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Species · California · BBS 2025 Release · 1966–2024

Cassin's Vireo Population Trend in California

All statesForest birds

Cassin's Vireo in California has surged: up 287% on the route-weighted index since 1970.

+287%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 →
156Routes In StateSurvey routesHow many standard-protocol BBS routes contributed counts. More routes means a steadier, better-sampled index; very thin coverage is suppressed.Full methodology →
56YearsYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

Notable Cassin's Vireo Trends in CaliforniaNotable 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 California: up 287% on the route-weighted index since 1970.

Cassin's Vireo Population Forecast in California

If the recent trend holds, Cassin's Vireo in California is projected to rise about 117% by 2029 — from 1.7 in 2024 to a central estimate of 3.8 (95% range 2.4–5.2). A 5-year backtest shows a typical error of ±111.4%, with 0% of held-out values landing inside the 95% band.

+117%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 →
3.8Projected 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 →
2.4–5.295% 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 →
±111.4%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 in California is projected to rise about 117% by 2029 — from 1.7 in 2024 to a central estimate of 3.8 (95% range 2.4–5.2). A 5-year backtest shows a typical error of ±111.4%, with 0% 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 →
20253.62.25.0
20263.72.35.1
20273.72.35.1
20283.72.35.1
20293.82.45.2

Cassin's Vireo Survey Routes in California

Routes recording Cassin's Vireo in California.
Recent countThe raw number of individuals recorded on this route in its most recent survey year. A single-route tally, not a trend.Full methodology →
Horse Creek4420191970
Forest Hill2720221997
Butte Meadow2620141972
Orleans 22320142014
Alderpoint2120121974
Stonyford2020182015
Forest Hill 21820052005
Lumberyard1820241997
Martins Fy1720091976
Riverton1720231997
Longvale1420231972
Downieville1420241975
Bartle1420211972
Genesee1320241972
Bartlett Sps1320181974
Hough Springs1320071999
Shasta Lake1220241972
Placerville1220241997
Friday Ridge1220241994
Hullville1120191968
Chester1120171997
Orleans1020121972
Hay Fork1020172004
Eagle Lake920241972
Westville920241971

Cassin's Vireo Population Trend in Other States

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Source: USGS North American Breeding Bird Survey, retrieved 2026-05-22.