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

Double-crested Cormorant Population Trend in Illinois

All statesWetland birds

Double-crested Cormorant in Illinois has surged: up 721% on the route-weighted index since 1984.

+721%Since 1984TrendPercent 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 →
48Routes 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 →
37YearsYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

Notable Double-crested Cormorant 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 →

Double-crested Cormorant has surged in Illinois: up 721% on the route-weighted index since 1984.

Double-crested Cormorant Population Forecast in Illinois

If the recent trend holds, Double-crested Cormorant in Illinois is projected to rise about 353% by 2029 — from 0.12 in 2024 to a central estimate of 0.54 (95% range 0.05–1.0). A 5-year backtest shows a typical error of ±467.9%, with 40% of held-out values landing inside the 95% band.

+353%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 →
0.54Projected 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 →
0.05–1.095% 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 →
±467.9%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, Double-crested Cormorant in Illinois is projected to rise about 353% by 2029 — from 0.12 in 2024 to a central estimate of 0.54 (95% range 0.05–1.0). A 5-year backtest shows a typical error of ±467.9%, with 40% of held-out values landing inside the 95% band.19742029
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 →
20250.510.021.0
20260.520.021.0
20270.530.031.0
20280.540.041.0
20290.540.051.0

Double-crested Cormorant Survey Routes in Illinois

Routes recording Double-crested Cormorant 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 →
Karnak2420132013
Nilwood2319951995
Barrington1020121990
Yorkville820192001
Dudleyville720212008
Sandusky720162016
Atkinson520192019
St Morgan520222009
Seneca420192009
Itasca320222004
Wayne320241990
Janesville320222019
Lake Summerset320132008
Flora319951995
Bartonville220222022
Monticello220212021
Belleview220072007
Keithsburg220052005
Delwood220241991
Royalton220022002
Shannon220222002
Guilford120182003
Geneseo120151993
Putnam120222018
Sheffield120092004

Double-crested Cormorant 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.