have a variety of methods of communication and perception. Birds are best able to hear frequencies between 2 and 5 kHz, but songbirds like vesper sparrows have a better ability to hear high frequencies compared to non-songbirds. Vesper sparrows use songs to establish and defend territories, and for mate selection. Vesper sparrows can have many songs that vary slightly, but the basic structure is two to four whistles followed by up to seven trills. Additionally, Wells and Vickery (1994) describe longer songs called "extended flight songs" that last about 6 to 10 seconds and are usually given in flight. The four subspecies apparently have weak dialects (differences across geographic regions).

Passerine birds have excellent sight. In addition to the visible spectrum (ca. 380 to 740nm), birds such as vesper sparrows see in the ultraviolet light range. This allows them to better select mates, find food, and avoid predation.

Chemical pheromones influence mate choice in vesper sparrows and play a role in intraspecific communication. Their olfactory senses may also aid in predator detection.

Males and females communicate using visual and auditory cues while communicating. Males move around females while raising their wings and tails. During courtship, the male will also occasionally take flight and sing. Furthermore, vesper sparrows, like other passerines, typically use tactile communication when males feed females. This is a behavior known as courtship feeding that may also occur outside of courtship. (Caro and Balthazart, 2010; Kroodsma, 1972; Ödeen, et al., 2011; Okanoya and Dooling, 1987; Ritchison, 1981; Smith, 1980; Wells and Vickery, 1994)

Food Habits

In winter, vesper sparrows eat primarily grains and seeds of varying sizes, weighing up to about 2 mg. Five of the most common seeds that vesper sparrows eat include the following genera: amaranth (Amaranthus), cupgrass (Erichloa), dropseed (Sporobolus), fingergrass (Chloris), and panicum (Panicum). They eat other plants in winter, including threeawn (Aristada), stinkgrass (Eragrostis), carpetweed (Mollugo), purslane (Portulaca), vervain (Verbana), clover (Trifolium), neckweed (Veronica), goosefoot (Chenipodium), and knotweeds (Polygonum) and (Fallopia).

In summer, vesper sparrows feed on insects in addition to grain, and feed their nestlings almost exclusively insects. Some of the most commonly eaten insects are grasshoppers (order Orthoptera), beetles (order Coleoptera), and caterpillars (order Lepidoptera). Additionally, they have been found to eat spiders (order Araneae), membrane-winged insects (order Hymenoptera), dragonflies (order Odonata), lice (order Phthiraptera), true bugs (order Heteroptera), flies (order Diptera), leafhoppers (suborder Auchenorrhyncha), jumping plant bugs (suborder Sternorrhyncha), barklice (order Psocoptera), millipedes, (order Julidae), windscorpions (order Solifugae), and pseudoscorpions (order Pseudoscropiones).

In addition to eating insects during the breeding season, vesper sparrows have been documented eating the following: panic grass (Panicum), amaranth (Amaranthus), dropseed (Sporobolus), bluegrass (Poa), needle and thread grass (Hesperostipa), goosefoots (Chenopodium), muhly (Mulenbergia), peppergrass (Lepidium), wedgescales (Sphenopholis), papyrus sedges (Cyperus), clover (Trifolium), dead-nettles (Lamium), beard grass (Polypogon), spurges (Euphorbia), grama grass (Bouteloua), fleaworts (Plantago), sedges (Carex), violets (Viola), rock jasmines (Androsace), pennycress (Thlaspi), false flax (Camelina), purslanes (Portulaca), oatgrass (Danthonia), hair grass (Deschampsia), bristle grasses (Setaria), indiangrass (Sorghastrum), milkweeds (Asclepias), wood sorrels (Oxalis) and hair moss (Polytrichum).

Seasonal variations between spring and summertime diets have been recorded. This includes changes in the percentages of grain vs animal foods that make up their diet and varying composition of organisms eaten. Evans (1964) found that diets of vesper sparrows change from 59 to 44 percent plant materials from spring to summer, respectively. Rotenberry (1980) found that during breeding season, the diets of vesper sparrows consisted of at least 70 percent insects. Furthermore vesper sparrows ate proportionally more grasshoppers as they became abundant; usually constituting over half of their diet during this time period (June to August). Feist (1968) found that grasshoppers, leaf beetles, and snout beetles made up 59 percent of their diet in the summer of 1966, and 42 percent of their diet in the summer of 1967, with grasshoppers making up 30 and 33 percent of their diet in those years, respectively.

Most (63 to 100 percent) adult vesper sparrows eat grit and small rocks as a digestive aid.

Vesper sparrows forage for food by walking or flying to different sites within their territory, traveling further away as resources become scarcer. In croplands and other areas, they are most commonly found foraging where vegetation is not very dense. (Desmond, et al., 2008; Evans, 1964; Feist, 1968; Jones and Cornely, 2002; Rodenhouse and Best, 1994; Rotenberry, 1980)

  • Animal Foods
  • insects
  • terrestrial non-insect arthropods
  • Plant Foods
  • seeds, grains, and nuts
  • bryophytes

Predation

Vesper sparrows are a known prey item of Richardson’s merlins (Falco columbarius richardsonii) in southeastern Montana. They are eaten by northern Pacific rattlesnakes (Crotalus oreganus). The remains of a vesper sparrow were also found in a pellet from a short-eared owl (Asio flammeus). Grant et al. (2005) noted that thirteen-lined ground squirrels (Spermophilus tridecemlineatus) prey on vesper sparrows.

Predators of wintering sparrows in Arizona include Cooper’s hawks (Accipiter cooperi), prairie falcons (Falco mexicanus), American kestrels (Falco sparverius), and loggerhead shrikes (Lanius ludovicanus).

Based on field observations, Rodenhouse and Best (1983) predicted that potential nest predators include American crows (Corvus brachyrhynchos), blue jays (Cyanocitta cristata), garter snakes (Thamnophis), fox snakes (Pantherophis vulpinus), deer mice (Peromyscus), coyotes (Canis latrans), and domestic dogs (Canis lupus familiaris) and cats (Felis catus).

Jones and Cornely (2002) state that probable predators include red foxes (Vulpes vulpes), eastern spotted skunks (Spilogale putorius), striped skunks (Mephitis mephitis), and raccoons (Procyon lotor).

Grant et al. (2005) predicted that early nesting may be an adaptation by vesper sparrows to avoid predators, which become more common later in the breeding season.

In order to avoid nest predation, ground-nesting birds such as vesper sparrows frequently camouflage and conceal their nests in vegetation. (Becker, 1985; Grant, et al., 2005; Johnston, 1956; Jones and Cornely, 2002; MacArtney, 1989; Pulliam and Mills, 1977; Rodenhouse and Best, 1983; Stephenson, et al., 2019)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Vesper sparrows serve as both prey items and parasite carriers within their ecosystems. Wray and Whitmore (1979) reported a fledging success rate of 38%, with all failed nests having been lost to predation. Rodenhouse and Best (1983) observed nest parasitism by brown-headed cowbirds (Molothrus ater), which was higher during early breeding season, accounting for 11% of failed nests. Rates of predation were found to be roughly the same, causing about 29% of lost nests.

In addition to having their nests parasitized by cowbirds, vesper sparrows have been found to carry ectoparasites, including louse flies Orinthoica vicina. They also carry protozoan parasites, including the avian malaria parasite Plasmodium elongatum, Trypanosoma, which causes sleeping sickness and other livestock diseases, and Haemoproteus and Leucocytozoon, two genera of parasitic alveolates. (Grant, et al., 2005; Herman, 1944; Main and Anderson, 1970; Rodenhouse and Best, 1983; Wray and Whitmore, 1979)

Commensal/Parasitic Species
  • Brown-headed cowbirds Molothrus ater
  • Louse flies Orinthoica vicina
  • Avian malarian parasite Plasmodium elongatum
  • Kinetoplastids Trypanosoma
  • parasitic alveolates Haemoproteus
  • parasitic alveolates Leucocytozoon

Economic Importance for Humans: Positive

Vesper sparrows contribute to the bird watching sector of the ecotourism industry in America. During the mid-80s, the bird watching industry was estimated to be worth about $20 billion. Point Pelee National Park in Canada, part of the breeding range of vesper sparrows, was number six out of America's top twelve most popular birding sites. In 1987 alone, Point Pelee National Park spent an estimated $5.4 million in association with bird-watching. (Hvenegaard, et al., 1989; "Status and management of neotropical migratory birds", 1993)

Economic Importance for Humans: Negative

There are no known adverse effects of vesper sparrows on humans.

Conservation Status

The IUCN Red List of Threatened Species lists vesper sparrows in the category of Least Concern, although populations are thought to be decreasing. Vesper sparrows are protected under the US Migratory Bird Act, making it illegal to sell, possess, or transport them without a valid Federal Permit. Vesper sparrows have no special status under CITES or in the state of Michigan.

The American Bird Conservancy has petitioned for Oregon vesper sparrows (Pooecetes gramineus affinis) to be listed as endangered or threatened under the U.S. Endangered Species Act. Threats include loss of natural habitats and negative impacts of land management, such as mowing or overgrazing on nesting vesper sparrows. These threats play a role in declining populations. If listed as endangered, Oregon vesper sparrows would be afforded more protections, such as having protected land, mitigating the negative impacts of habitat loss. Oregon vesper sparrows are already listed as birds of Conservation Concern by the US Fish and Wildlife Service. This does not offer any extra protections, but it identifies them as at risk of becoming endangered in hopes of stimulating conservation actions.

Vesper sparrows are critically imperiled in the states of Connecticut, Kentucky, Massachusetts, New Jersey, and Tennessee. Vesper sparrows are listed as imperiled in Kansas, New Hampshire, North Carolina, Vermont, and Wisconsin. They are considered vulnerable in the states of Delaware, Maine, New York, and West Virginia. Different states have different plans of action for protecting vesper sparrows, some of which are as simple as monitoring the amount of birds in the state.

As an example of a more protective state policy, Connecticut's Wildlife Action Plan includes measures to help upland herbaceous grassland species, such as creating, restoring, and maintaining herbaceous grasslands, increasing the number of prescribed burns, working with the department of transportation to limit habitat fragmentation, and developing incentives for towns to preserve herbaceous grasslands. ("2015 Connecticut Wildlife Action Plan", 2015; "Availability of Birds of Conservation Concern 2008", 2009; BirdLife International, 2016; NatureServe, 2019; "Petition to list Oregon vesper sparrow (Pooecetes gramineus affinis) as an endangered or threatened species under the U.S. Endangered Species Act", 2016)

Contributors

Abigail Ouellette (author), Radford University, Lauren Burroughs (editor), Radford University, Logan Platt (editor), Radford University, Karen Powers (editor), Radford University, Galen Burrell (editor).

Glossary

Nearctic

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.

World Map

acoustic

uses sound to communicate

agricultural

living in landscapes dominated by human agriculture.

altricial

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.

bilateral symmetry

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.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

diurnal
  1. active during the day, 2. lasting for one day.
ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

endothermic

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.

female parental care

parental care is carried out by females

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

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).

male parental care

parental care is carried out by males

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

polygynous

having more than one female as a mate at one time

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

tropical savanna and grassland

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.

savanna

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.

temperate grassland

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.

visual

uses sight to communicate

References

Department of Energy and Environmental Protection. 2015 Connecticut Wildlife Action Plan. None. Hartford, CT: State of Connecticut. 2015.

U. S. Fish and Wildlife Service. Availability of Birds of Conservation Concern 2008. 74 FR 11128. Arlington, VA: U.S. Fish and Wildlife Service. 2009.

U.S. Fish and Wildlife Service. Migratory Bird Treaty Act. None. Falls Church, Virginia: U.S. Fish and Wildlife Service. 2018.

American Bird Conservancy. Petition to list Oregon vesper sparrow (Pooecetes gramineus affinis) as an endangered or threatened species under the U.S. Endangered Species Act. FWS-R1-ES-2018-0019. The Plains, Virginia: American Bird Conservancy. 2016.

U.S. Department of Agriculture, Forest Service. Status and management of neotropical migratory birds. RM-GTR-229. Fort Collins, CO: U.S. Department of Agriculture. 1993.

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