- 4.3–5.5 in
- 9.4 in
- 0.5–0.6 oz
- Solitary Vireo (western form)
- Viréo de Cassin (French)
- Vireo de Cassin (Spanish)
- The Cassin's Vireo is the western-most form in the "Solitary Vireo" complex. Formerly considered one species, three species now are recognized. In appearance it is intermediate between the brightly colored Blue-headed Vireo and the dull gray Plumbeous Vireo. Its song also is intermediate between the other two species, being much hoarser than Blue-headed Vireo, but slightly higher and with more clear phrases than Plumbeous Vireo.
- Two subspecies of Cassin's Vireo are recognized. One is widespread in western North America from Canada to the northern part of Baja California. The other form is found only on the very southern tip of Baja California more than 800 km (500 mi) away.
- The Cassin's Vireo is a fearless defender of its nest. Both the male and female will vigorously scold a predator and dive at it. The female often will not leave her nest and sometimes can be picked up off of it by a human observer.
Breeds in dry, open forests of mountains and foothills.
- Clutch Size
- 2–5 eggs
- Egg Description
- Whitish with a few brown spots around large end.
- Condition at Hatching
Open cup suspended by rim from a fork of a branch of a tree or sapling. Woven of grasses, dead leaves, moss, and hair, decorated with lichen, spider egg cases, cocoons, pieces of wasp nest and paper. Inner lining of grasses, plant fibers, and hair.
Male performs nest-building display while slightly crouched with body horizontal, usually without nest material in bill.Forages in middle and lower levels of forest. Gleans insects from outer twigs and foliage. Forages in slow and deliberate manner.
Cassin's Vireo populations slightly increased overall between 1966 and 2015, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 4 million with 56% spending some part of the year in the U.S., 92% in Mexico, and 44% breeding in Cananda. The species rates a 10 out of 20 on the Continental Concern Score. Cassin's Vireo is not on the 2016 State of North America's Birds' Watch List.