BirdAnalyst
Species · BBS 2026 Release · 1966–2025

Fox Sparrow Population Analysis

PasserellidaeForest birdsPasserella iliaca

Fox Sparrow has surged: up 650% on the route-weighted index since 1970.

+650%Since 1970TrendPercent 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 →
491RoutesSurvey routesHow many standard-protocol BBS routes contributed counts. More routes means a steadier, better-sampled index; very thin coverage is suppressed.Full methodology →
57Years SurveyedYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

About the Fox Sparrow

The Fox Sparrow (Passerella iliaca) is a North American member of the New World Sparrows (Passerellidae). In this analysis it is grouped with the forest birds.

Size
4.5–7.5 in long (12–19 cm) — a small songbird (typical for the family)
Habitat
Woodlands and forest edges, including wooded suburbs and parks.
Diet
Insects and spiders gleaned from foliage and bark, with seeds and berries in season.
Range
Recorded on 491 Breeding Bird Survey routes across 11 states, most concentrated in the Northern Rockies.
Family
Passerellidae · Forest birds

Notable Fox Sparrow TrendsNotable 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 →

Fox Sparrow has surged in surveyed states: up 650% on the route-weighted index since 1970.

Fox Sparrow Population Forecast

If the recent trend holds, Fox Sparrow is projected to rise about 69% by 2030 — from 0.60 in 2025 to a central estimate of 1.0 (95% range 0.76–1.3). A 5-year backtest shows a typical error of ±59.8%, with 0% of held-out values landing inside the 95% band.

+69%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 →
1.0Projected 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 →
0.76–1.395% 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 →
±59.8%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, Fox Sparrow is projected to rise about 69% by 2030 — from 0.60 in 2025 to a central estimate of 1.0 (95% range 0.76–1.3). A 5-year backtest shows a typical error of ±59.8%, with 0% of held-out values landing inside the 95% band.19682030
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 →
20260.960.711.2
20270.980.721.2
20280.990.741.2
20291.00.751.3
20301.00.761.3

Where the Fox Sparrow Is Detected

BBS routes recording Fox Sparrow, sized by most recent count.

Fox Sparrow Population Trend by State

Fox Sparrow population trend by state.
TrendPercent 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 →Baseline yearThe first year of the smoothed window the trend is measured from. An earlier baseline means a longer record stands behind the number.Full methodology →Survey routesHow many standard-protocol BBS routes contributed counts. More routes means a steadier, better-sampled index; very thin coverage is suppressed.Full methodology →
Alaska+77%1975128
California+205%197097
Colorado+433%197252
Idaho+326%197733
Maine+15%199511
Montana+217%197122
Nevada-33%199512
Oregon-65%197061
Utah-46%197921
Washington+41%197031
Wyoming-19%197023

Fox Sparrow Population Trend by Region

Bird Conservation Regions are the ecological unit for trends.

Fox Sparrow population trend by Bird Conservation Region.
TrendPercent 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 →Baseline yearThe first year of the smoothed window the trend is measured from. An earlier baseline means a longer record stands behind the number.Full methodology →Survey routesHow many standard-protocol BBS routes contributed counts. More routes means a steadier, better-sampled index; very thin coverage is suppressed.Full methodology →
BCR 2-33%198523
BCR 3+143%19964
BCR 4-41%197666
Northern Pacific Rainforest+46%197080
Great Basin+10%197073
Northern Rockies+58%197098
Atlantic Northern Forest-8%199511
Sierra Nevada+166%197038
Southern Rockies / Colorado Plateau+856%197269
Coastal California-83%197127

Fox Sparrow Conservation Status

Our route-weighted index shows it up about 650% since 1970.

Source: USGS North American Breeding Bird Survey, retrieved 2026-08-14. Trend is a route-weighted relative-abundance index, not an absolute population.