BirdAnalyst
Species · California · BBS 2025 Release · 1966–2024

Bell's Vireo Population Trend in California

All statesForest birds

Bell's Vireo in California has surged: up 725% on the route-weighted index since 1976.

+725%Since 1976TrendPercent 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 →
18Routes 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 →
30YearsYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

Notable Bell'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 →

Bell's Vireo has surged in California: up 725% on the route-weighted index since 1976.

Bell's Vireo Population Forecast in California

If the recent trend holds, Bell's Vireo in California is projected to fall about 36% by 2029 — from 0.18 in 2024 to a central estimate of 0.12 (95% range 0.03–0.20). A 5-year backtest shows a typical error of ±60%, with 60% of held-out values landing inside the 95% band.

-36%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 →
0.12Projected 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 →
0.03–0.2095% 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 →
±60%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, Bell's Vireo in California is projected to fall about 36% by 2029 — from 0.18 in 2024 to a central estimate of 0.12 (95% range 0.03–0.20). A 5-year backtest shows a typical error of ±60%, with 60% of held-out values landing inside the 95% band.19712029
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 →
20250.110.020.19
20260.110.020.20
20270.110.030.20
20280.110.030.20
20290.120.030.20

Bell's Vireo Survey Routes in California

Routes recording Bell'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 →
Jofegan 21620242024
San Ysidro520191992
Tecopa320231995
Cp. Pendleton220031975
Flynn119921992
Mesa Grande120102010
Creston119761976
Moreno120122012
Oak Grove120182010
Anza Borrego120241979
Jofegan120222015
Coalinga119761976
Orange Cove119971975
Atwater120122012
Cuyamaca Pk119711971
Poway119791979
Death Valley119961996
Wildrose120042004

Bell's Vireo Population Trend in Other States

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