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

Brown-headed Cowbird Population Trend in Illinois

All statesGeneralists

Brown-headed Cowbird in Illinois has surged: up 156% on the route-weighted index since 1968.

+156%Since 1968TrendPercent 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 →
105Routes 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 →
59YearsYears 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 IllinoisNotable 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 surged in Illinois: up 156% on the route-weighted index since 1968.

Brown-headed Cowbird Population Forecast in Illinois

If the recent trend holds, Brown-headed Cowbird in Illinois is projected to stay roughly flat through 2030, near 23 (95% range 19–28). A 5-year backtest shows a typical error of ±10.9%, with 80% of held-out values landing inside the 95% band.

+1%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 →
23Projected 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 →
19–2895% 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 →
±10.9%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 Illinois is projected to stay roughly flat through 2030, near 23 (95% range 19–28). A 5-year backtest shows a typical error of ±10.9%, with 80% of held-out values landing inside the 95% band.19662030
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 →
2026231827
2027231827
2028231828
2029231928
2030231928

Brown-headed Cowbird Survey Routes in Illinois

Routes recording Brown-headed Cowbird in Illinois.
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 →
Belleview9420251966
Monticello7620251976
Humphrey5120251972
Fairland5120251967
Fairbury4720251976
Gulfport4420251971
Wayne4320251966
Buda4220231977
Chandlervill4120251971
Pontiac4120251978
Cave In Rock4120241993
Atkinson4020241966
Stillwell3920251971
Gridley3620252001
Guilford3520231967
Humboldt3520251970
Le Roy3520251993
New Lenox3420191966
Grape Creek3320252001
Streator3220251972
Newark3220221969
Holcomb3120251971
Geneseo3120251967
Seneca3120251993
Sheffield3020241966

Brown-headed Cowbird Population Trend in Other States

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

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