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

Brown Thrasher Population Trend in Iowa

All statesGeneralists

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 →
57YearsYears 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 56% 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 21% by 2029 — from 3.7 in 2024 to a central estimate of 4.4 (95% range 2.2–6.7). A 5-year backtest shows a typical error of ±33%, with 100% of held-out values landing inside the 95% band.

+21%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 →
4.4Projected 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 →
2.2–6.795% 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 →
±33%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 21% by 2029 — from 3.7 in 2024 to a central estimate of 4.4 (95% range 2.2–6.7). A 5-year backtest shows a typical error of ±33%, 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 →
20254.82.57.0
20264.72.56.9
20274.62.46.8
20284.52.36.8
20294.42.26.7

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 Hamill1820241967
Washta1620141967
Bedford1520181968
College Spgs1420041967
Red Rock1020241968
College Spgs 2920232005
Zearing819951967
Wales720181967
Maquoketa720241967
Davis Corner620241969
Cedar Valley519971967
Panther520241967
Titonka520241968
Pierson420191968
Burchinal420241995
Melrose320241974
West Liberty320241968
Brooklyn320221967
Alice319991967
Scranton320241967
Clare320181967
Sioux City320241968
Rock Valley320191968
Mccallsburg320241996
Exline220241974

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 2026-05-22.