The western tanager was illustrated and formally described by American ornithologist Alexander Wilson in 1811 under the binomial name Tanagra ludoviciana from a specimen collected on the Lewis and Clark Expedition (1804–1806).

[7] Measurements: Adults have pale, stout pointed bills, yellow underparts, and light wing bars.

Adult males have a bright red face and a yellow nape, shoulder, and rump, with black upper back, wings, and tail; in non-breeding plumage, the head has no more than a reddish cast and the body has an olive tinge.

[10][11][12] The western tanager's wintering range stretches from central Costa Rica north through Nicaragua, Honduras, El Salvador, and Guatemala to southern Baja California Sur and extreme southeastern Sonora in western Mexico and to southern Tamaulipas in northeastern Mexico.

Cutleaf burnweed (Erechtites glomerata) was characteristic of the youngest age class, while slightly older sites were composed predominantly of tanoak (Lithocarpus densiflorus) with smaller amounts of snowbrush ceanothus (Ceanothus velutinus), whitebark raspberry (Rubus leucodermis), and Sierra gooseberry (Ribes roezlii).

[14] In addition, western tanagers were captured along the Rio Grande in New Mexico during spring and fall migration in an agricultural area composed primarily of alfalfa (Medicago sativa) and corn (Zea mays).

[15] Western tanagers have also been observed in saltcedar (Tamarix species) communities [15] and in Russian olive (Elaeagnus angustifolia) vegetation.

[15] Ten western tanagers were observed among three sites composed of Russian olive in Colorado, Utah, and Idaho.

All sites were dominated by Russian olive with cheatgrass (Bromus tectorum) comprising a substantial portion of the understory.

[11][12] In the Sandia Mountains of north-central New Mexico, western tanagers were heard singing beginning in late May, and the first nest was found in early June.

[11] Cup nests are built by the female, take about four or more days to construct, and are made from twigs, rootlets, grasses, and pine needles.

[18] During the breeding season, western tanagers are found primarily in relatively open coniferous forests and mixed woodlands.

Of 9 western tanager nests in an Alberta study site, eight occurred in white spruce (Picea glauca) and one was found in quaking aspen.

[11] Western tanager nests on a north-central New Mexico site occurred at heights from 8 to 15 ft (2–5 m), typically in white fir (Abies concolor) located in open areas.

[17] Western tanagers forage in many habitats, in all successional stages from grass-forb communities to stands of large trees with greater than 70% cover.

In a California study of foraging and habitat relationships of insect-gleaning birds in mixed conifer-oak forest, they used white fir more and incense-cedar (Calocedrus decurrens) less than would be expected from their availability.

[28] In addition, western tanagers were significantly positively associated with large saw timber (>20% cover, >21 in [>53.2 cm] mean dbh) and significantly negatively associated with pole timber (>20% cover; conifers >10 ft [>3 m] tall and 4–12 inh (10.2–30.4 cm) mean dbh; hardwoods 10–50 ft (3–15 m) tall and 4–12 in mean dbh) stands dominated by Douglas fir, western hemlock (Tsuga heterophylla), and red alder in the central Oregon Coast Ranges.

In sapling/pole and mature ponderosa pine habitats of the Black Hills in South Dakota, western tanagers occurred at the highest densities in stands with intermediate (40%-70%) canopy cover.

In British Columbia, western tanagers occurred at significantly higher densities after "light" logging on a site containing Douglas fir and ponderosa pine.

In the Sierra Nevada of California, western tanagers occurred at a higher density in an open-canopied (602 trees >10 cm dbh/ha) mixed-conifer stand consisting of Jeffrey pine (Pinus jeffreyi), lodgepole pine (P. contorta), white fir, and incense-cedar compared to a closed-canopied (994 trees >10 cm dbh/ha) mixed conifer stand of incense-cedar and white fir.

In the Black Hills of South Dakota, they were significantly more abundant in multistoried habitats with bur oak (Q. macrocarpa) and quaking aspen/paper birch (Betula papyrifera) under a ponderosa pine canopy than in sapling/pole or mature ponderosa pine stands with varying canopy cover.

[29] Reviews assert the importance of a diverse forest structure [28] and a dense deciduous understory [21] for western tanagers.

For example, removal of incense-cedar and white fir from 1 to 10 ft (0.3–3 m) tall in giant sequoia forests had little impact on western tanager density.

[26] Western tanagers' preference for multistoried habitats in the Black Hills may be related to the bur oak and quaking aspen/paper birch midstory.

[29] Western tanagers were not significantly related with abundance of pineland dwarf mistletoe (Arceuthobium vaginatum ssp.

The western tanager species was negatively associated with subalpine fir (A. lasiocarpa) cover in northern Rocky Mountain conifer forests.

For instance, in a California mixed conifer-oak forest consisting mainly of white fir, Douglas fir, incense-cedar, and California black oak, about 47% of western tanager foraging observations were gleaning, about 40% were hawking, and lunging and hovering occurred in about 6% and 7% of observations, respectively.

[24] In contrast, in the mainly Douglas fir-dominated communities of interior British Columbia, gleaning constituted 93.2% of western tanager foraging observations.

[31] In southwestern Idaho, western tanager remains were reported in one of over 170 prairie falcon (Falco mexicanus) nests observed.

Other reported nest predators include black bears (Ursus americanus), prairie rattlesnakes (Crotalus viridis), and bullsnakes (Pituophis catenifer).