There are three subspecies, found in disjunct populations, which differ in the color of their ear patch and in the brightness and tone of their body plumage.

Although this bird is considered to be a least-concern species according to the International Union for Conservation of Nature (IUCN), its numbers are thought to be declining due to habitat destruction.

English jeweler and naturalist William Bullock and his son traveled to Mexico soon after its independence, spending six months in 1823 collecting archaeological artifacts and many bird and fish species new to science.

In 1873, English naturalists Philip Lutley Sclater and Osbert Salvin moved the species to the genus Ergaticus, where it remained for more than a century.

The authors recommended moving the red and pink-headed warblers to the genus Cardellina,[7] a suggestion which has been adopted by the International Ornithologists' Union (IOC).

The genus name Cardellina is the diminutive of the Italian cardella, a regional name for the European goldfinch,[14] while its specific name, rubra, is Latin for "red".

[5] 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.

[9] The red warbler has several common calls, including a high, thin tsii and a stronger pseet.

[23] 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.

[9] Youngsters probably choose mates in the autumn of their first year, and pairs remain together year-round,[5] except during severe weather and during post-breeding molt.

[20] The red warbler breeds primarily in early spring, from February until May,[5] though at least one nest containing young has been found as late as the end of June.

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.

It gleans primarily in understory shrubs at low to middle levels,[5] moving slowly and deliberately through more open areas of the vegetation,[30] and feeding with quick jabs into cracks in bark and pine needle clusters.

[20] Though it lacks any obvious adaptations for climbing, it regularly does so in its search for prey items on bark and epiphytes on branches,[31] often hanging head-down as it probes.

[20] 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 often feeds alongside other birds with no signs of conflict, displaying no hostility towards other species—such as the slate-throated whitestart (Myioborus miniatus)—with which it competes.

[27] Isospora cardellinae is a protozoan species that has been isolated from a red warbler from Nevado de Toluca National Park, Mexico.

[35] 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.