Cassin's Vireo Life History

Habitat

Habitat Forests

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, horsechestnut, 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, thornforest, tropical deciduous, oak, pine-oak, pine, and pine-oak-fir forests, riparian woodlands, mesquite bosques, mangrove forest, and desert arroyos.

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Food

Food Insects

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. They 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, squashbugs, 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.

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Nesting

Nest Placement

Nest Tree

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 large tree.

Nest Description

It’s likely that both male and female 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 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 the interior diameter 2.2 inches and depth 1.6 inches.

Nesting Facts
Clutch Size:2-5 eggs
Egg Description:

Whitish with a few brown spots around large end.

Condition at Hatching:

Helpless.

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Behavior

Behavior Foliage Gleaner

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 Cassin’s 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 male and female share incubation and chick-feeding duties. After the young fledge, the family may join roving flocks of woodland birds before migrating southward in autumn.

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Conservation

Conservation Low Concern

According to the North American Breeding Bird Survey, Cassin's Vireo populations grew by an average of 1% per year between 1968 and 2015. Partners in Flight estimates a global breeding population of 5 million and rates the species a 9 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. This species disappears when forests are clearcut, 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.

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Credits

Goguen, Christopher B. and David R. Curson. (2002). Cassin's Vireo (Vireo cassinii), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2017). The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, USA.

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