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

Brown-headed Cowbird Population Trend in Illinois

All statesGeneralists

Brown-headed Cowbird in Illinois has surged: up 155% on the route-weighted index since 1968.

+155%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 →
58YearsYears 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 155% 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 2029, near 23 (95% range 19–28). A 5-year backtest shows a typical error of ±11.7%, with 80% of held-out values landing inside the 95% band.

+1%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 →
23Projected 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 →
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 →
±11.7%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 Illinois is projected to stay roughly flat through 2029, near 23 (95% range 19–28). A 5-year backtest shows a typical error of ±11.7%, with 80% of held-out values landing inside the 95% band.19662029
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 →
2025221827
2026231827
2027231827
2028231828
2029231928

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 →
Belleview8220241966
Chandlervill6520241971
Fairland5620241967
Monticello4820241976
Stillwell4520241971
Grape Creek4420242001
Mill Creek4320241966
Terre Haute4320241966
Buda4220231977
Cave In Rock4120241993
Atkinson4020241966
Sherrard4020241993
Milford3920231966
Duncan Mills3920241993
Streator3820241972
Camp Point3820241993
Harristown3720242001
Guilford3520231967
Pulaski3520241970
York3520241993
New Lenox3420191966
Fairbury3320241976
Janesville3320241972
Delwood3320241970
Gridley3320242001

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 2026-05-22.