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

Blue Grosbeak Population Trend in Illinois

All statesGeneralists

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 →
77Routes 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 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 43% by 2029 — from 4.8 in 2024 to a central estimate of 2.8 (95% range 1.6–3.9). A 5-year backtest shows a typical error of ±43.6%, with 20% of held-out values landing inside the 95% band.

-43%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 →
2.8Projected 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 →
1.6–3.995% 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 →
±43.6%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 43% by 2029 — from 4.8 in 2024 to a central estimate of 2.8 (95% range 1.6–3.9). A 5-year backtest shows a typical error of ±43.6%, with 20% 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 →
20252.61.43.7
20262.61.43.8
20272.71.53.8
20282.71.53.9
20292.81.63.9

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 →
Karnak1920242001
Bremen1620241970
Beaverdam Lk1520231970
Mt Vernon1420241973
Brownville1420241993
Kell1420242001
Royalton1420242001
Delwood1320241970
Belleview1220241968
Janesville1220242001
Flora1220241981
Belknap1120241970
Sandusky1120231993
Marshall1120242016
Brnt Prairie1020241980
York1020242017
Cave In Rock1020241993
Payson1020242004
Bradbury920241997
Olney920241983
Dudleyville920242001
Mascoutah Lk820242005
Jamestown820241988
Robinson820232021
Monticello720241999

Blue Grosbeak Population Trend in Other States

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