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Species · Illinois · BBS 2026 Release · 1966–2025

Blue Grosbeak Population Trend in Illinois

All statesForest birds

Blue Grosbeak in Illinois has surged: up 46× on the route-weighted index since 1968.

46×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 →
78Routes 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 Blue Grosbeak 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 →

Blue Grosbeak has surged in Illinois: up 46× on the route-weighted index since 1968.

Blue Grosbeak Population Forecast in Illinois

If the recent trend holds, Blue Grosbeak in Illinois is projected to fall about 22% by 2030 — from 3.7 in 2025 to a central estimate of 2.9 (95% range 1.7–4.1). A 5-year backtest shows a typical error of ±49.8%, with 0% of held-out values landing inside the 95% band.

-22%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 →
2.9Projected 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 →
1.7–4.195% 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 →
±49.8%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, Blue Grosbeak in Illinois is projected to fall about 22% by 2030 — from 3.7 in 2025 to a central estimate of 2.9 (95% range 1.7–4.1). A 5-year backtest shows a typical error of ±49.8%, with 0% 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 →
20262.71.53.9
20272.71.54.0
20282.81.64.0
20292.81.64.1
20302.91.74.1

Blue Grosbeak Survey Routes in Illinois

Routes recording Blue Grosbeak 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 →
Bremen1820251970
Beaverdam Lk1520231970
Brownville1520251993
Mt Vernon1320251973
Chandlervill1220251973
Belleview1220251968
Janesville1220252001
Flora1220241981
Dudleyville1220252001
Pulaski1120251970
Sandusky1120231993
Kell1120252001
Mascoutah Lk1020252005
Brnt Prairie1020251980
Cave In Rock1020241993
Marshall1020252016
Olney920241983
Jamestown820251988
Robinson820232021
Bradbury720251997
Beaucoup720251994
Delwood720251970
Duncan Mills720252018
Eldred720252000
Terre Haute620251999

Blue Grosbeak Population Trend in Other States

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