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

Northern Harrier Population Trend in Illinois

All statesGeneralists

Northern Harrier in Illinois has fallen sharply: down 70% on the route-weighted index since 1972.

-70%Since 1972TrendPercent 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 →
53Routes 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 →
47YearsYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

Notable Northern Harrier 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 declinecomputed 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 →

Northern Harrier has fallen sharply in Illinois: down 70% on the route-weighted index since 1972.

Northern Harrier Population Forecast in Illinois

If the recent trend holds, Northern Harrier in Illinois is projected to stay roughly flat through 2029, near 0.03 (95% range 0.00–0.09). A 5-year backtest shows a typical error of ±60.2%, with 100% of held-out values landing inside the 95% band.

n/aChange 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 →
0.03Projected 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 →
0.00–0.0995% 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 →
±60.2%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, Northern Harrier in Illinois is projected to stay roughly flat through 2029, near 0.03 (95% range 0.00–0.09). A 5-year backtest shows a typical error of ±60.2%, with 100% of held-out values landing inside the 95% band.19672029
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 →
20250.030.000.10
20260.030.000.10
20270.030.000.09
20280.030.000.09
20290.030.000.09

Northern Harrier Survey Routes in Illinois

Routes recording Northern Harrier 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 →
Mill Creek320101995
Grape Creek320072007
Holcomb220212021
Compton219721972
Streator220241984
Pontiac219871987
Clarksdale219951990
Mascoutah Lk219671967
Jamestown220232004
Guilford119981998
Payson120042004
Sheffield120181988
Buda120211997
Wayne119681968
Essex120191986
Newark119911976
Gulfport120112011
Stillwell120111991
Williamsfld119891989
Monica120171985
Minonk120111989
Monticello120151990
Fairbury120211978
Dailey120161988
Milford120191967

Northern Harrier Population Trend in Other States

alabamaalaskaarizonaarkansascaliforniacoloradodelawaregeorgiaidahoindianaiowakansaskentuckylouisianamainemarylandmassachusettsmichiganminnesotamissourimontananebraskanevadanew-hampshirenew-jerseynew-mexiconew-yorknorth-carolinanorth-dakotaohiooklahomaoregonpennsylvaniarhode-islandsouth-dakotatennesseetexasutahvermontvirginiawashingtonwest-virginiawisconsinwyoming

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