Cassin's Vireo
Cassin's Vireo has surged: up 213% on the route-weighted index since 1970.
About the Cassin's Vireo
The Cassin's Vireo (Vireo cassinii) is a North American member of the Vireos (Vireonidae). In this analysis it is grouped with the forest birds.
- Size
- 4.5–6 in long (11–15 cm) — a small, deliberate songbird (typical for the family)
- Habitat
- Woodlands and forest edges, including wooded suburbs and parks.
- Diet
- Insects and spiders gleaned from foliage and bark, with seeds and berries in season.
- Range
- Recorded on 388 Breeding Bird Survey routes across 6 states, most concentrated in the Northern Pacific Rainforest.
- Family
- Vireonidae · Forest birds
Notable Cassin's Vireo Trends
Cassin's Vireo has surged in surveyed states: up 213% on the route-weighted index since 1970.
Cassin's Vireo Population Forecast
If the recent trend holds, Cassin's Vireo is projected to rise about 52% by 2029 — from 0.23 in 2024 to a central estimate of 0.36 (95% range 0.25–0.46). A 5-year backtest shows a typical error of ±49.7%, with 20% of held-out values landing inside the 95% band.
| Year | Projected index | 95% low | 95% high |
|---|---|---|---|
| 2025 | 0.34 | 0.24 | 0.45 |
| 2026 | 0.35 | 0.24 | 0.45 |
| 2027 | 0.35 | 0.25 | 0.45 |
| 2028 | 0.35 | 0.25 | 0.46 |
| 2029 | 0.36 | 0.25 | 0.46 |
Where the Cassin's Vireo Is Detected
BBS routes recording Cassin's Vireo, sized by most recent count.
Cassin's Vireo Population Trend by State
| Alaska | insufficient data | n/a | 2 |
| California | +287% | 1970 | 156 |
| Idaho | +166% | 1973 | 27 |
| Montana | -6% | 1970 | 33 |
| Oregon | +21% | 1970 | 95 |
| Washington | -14% | 1970 | 75 |
Cassin's Vireo Population Trend by Region
Bird Conservation Regions are the ecological unit for trends.
| Northern Pacific Rainforest | -19% | 1970 | 114 |
| Great Basin | +100% | 1970 | 77 |
| Northern Rockies | -21% | 1970 | 95 |
| Sierra Nevada | +21% | 1972 | 37 |
| Coastal California | +155% | 1970 | 60 |
Cassin's Vireo Conservation Status
Our route-weighted index shows it up about 213% since 1970.
Source: USGS North American Breeding Bird Survey, retrieved 2026-05-22. Trend is a route-weighted relative-abundance index, not an absolute population.