|Cardellina ruber melanauris
|Range of the red warbler|
The red warbler (Cardellina ruber) is a small passerine bird endemic to the highlands of Mexico, north of the Isthmus of Tehuantepec. It is closely related to, and forms a superspecies with, the pink-headed warbler of southern Mexico and Guatemala. There are three subspecies, found in disjunct populations, which differ primarily in the color of their ear patch and in the brightness and tone of their body plumage. The adult is bright red, with a white or gray ear patch, depending on the subspecies; young birds are pinkish-brown, with a whitish ear patch and two pale wingbars.
Like all New World warblers, the red warbler is an insectivore. It gleans primarily in understory shrubs. Breeding typically occurs between February and May. The female lays three or four eggs in a domed nest, which she builds on the ground. Though she alone incubates the eggs, both sexes feed the young and remove fecal sacs from the nest. The young fledge within 10–11 days of hatching.
When he first described the red warbler in 1827, naturalist William John Swainson assigned it to the genus Setophaga. Over the next half century, other naturalists moved it to Cardellina, with the red-faced warbler, and to the widespread tropical warbler genus Basileuterus, as well as to the Old World warbler genus Sylvia and the Old World tit genus Parus. In 1873, naturalists Philip Lutley Sclater and Osbert Salvin moved the species to the genus Ergaticus, where it remained for more than a century.
There are three subspecies, which differ slightly in appearance:
- C. r. ruber, described by Swainson in 1827, has white ear patches and is found from southern Jalisco and southern Hidalgo to Oaxaca.
- C. r. melanauris, which was described by Robert Thomas Moore in 1937, has dark gray ear patches and somewhat more scarlet upperparts than C. r. ruber. It is found from southwestern Chihuahua to northern Nayarit.
- C. r. rowleyi was described by R. T. Orr and J. D. Webster in 1968. It has white ear patches and ruby-red upperparts (brightest of the three subspecies), and is found in the Sierra Madre del Sur, from Guerrero to southern Oaxaca.
The red warbler forms a superspecies with the pink-headed warbler of Chiapas and Guatemala, to which it is closely related. Despite their disjunct ranges and considerably different plumages, the two have sometimes been considered conspecific. Conversely, it has also been suggested that the red warbler should be split into a northern gray-eared species (C. melanauris) and a southern white-eared species (C. ruber).
The red warbler's common name is a straightforward reference to its color. The genus name Cardellina is the diminutive of the Italian cardella, a regional name for the European goldfinch, while its specific name, ruber, is Latin for "red".
The red warbler is a small passerine, measuring 12.5–13.5 cm (4.9–5.3 in) in length,[nb 1] and weighing from 7.6 to 8.7 g (0.27 to 0.31 oz). As an adult, it is red overall, with either a white or dark gray (depending on the subspecies) auricular patch on each side of its head. Its slightly darker wings and tail are edged in pinkish-red. Its legs are flesh-colored, and its flesh-colored bill shows a dark tip. The feathers of this species contain alkaloids, which render the bird unpalatable; humans find it inedible.
As a juvenile, the red warbler is pinkish-brown with a whitish auricular patch. Its darker wings and tail show pinkish-cinnamon edges, with two paler wingbars on the former.
The red warbler has several common calls, including a high, thin tsii and a stronger pseet. Its song is a mix of short trills and richer warbles, interspersed with high-pitched chips. Unlike other species in the same habitat zone, it tends to sing only during bright morning hours during the breeding season; regardless of season, it does not sing – and even its calling frequency decreases – in cloudy weather.
Habitat and range
Endemic to the highlands of Mexico north of the Isthmus of Tehuantepec, the red warbler has three disjunct populations: from southwestern Chihuahua to northern Nayarit, from southern Jalisco and southern Hidalgo to Oaxaca, and from Guerrero into southern Oaxaca in the Sierra Madre del Sur. It is fairly common to common in the interior and on adjacent slopes, where it occurs at elevations ranging from 1,800 to 3,900 metres (5,900 to 12,800 ft) above sea level. It is an altitudinal migrant, moving from higher humid or semi-humid pine, pine-oak and fir forests in the breeding season to lower elevations, often in oak forests, in the winter. It is among the most common of the small birds in its woodland habitat, second only to the golden-crowned kinglet in fir forests in one study and the third most common warbler in oak-conifer woodlands in another.
Though the species was reported to have been collected in Texas in the late 19th century, the record's location was not widely believed, and there is no strong evidence that it ever occurred there.
Though it sometimes joins mixed-species flocks, the red warbler is more typically found alone or in pairs. Youngsters probably choose mates in the autumn of their first year, and pairs remain together year-round, except during severe weather and during post-breeding molt.
Food and feeding
The red warbler is an insectivore. It gleans primarily in understory shrubs at low to middle levels, moving slowly and deliberately through more open areas of the vegetation, and feeding with quick jabs into cracks in bark and pine needle clusters. It sometimes hover gleans to feed at pine needle clusters. Though it lacks any obvious adaptations for climbing, it regularly does so in its search for prey items on bark and epiphytes on branches, often hanging head-down as it probes. In areas of deciduous growth, it typically flycatches, making brief aerial sorties from a perch in pursuit of flying insects. While it seldom associates with mixed-species flocks, it feeds alongside other birds with no signs of conflict, displaying no hostility towards other species with which it competes. Its foraging area is quite small, often amounting to only a few dozen square meters (several hundred square feet) per day. Late in the afternoon, its rate of foraging declines, and it rests, often taking brief naps, in the forest understory. Though it does not generally feed after sunset, it may do so to take advantage of transient food sources, such as hatching Neuroptera.
The red warbler breeds primarily in early spring, from February until May, though at least one nest containing young has been found as late as the end of June. The female alone builds the nest, a task which typically takes 4–6 days. She chooses a sunlit area, such as an area of windfall, the brushy edge of a trail or water course, or a small clearing, for its location. Tightly woven of plant material, the nest is hidden in ground vegetation and anchored to the stalks of surrounding vegetation. Bulky and untidy on the outside, it is typically constructed primarily of dead pine needles and dead grass, though gray lichens, green moss, dead leaves, shreds of bark and tips of fern fronds are also used; most of these materials are gathered from the ground close to the nest, though some is picked from low branches or further away. The nest, which is usually oven-shaped with a side or upward-facing entrance, measures roughly 6 in (15 cm) wide by 7 in (18 cm) long by 4.5 in (11 cm) high. A small number of nests are only cups, lacking the roof of the more typical structures. Inside, the nest is tidy and compact, lined with fine grasses and plant fluff, which is generally gathered some distance from the nest.
Early in the breeding season, as many as 11 days may transpire between the completion of the nest and the laying of the first egg. Later in the season, this time decreases so that the first egg is laid as soon as the nest is ready. The female normally lays three eggs, though clutches of up to four have been recorded. The eggs, which are variously described as pale pink with evenly distributed brown spots or white with cinnamon and rust spots densely ringing the larger end of the egg, measure 16–17 mm (0.63–0.67 in) by 13 mm (0.51 in) and weigh 1–1.4 g (0.035–0.049 oz). The female alone incubates the eggs for 16 days; the male does not even approach the nest until several days after the eggs hatch. She sits facing the back wall of the nest, with her head and body sheltered by its roof and her tail sticking out the opening. She sits tight at the approach of danger, typically not flying until a potential predator actually makes contact with the nest.
Both adults feed the nestlings and remove fecal sacs, though the female removes far more than the male does. The pair move deceptively when approaching the nest, foraging – or pretending to forage – in nearby vegetation. They stay only a few seconds in any one spot, including at the nest, making it more difficult for a predator to locate the young. The nestlings, which make a rapid, high-pitched peeping call as an adult approaches carrying food, fledge within 10–11 days of hatching.
Conservation and threats
The red warbler is currently rated as a species of least concern by the International Union for the Conservation of Nature. Though there is evidence that its numbers are decreasing, the decline has not been precipitous, and the population remains quite large, with estimates ranging from 50,000 to 499,999 birds. The forested areas in which it occurs, however, include some of the most threatened habitats in Mexico, with logging, agricultural expansion, firewood gathering, road building, tourist development, overgrazing and intensive urbanization among the many things contributing to the destruction of the forests. There is some evidence that selective logging in pine forests may actually favor this species, which prefers more open, sunlit areas in which to breed.
- By convention, length is measured from the tip of the bill to the tip of the tail on a dead bird (or skin) laid on its back.
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- Red warbler photos on the Academy of Natural Sciences' Visual Resources for Ornithology website
- Red warbler videos on the Internet Bird Collection website
- Red warbler vocalizations on the Macauley Library's (Cornell Laboratory of Ornithology) website
- Red warbler vocalizations on the xeno-canto.org website