BirdAnalyst
Species · BBS 2025 Release · 1966–2024

Cassin's Vireo

VireonidaeForest birdsVireo cassinii

Cassin's Vireo has surged: up 213% on the route-weighted index since 1970.

+213%Since 1970TrendPercent change in the route-weighted abundance index between a smoothed baseline window and the most recent one. It tracks direction, not absolute population.Full methodology →
388RoutesSurvey routesHow many standard-protocol BBS routes contributed counts. More routes means a steadier, better-sampled index; very thin coverage is suppressed.Full methodology →
56Years SurveyedYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

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 TrendsNotable signalsLong-arc shifts the engine flags automatically — sustained declines or increases large enough to stand out from year-to-year noise.Full methodology →

long arc increasecomputed indexTrend sourceWhether the figure is our own computed route-weighted index or an official USGS modeled estimate. The current build labels every trend as computed.Full methodology →

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.

+52%Change by 2029TrendPercent change in the route-weighted abundance index between a smoothed baseline window and the most recent one. It tracks direction, not absolute population.Full methodology →
0.36Projected 2029 indexProjected indexThe central forecast of the abundance index if the recent trend continues. A projection of the current trajectory, not a prediction.Full methodology →
0.25–0.4695% range95% rangeThe 95% uncertainty band around the projection at the forecast horizon. The true value should land inside it most of the time.Full methodology →
±49.7%Backtest errorBacktest errorTypical error when we hold out the most recent years, forecast them, and compare to what actually happened. Lower means a more trustworthy projection.Full methodology →
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.19682029
Projection of the recent trend (dashed) with 80/95% bands — a projection, not a prediction. Habitat, climate, and land use are not modeled.
YearProjected indexProjected indexThe central forecast of the abundance index if the recent trend continues. A projection of the current trajectory, not a prediction.Full methodology →95% low95% rangeThe 95% uncertainty band around the projection at the forecast horizon. The true value should land inside it most of the time.Full methodology →95% high95% rangeThe 95% uncertainty band around the projection at the forecast horizon. The true value should land inside it most of the time.Full methodology →
20250.340.240.45
20260.350.240.45
20270.350.250.45
20280.350.250.46
20290.360.250.46

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

Cassin's Vireo population trend by state.
TrendPercent change in the route-weighted abundance index between a smoothed baseline window and the most recent one. It tracks direction, not absolute population.Full methodology →Baseline yearThe first year of the smoothed window the trend is measured from. An earlier baseline means a longer record stands behind the number.Full methodology →Survey routesHow many standard-protocol BBS routes contributed counts. More routes means a steadier, better-sampled index; very thin coverage is suppressed.Full methodology →
Alaskainsufficient datan/a2
California+287%1970156
Idaho+166%197327
Montana-6%197033
Oregon+21%197095
Washington-14%197075

Cassin's Vireo Population Trend by Region

Bird Conservation Regions are the ecological unit for trends.

Cassin's Vireo population trend by Bird Conservation Region.
TrendPercent change in the route-weighted abundance index between a smoothed baseline window and the most recent one. It tracks direction, not absolute population.Full methodology →Baseline yearThe first year of the smoothed window the trend is measured from. An earlier baseline means a longer record stands behind the number.Full methodology →Survey routesHow many standard-protocol BBS routes contributed counts. More routes means a steadier, better-sampled index; very thin coverage is suppressed.Full methodology →
Northern Pacific Rainforest-19%1970114
Great Basin+100%197077
Northern Rockies-21%197095
Sierra Nevada+21%197237
Coastal California+155%197060

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.