BirdAnalyst
Species · California · BBS 2026 Release · 1966–2025

Brown-headed Cowbird Population Trend in California

All statesGeneralists

Brown-headed Cowbird in California has risen sharply: up 58% on the route-weighted index since 1970.

+58%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 →
245Routes 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 →
57YearsYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

Notable Brown-headed Cowbird 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 →

Brown-headed Cowbird has risen sharply in California: up 58% on the route-weighted index since 1970.

Brown-headed Cowbird Population Forecast in California

If the recent trend holds, Brown-headed Cowbird in California is projected to rise about 41% by 2030 — from 5.0 in 2025 to a central estimate of 7.1 (95% range 3.6–11). A 5-year backtest shows a typical error of ±93.7%, with 20% of held-out values landing inside the 95% band.

+41%Change by 2030TrendPercent 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 →
7.1Projected 2030 indexProjected indexThe central forecast of the abundance index if the recent trend continues. A projection of the current trajectory, not a prediction.Full methodology →
3.6–1195% 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 →
±93.7%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, Brown-headed Cowbird in California is projected to rise about 41% by 2030 — from 5.0 in 2025 to a central estimate of 7.1 (95% range 3.6–11). A 5-year backtest shows a typical error of ±93.7%, with 20% of held-out values landing inside the 95% band.19682030
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 →
20267.03.511
20277.13.511
20287.13.511
20297.13.511
20307.13.611

Brown-headed Cowbird Survey Routes in California

Routes recording Brown-headed Cowbird 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 →
Dorris20020191972
Cedarville6020251974
Tipton4919971972
Atwater3320121973
Caruthers3119871972
Likely3120251972
Jackson3019921972
Cordova2819801974
Iron Gate2820191988
Trona2719771977
Staten Is.2720251994
Chilcoot2220241998
Crowder Flat2120192002
Horse Creek2020191971
Collegeville2020041973
Hollister2020111974
Palo Verde1920251973
Big Bear1919861969
Coyote1920251973
Yokohl Val1819971973
Collegeville 21820232007
White River1719921972
Crowley Lake1720251973
Yreka1720191973
Tracy1720091992

Brown-headed Cowbird Population Trend in Other States

alabamaalaskaarizonaarkansascoloradoconnecticutdelawarefloridageorgiaidahoillinoisindianaiowakansaskentuckylouisianamainemarylandmassachusettsmichiganminnesotamississippimissourimontananebraskanevadanew-hampshirenew-jerseynew-mexiconew-yorknorth-carolinanorth-dakotaohiooklahomaoregonpennsylvaniarhode-islandsouth-carolinasouth-dakotatennesseetexasutahvermontvirginiawashingtonwest-virginiawisconsinwyoming

Source: USGS North American Breeding Bird Survey, retrieved 2026-08-14.