BirdAnalyst
Species · Iowa · BBS 2026 Release · 1966–2025

Brown Thrasher Population Trend in Iowa

All statesForest birds

Brown Thrasher in Iowa has fallen sharply: down 57% on the route-weighted index since 1969.

-57%Since 1969TrendPercent 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 →
39Routes 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 Thrasher Trends in IowaNotable 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 →

Brown Thrasher has fallen sharply in Iowa: down 57% on the route-weighted index since 1969.

Brown Thrasher Population Forecast in Iowa

If the recent trend holds, Brown Thrasher in Iowa is projected to rise about 19% by 2030 — from 3.6 in 2025 to a central estimate of 4.3 (95% range 2.0–6.5). A 5-year backtest shows a typical error of ±39.4%, with 100% of held-out values landing inside the 95% band.

+19%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 →
4.3Projected 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 →
2.0–6.595% 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 →
±39.4%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 Thrasher in Iowa is projected to rise about 19% by 2030 — from 3.6 in 2025 to a central estimate of 4.3 (95% range 2.0–6.5). A 5-year backtest shows a typical error of ±39.4%, with 100% of held-out values landing inside the 95% band.19672030
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 →
20264.62.46.8
20274.52.36.8
20284.42.26.7
20294.42.16.6
20304.32.06.5

Brown Thrasher Survey Routes in Iowa

Routes recording Brown Thrasher in Iowa.
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 →
Mt Hamill1720251967
College Spgs1420041967
Scranton1120251967
Exline1020251974
Maquoketa820251967
Zearing819951967
Bedford720251968
Festina620251968
Davis Corner620251969
Cedar Valley519971967
Melrose420251974
West Liberty420251968
Wales420251967
Hudson420251967
Burchinal420241995
College Spgs 2420252005
Bryant320251967
Red Rock320251968
Panther320251967
Alice319991967
Clare320181967
Rock Valley320191968
Washington220241967
Neola220251968
Kanawha220251967

Brown Thrasher Population Trend in Other States

alabamaarkansascoloradoconnecticutdelawarefloridageorgiaillinoisindianakansaskentuckylouisianamainemarylandmassachusettsmichiganminnesotamississippimissourimontananebraskanew-hampshirenew-jerseynew-mexiconew-yorknorth-carolinanorth-dakotaohiooklahomapennsylvaniarhode-islandsouth-carolinasouth-dakotatennesseetexasvermontvirginiawest-virginiawisconsinwyoming

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