- Vireo cassinii
Length: 11-14 cm (4.3-5.3 in)
- Grey head, flanks, and whitish underparts
- Yellow flanks
- Olive green-grey back
- Bright green rump
- Brown black wings and tail
- Dull olive green side to chest
- Solid white eye spectacle
- 2 white wing bars
- Brown iris
- Black, grey based bill
- Grey blue legs
Sexes similar, female slightly duller
Blue-headed Vireo usually has brighter yellow flanks and more contrast between the face and malar. Plumbeous Vireo is duller with little contrast between the head and the back and lacks yellow on the flanks.
Breeds from British Columbia and southwestern Alberta through central Idaho, across to coastal Washington and Oregon, south to southern California and in Baja California, Mexico. Recent results suggest that at least part of the US population after having bred in early parts of summer migrates to north-western Mexico where a second round of breeding takes place.
Two subspecies are recognized:
- V. c. cassinii:
- V. c. lucasanus:
- Mountains of southern Baja California (Sierra de la Laguna)
Sluggish; moves slowly through the vegetation.
The diet includes insects.
The nest is a cup made from bark strips and down. The clutch consists of 2 to 5 white, brown spotted eggs.
Primary song, by male alone, a series of disjointed phrases, each consisting of 2–4 burry notes, interspersed with pauses similar to song of Plumbeous Vireo. Often described as a "question and answer" song. Also an extended rasping chatter, sometimes turning into a loud grating call described by Dawson as a “rasping, nerve-grating war-cry.”
Southern Baja California race (V. c. lucasanus) sedentary. Nominate race wholly migratory, with discrete breeding and wintering areas.
- Clements, J. F., T. S. Schulenberg, M. J. Iliff, S. M. Billerman, T. A. Fredericks, B. L. Sullivan, and C. L. Wood. 2019. The eBird/Clements Checklist of Birds of the World: v2019. Downloaded from http://www.birds.cornell.edu/clementschecklist/download/
- Rohwer, S., K.A. Hobson, V.G. Rohwer (2009) Migratory double breeding in Neotropical migrant birds. Proceedings of the National Academy of Sciences. pnas.0908121106; DOI: 10.1073/pnas.0908121106
- Wikipedia contributors. (2020, April 18). Cassin's vireo. In Wikipedia, The Free Encyclopedia. Retrieved 06:04, April 26, 2020 from https://en.wikipedia.org/w/index.php?title=Cassin%27s_vireo&oldid=951779066
- Cornell Lab of Ornithology. 2019. All About Birds. Cornell Lab of Ornithology, Ithaca, New York. https://www.allaboutbirds.org/guide/Cassins_Vireo/ Accessed on 26 April 2020
- Goguen, B. and D. R. Curson (2020). Cassin's Vireo (Vireo cassinii), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.casvir.01
- Brewer, D. (2020). Cassin's Vireo (Vireo cassinii). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/61257 on 26 April 2020).
- BirdForum Opus contributors. (2023) Cassin's Vireo. In: BirdForum, the forum for wild birds and birding. Retrieved 30 November 2023 from https://www.birdforum.net/opus/Cassin%27s_Vireo
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