The Common Bush-tanager (Chlorospingus ophthalmicus) is a small passerine bird. It is a resident breeder in the highlands from central Mexico south to Bolivia and northwest Argentina.
The adult is 13.5 cm long and weighs 20g. They have a brown head with a (usually) thin supercilium and a white spot behind the eye and a light throat. The upperparts are olive and the underparts yellow, becoming white on the belly. Immatures are browner above, darker below, and have a duller olive eye spot. Coloration, especially of the cheeks, throat and eye region, is very variable across the wide range, giving weight to the theory that this is actually a superspecies.
The call is a squeaky tseeet, and the song is a high thin whichis whichis witchery tsee tseep seeur with many variations.
The related Chlorospingus pileatus has a blacker head with a bold white supercilium rather than an eye spot.
This bird is found in cloud forests and adjacent bushy clearings, typically from 400 m to 2300 m altitude. It is usually encountered in small groups or as part of a mixed-species feeding flock, and is rather sedentary. This passerine feeds on insects, spiders small fruits and nectar. The menoponid chewing louse Myrsidea ophthalmici was described from a Venezuelan specimen of this bird; it is not known from other hosts to date. The Venezuelan population of the Common Bush-tanager would, if this taxon is split up, be assigned to a distinct species.
The bulky cup nest is built on bank, slope, or hidden amongst epiphytes up to 15 m high in a tree. The normal clutch is two brown-marked white eggs, and this species is regularly double-brooded.
Systematics and taxonomy
Traditionally, the genus Chlorospingus was placed with the tanager family, Thraupidae. More recent research suggests that they are actually aberrant brush-finches in the Emberizidae and particularly close to Arremonops. Hence, as a common name "chlorospinguses" is arguably better than that the widely used "brush-tanagers", as the former is equally well suited to either placement. The use of chlorospingus has yet to be proposed.
The wide range and considerable morphological variation displayed by C. ophthalmicus has been a problem for ornithologists for many decades. Initially, many taxa presently united in this species were considered distinct, but they were eventually all lumped together. More recent results suggest that the "Common Bush-tanager" is actually a superspecies.
A preliminary study of allozyme data found pronounced divergences between the northern populations. mtDNA ATPase 8 sequence data found 5 major clades in the Mexican populations alone. These were subsequently confirmed by morphological comparisons. A formal splitting-up of this species into several seems to be the eventual outcome, pending data from the populations south of Mexico. In addition, it would be interesting to determine whether there are geographical variations in the song that would further strengthen the case for species status of the taxa. The morphological variation, though recognizable, is rather inconspicuous and probably more the consequence of genetic drift in freshly-isolated subpopulations than a cause for their separation.
It appears as if the group north of the Isthmus of Panama originated before the final bout of the Great American Interchange, by island-hopping across the emerging mountain ranges that now make up the isthmus, some 6 million years ago (mya). The direct descendants of the original colonizers of Mesoamerica are the southernmost Mesoamerican group, as is to be expected. Some 5.5 mya, the Sierra de los Tuxtlas population originated. The other groups west of the Isthmus of Tehuantepec radiated rather synchronously between 3 and 4 mya.
Notably, the divergence between the northern clades seems to have occurred far too early for the Pleistocene ice ages to have played a major part. Rather, it appears that the "Common Bush-tanager" evolved in and was always confined to montane cloud forest. Given its sedentary habits, the phylogeny seems to be a result of a combination of habitat expanding and constricting due to Pliocene climate changes and, less frequently, accidental dispersal (such as to the Sierra de los Tuxtlas and, earlier, to Mesoamerica in general)
The 5 northern clades are:
- Brown-headed Bush-tanager, Chlorospingus (ophthalmicus) ophthalmicus (Du Bus de Gisignies, 1847)
Sierra Madre Oriental, from south-east San Luis Potosí south to northern Oaxaca. May contain several subspecies. Status of birds from around Montserrate (westernmost Chiapas) unknown, but these are probably dwighti.
Crown olive. Throat whitish-grey, speckled and with indistinct "whiskers". Breast-band yellow to chartreuse yellow. Ear region dark. Eyestreak thin. Thin, incomplete white spectacles. Forehead spots indistinct.
- Dusky-headed Bush-tanager, Chlorospingus (ophthalmicus) postocularis Cabanis, 1866
Extreme SE Sierra Madre de Chiapas (Volcán Tacaná; probably west to El Triunfo) and southwards along the Pacific slope of the Central American mountains, probably to Nicaragua. If considered a separate species, includes subspecies honduratius and schistaceiceps.
Crown sepia. Throat almost white, speckled, no "whiskers". Breast-band yellow. Ear region whitish. Eyestreak thin, broader behind eye but no spectacles. Forehead unspotted.
- White-fronted Bush-tanager, Chlorospingus (ophthalmicus) albifrons Salvin & Godman, 1889
Sierra Madre del Sur in Guerrero and Oaxaca. May contain several subspecies such as persimilis of the Sierra de Miahuatlán.
Crown cinnamon. Throat light buff with conspicuous "whiskers" and speckled sides. Breast-band tangerine yellow. Ear region dark. Eyestreak thin, merging into incomplete white spectacles and hence shaped like a horizontal teardrop. Forehead with round white spots.
- Dwight's Bush-tanager, Chlorospingus (ophthalmicus) dwighti Underdown, 1931
Mountains of N Chiapas and easternmost Oaxaca, west to the Atlantic slope of Guatemala and probably the western Sierra Madre de Chiapas.
Crown olive. Throat almost white, speckled and with indistinct "whiskers". Breast-band chartreuse yellow shading to olive green. Ear region dark. Eyestreak thick. Conspicuous incomplete white spectacles. Forehead spots indistinct.
- Wetmore's Bush-tanager, Chlorospingus (ophthalmicus) wetmorei Lowery & Newman, 1949
Sierra de los Tuxtlas, Veracruz
Crown olive. Throat whitish-grey, speckled and with indistinct "whiskers". Breast-band yellow to chartreuse yellow. Ear region dark. Eyestreak thin. Thin but distinct incomplete white spectacles. White forehead spots V-shaped.