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The curve-billed thrasher (Toxostoma curvirostre) is a medium sized mimid that is a member of the Toxostoma genus, native to the southwestern United States and much of Mexico. Referred to as the default desert bird, it is a non-migratory species. Several subspecies have been classified since 1827, though a definite number has been disputed. Allopatric speciation is believed to have played a major role in the variations of the curve-billed. Along with its numerous subspecies is the striking similarities in appearance that it shares with is fellow Toxostoma member Bendire's thrasher. Nevertheless, it is recognized for its grey-like appearance and sickle-shaped bill. It generally resides in numerous desert plants within desert regions of the United States and Mexico, but can inhabit areas predominately populated by humans.
The demeanor of the curve-billed has been described as "shy and rather wild", but it allows humans to view it closely. It also has been spotted shrilling "Whit-Wheet!" while flying from bush to bush. The curve-billed also is very aggressive in driving out potential threats, whether for feeding themselves or protecting its chicks from predators. The curve-billed thrasher is also capable of mimicking several other species, though not as prominent as other mimids. Its songs are generally distinctive from one another, and its extensive repertoire of melodies has led it to be known as Cuitiacoache in Mexico, which means songbird.
The species was first named Toxostoma curvirostre by William John Swainson in 1827. Since then, six subspecies have been recognized, and have been divided into two branches. Genetic research has indicated that the ocellated thrasher is the closest species related to the curve-billed thrasher, and is a sister species to the brown thrasher, long-billed thrasher, and Cozumel thrasher.
T.c. celsum (R. T. Moore, 1941) 'plateau thrasher'. Range spans from SE Colorado, SW Kansas, extreme NW Oklahoma to S/SE Arizona, S New Mexico and W Texas to Northern Mexico (E Chihuahua to Guanajuato and Jalisco)
T. c. palmeri (Coues, 1872) Palmer's thrasher. Range is S Arizona, N Sorona and Chihuahua. This species is the representative of the western species, and was suggested to have enough variations to be considered a different species from T.c. curvirostre. T.c. palmeri was also the used in a 2009 proposal by the American Ornithologists' Union to be elevated to species status, along with the eastern T.c. curvisrostre representing the eastern curve-billed thrashers.
The curve-billed thrasher is generally 25 to 28 cm (10 to 12 inches) long, and there is no apparent sexual dimorphism within the species. It is immediately recognized for its long sickle-shaped bill, which is almost as long as its head width, and with a brownish-black color. The body is of a compact build with a large head, short wings, and a long tail. However, their tail is short relative to other thrasher species. The plumage is generally brownish-grey while the chest is typically mottled. The eyes are generally orange with golden shades in adulthood. Juveniles are lacking in pale tips, retrices, the abdominal feathers are unkempt, and the upper parts and chest are washed rufously.
The variations are markedly different with each subspecies. The eastern subspecies has more distinct spots on its chest, and with more white along the tips and retraces and obvious wing bars. T.c. palmeri has less spotting on the chest, tips, and less obvious wing bars. Other examples include T.c. curvirostre possessing longer wings and a tail than T.c. oberholseri, and that T.c. insularum is paler and has more grey than T.c. palmeri, and with more visible spotting on its breasts.
One study published in 2003 suggested that the speciation of the curve-billed was due to climate, and it could be explained with the molt and feather wear. It also suggested that the Sierra Madre Occidental acted as a barrier to initiate differentiation within the species.
Because of its similar coloration to Bendire's thrasher, the two birds are easily mistaken for one another. The bill of the curve-billed is all black, while that of Bendire's is paler down to the lower mandible, resembling a dark grey with a basal area that does not feature a stark contrast. The upper and lower mandibles of the curve-billed are curved, while the upper is curved and lower is essentially straight for the Bendires'. Although geographic variances for the curve-billed can make discerning chest patterns difficult, the curve-billed has a tendency to display larger and rounder spots overall on its chest, in contrast to the Bendires' smaller, more pronounced markings that are shaped like arrowheads. However, curve-billed thrashers are capable of presenting this feature as well. Bendires' typically have yellow eyes, and curve-billed eyes are typically orange, but Bendires' can have variations that appear yellow-orange, and some curve-billed eyes are red-orange, yellow-orange, or yellow. The curve-billed is heavier, and is more likely to inhabit suburban regions, and Bendires' are likely to avoid dense vegetation such as mesquite and prefers open grassland with scattered yucca or around hedgegrows in agricultural areas. The song call of Bendire's is said to be more melodic and continuous with a Chuck, while the curve-billed is known for its Whit-Wheet calls.
Habitat and range
The curve-billed thrasher is commonly found throughout the south-western United States, from the Sonoran Desert of Arizona and across New Mexico to west Texas, southeastern Colorado, southwestern Kansas as well as most of Mexico, from the Sonoran-Chihuahuan Deserts and south through the Mexican Plateau into Central Tamaulipas, inland to Oaxaca, and ranges on the coast of Sonora to Nayarit. Vagrants have been found in bordering states of its range, as far north as North Dakota, Alberta, and Manitoba and as far east as Florida. Palmeri types have been spotted in Florida, while curvirostre have been found in Iowa on several occasions. Other than the previously noted exceptions, the curve-billed is essentially a non-vagrant.
In comparison with other desert thrashers, the curve-billed is not as particular with habitats. The curve-billed can be found from ground level up to 3,000 m (9,800 ft). It generally resides where cholla and saguaro cacti, ocotillo, mesquites, palo verde, and creosote bushes are prevalent. It also can be located along woodland edges, piñon, dry desert bushland, and areas within its region where cacti are present. If there is an adequate amount of desert vegetation near human habitation along with feeders, the curve-billed can adapt within these type of environments.
The curve-billed thrasher can generally be found running rapidly from cover to cover, or flying near ground level from bush to bush. However, this bird is not shy about being in the open. The curved-billed is generally a forager on ground level, but can be very aggressive by routing out potential competitors for food at feeders, such the Inca dove and round-tailed ground squirrel. If it spots the nest of cactus wrens, it will usually destroy it.
Voice and song
The most distinctive voice of the curve-billed is the abrupt and brash Whit-Wheet, which sounds akin to a person whistling to get one's attention. When heard, it is usually chirped from high perches. The songs are similar to that of the northern mockingbird, but the notes do not end as abruptly, and its voice is regarded as more pleasant. The Curve-billed can utter its songs consecutively two or three times. The length of the songs span from 2 to 15 seconds. Some phrases are nasal and buzzy, but its large vocabulary enables it to conjure successive songs that are usually different from one another.
One recorded case of courtship behavior involving curve-billed thrashers described two males attacking each over vigorously, and resorting to purring and hissings sounds after neither bird appeared to have relented. The two males then puffed up their chests and strutted up and down in front of the female. They continued to alternate between purr and hiss and fighting one another until one became the victor. The victor flew towards the female, and both began to chirp melodically before copulation.
The breeding season for the curve-billed begins in February, and reaches an apex in March to May. However, new fledglings have been recorded as late as August. The dates vary within its range due to temperature and rainfall. The nests are generally built at conspicuous locations, and the cholla is the most frequent nesting plant. Other potential nesting sites range from desert plants such as mesquite, prickly pear, yucca to occasional occurrences of oak and pine trees. The nest is generally a deep cup but also can be flat excluding a depression for the eggs. The outer layer consists of thorny twigs, while the inner layer consist of smooth sticks, roots, and coarse grasses. Materials used to build the nest include fine grass, rootlets, or hair.
Both sexes build the nest, and nest building periods can be as brief as 3 days to as long as 4 weeks. The egg count differs by location; the number has been listed as low as 2 and as high as 5. The egg color also varies, as it can appear bluish-green to a pale yellowish-blue, spotted abundantly in reddish-brown. Both sexes incubate the eggs, but the female is mainly responsible for incubating as well as being the sole provider of brooding. Curve-billed parents defend their nests fervently against other species, but snakes are the top predators to eggs and nestlings, and are rarely victims of parasitism by cowbirds. In situations where food availability is scarce, parents may resort to feeding older fledglings and let the younger ones starve. The fledgling period ranges from 11 to 18 days.
The curve-billed thrasher is an omnivore. Its diet includes invertebrates such as beetles, moths, butterflies, arachnids, and snails. It also eats vegetable matter, and fruits from cacti, prickly pear, hackberries, and anacua, among other plants. The curve-billed has also been spotted eating dog food, and will feed it to their chicks.
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