Cassin’s Vireo

A delicately beautiful songbird with a bold white ring encircling the eye, Cassin’s Vireo sings a loud, burry song while foraging slowly through the lower and middle parts of trees. It breeds in dry, open forests of far western North America. Outside the breeding season, this species often joins mixed-species flocks of woodland birds. Until 1997, Cassin’s Vireo was lumped with Blue-headed Vireos and Plumbeous Vireos into a single species called Solitary Vireo. Cassin’s is less colorful than Blue-headed Vireos but richer in olive tones than the mostly Gray Plumbeous.

About Cassin’s Vireos

Named in honor of the nineteenth-century ornithologist, John Cassin, who published the first comprehensive work on the birds of the western United States. The Cassin’s Vireo is a common member of many coniferous and mixed-forest bird communities of far western North America. Although somewhat drab in appearance, and slow and deliberate in motion, Cassin’s Vireo is a conspicuous element of the forest, both in its loud and tireless singing and its raucous scolding call, given in defense of nests.

Unlike many other vireos and migratory songbird species, Cassin’s Vireo appears to be steadily increasing in abundance throughout most of its breeding range, particularly over the past 20 years. Today, we will be talking a little more about these birds that only recently got their own identities. We will be covering:

● Cassin’s Vireo Photos, Color Pattern, Song
● Cassin’s Vireo Size, Eating Behavior, Habitat
● Cassin’s Vireo Range and Migration, Nesting


Cassin’s Vireo Color Pattern

These birds have dull olive-green upperparts that change into a brownish-gray on the crown and auriculars. Their brownish-gray face contrasts with the bold, white rings that encircle their eyes. This makes it seem that they are wearing spectacles. The wings and tail are brownish-black, while there are two broad and yellowish-white wing bars that mark their wings. Their remiges and retrices have fine edges with olive-yellow or grayish olive. The outer rectrix has a fine edge of white. Their rump is a brighter olive green than their mantle. While their underparts are a dingy white with the sides of the breast has a little bit of a dull, olive-green. The flanks are paler and yellowish-olive, while their legs are a grayish-blue. The irises are brown, and the bill is black with a bluish-gray base. Both sexes are identical to each other in plumage, though females tend to be slightly duller.

Juveniles are not easy to distinguish from adults, but the whitish to the grayish-white lining of their upper mandible gives them away. This lining is black in adults. They also tend to have more brown and less bright colors, looking far drabber than the adults. These birds look very similar to Blue-headed Vireos in appearance, but tend to look grayer overall.

Description and Identification

Cassin’s Vireo nests in many open woodlands, from coastal forests to woodlands near treeline. Listen for its halting, rising-and-falling song, with its distinctly burry tone. The species also frequently makes a loud, fussing call, rendering it a fairly conspicuous bird within its habitat. Cassin’s Vireos forage at fairly low levels, on the outer parts of trees, so
spotting one can be easy. This species has an attraction to pishing sounds.

Cassin’s Vireo Song

The vocalizations of these birds require further study, but a few different calls have been recorded. The primary song is given only by the male and mostly functions in territorial establishment and defense, mate attraction, and inter-pair communication. It consists of a series of jerky and burry phrases, with each phrase have 2 to 4 notes. It sounds like “chreu chree chooreet”, “chree ch-ri-chi-roo”, or “ch-ree ree e eu ree u yuh ree-ruh”. They repeat this song constantly, especially if the male doesn’t have a mate during the breeding season. They rarely ever sing the same phrase twice in succession.

The next call is a chatter call, also known as a scolding call. It might be the most memorable call by these Vireos, with naturalist William Dawson describing it as a “rasping, nerve-grating war-cry” in 1923. This is used in hostile situations where the birds may encounter potential predators and may act as a long-range alarm call between pair members. It is best rendered as a “cheh cheh cheh cheh cheh chee” or a “ch-chi ch-chi ch-chi ch-chi ch-chi”, often with the last note extended and emphasized.

They also have contact calls that are of low volume and are rather simple. They are given between members of a pair during close contact or interaction. While there is little information on them, there are most likely numerous other call types that have been found in the closely related Blue-headed Vireo.

Cassin’s Vireo Size

Cassin’s Vireos are small birds that are around 4.3–5.3 inches long. They weigh around 0.5–0.6 ounces and have a wingspan of roughly 9.4 inches. They are small but sturdy songbirds, with a thick and short bill. Their wings and tail are both relatively long, giving them a graceful but stout appearance.

Cassin’s Vireo Behavior

Males of the migratory subspecies begin singing to establish territories as soon as they arrive back on the nesting grounds. Territorial displays aren’t known for Cassin’s Vireos, but males of the closely related Blue-headed Vireo confront each other by “countersinging” (each male singing in turn) and threat displays involve ruffling of feathers, holding the body horizontally in a “threat posture” while facing a rival, and aerial chases. They also sing rapidly and make scolding calls. Cassin’s Vireo probably has very similar behaviors. Prior to nest-building by females, the male performs a display in which he fluffs up his plumage and presents nest material to her with jerky, mechanical movements. Cassin’s Vireo is probably monogamous in its mating system. Both males and females share incubation and chick-feeding duties. After the young fledge, the family may join roving flocks of woodland birds before migrating southward in autumn.

Cassin’s Vireo Diet

Cassin’s Vireos consume mostly insects, including larvae. They also take spiders, seeds, and small fruits. They forage mostly in lower and middle levels in trees, in the outer portions of the tree. Cassin’s Vireos search their surroundings slowly and methodically, then hop or fly to seize prey with the bill. They glean most of their insect prey from leaves and twigs but also pursue insects in flight, either hovering or hawking to capture them. Insect prey items include moths, butterflies, caterpillars, stinkbugs, squash bugs, shieldbugs, ladybird beetles, leafhoppers, treehoppers, scale insects, weevils, bees, wasps, ants, grasshoppers, and crickets. Fruits and seeds apparently make up a small part of the diet, mostly in winter.

Cassin’s Vireo Habitat

Cassin’s Vireos nest in a remarkable variety of coniferous, deciduous, and mixed forests from sea level to about 8,000 feet elevation. In coastal areas, they favor oak forests, and in the interior, they generally nest more in pine, fir, Douglas-fir, and mixed forests. They typically use fairly dry and open woodlands, with key trees over their large range including ponderosa pine, lodgepole pine, sugar pine, and species of larch, birch, aspen, maple, horse chestnut, manzanita, madrone, spruce, cedar, redcedar, dogwood, and cottonwood. In the United States and Canada, Cassin’s Vireos sometimes nest in leafy suburbs. In Baja California Sur, Mexico, birds of the subspecies lucusanus nest from dry thorn forest at lower elevations up through pine-oak cloud forest at higher elevations. Migrating Cassin’s Vireos may turn up in almost any habitat, from lowland shrub-scrub to subalpine forest. As with many migrant passerines in western North America, small “oases” of habitat in the deserts, including verdant draws and washes, can be good places to look for them. Wintering birds in Mexico inhabit almost every wooded habitat, including second-growth, tropical evergreen, thorn forest, tropical deciduous, oak, pine-oak, pine, and pine-oak-fir forests, riparian woodlands, mesquite bosques, mangrove forest, and desert arroyos.

Range and Migration

These birds are found breeding from southern British Columbia in Canada, through the western coastal states of the United States. As the bird migrates, their routes take them towards the warmer regions of the Sonoran Desert in southern Arizona, and toward southern Mexico. They are medium to long-distance migrants. However, populations that breed in southern Baja California are permanent residents of their breeding habitats and do not migrate.

Cassin’s Vireo Lifecycle

These birds lay their only brood of the year during the breeding seasons, with the females laying 3–5 whitish brown spotted eggs. Both parents incubate the birds for 12–14 days, after which the chicks emerge in a completely helpless state. However, their nests are often parasitized by Cowbirds. They are fed by both parents, who tend to the young ones until they are independent. The birds are capable of leaving the nest around 2 weeks after they hatch.


Male and female visit potential nest sites together; the female probably selects the site, usually in a fork near the tip of a branch, in the middle level of a medium-sized to a large tree. It is likely that both males and females build the nest, a bulky, messy-looking cup suspended from a fork in the branches. The cup is made of leaves, grasses, and moss, and lined with grasses, plant fibers, and hair. As with other Vireo species, Cassin’s Vireo adorns the outer part of the nest with bits of paper, lichen, spider eggs, and pieces of hornets’ nests. Nests average about 3.2 inches across and 2.2 inches tall, with an interior diameter of 2.2 inches and depth of 1.6 inches.

Anatomy of a Cassin’s Vireo

Cassin’s Vireos are small birds that are around 4.3–5.3 inches long. They weigh around 0.5–0.6 ounces and have a wingspan of roughly 9.4 inches. They are small but sturdy songbirds, with a thick and short bill. Their wings and tail are both relatively long, giving them a graceful but stout appearance.

Final Thoughts

According to the North American Breeding Bird Survey, Cassin’s Vireo populations grew by an average of 1% per year between 1968 and 2015, making them a species of low environmental concern. This species disappears when forests are clear-cut, but in some areas where trees are selectively thinned, its populations have not declined and in some cases have increased. Deforestation in tropical wintering grounds has negative impacts on populations of this and other woodland species.

However, their numbers seem to be gradually increasing rather than decreasing, as is the case with many migratory songbirds. Although they continue to be susceptible to parasitism by Brown-headed Cowbirds, their populations flourish in regions that are not affected by these brood parasites. Despite their growing numbers, most aspects of their life remain poorly studied, mainly due to their former status as another species. As an intermediate between Blue-headed Vireos and Plumbeous Vireos, missing gaps of information can sometimes be reconstructed from the other two species. So, are you confident enough to try and identify each of these unique birds? Try it out! As you learn to identify them, the feeling of accomplishment can be unparalleled.


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At the Bird Watching Academy & Camp we help kids, youth, and adults get excited and involved in bird watching. We have several monthly subscription boxes that you can subscribe to. Our monthly subscription boxes help kids, youth, and adults learn about birds, bird watching, and bird conservation.

Bird Watching Binoculars for IdentifyingCassin’s Vireos

The most common types of bird watching binoculars for viewing Cassin’s Vireos are 8×21 binoculars and 10×42 binoculars. Bird Watching Academy & Camp sells really nice 8×21 binoculars and 10×42 binoculars. You can view and purchase them here.

Cassin’s Vireo Stickers

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Bird Feeders ForCassin’s Vireos

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Bird Houses ForCassin’s Vireos

There are many types of bird houses. Building a bird house is always fun but can be frustrating. Getting a bird house for kids to watch birds grow is always fun. If you spend a little extra money on bird houses, it will be well worth every penny and they’ll look great.

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