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

Tree Swallow Population Trend in California

All statesAerial insectivores

Tree Swallow in California has surged: up 89% on the route-weighted index since 1970.

+89%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 →
174Routes 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 Tree Swallow Trends in CaliforniaNotable 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 →

Tree Swallow has surged in California: up 89% on the route-weighted index since 1970.

Tree Swallow Population Forecast in California

If the recent trend holds, Tree Swallow in California is projected to rise about 13% by 2030 — from 3.2 in 2025 to a central estimate of 3.6 (95% range 2.0–5.2). A 5-year backtest shows a typical error of ±85.8%, with 40% of held-out values landing inside the 95% band.

+13%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 →
3.6Projected 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 →
2.0–5.295% 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 →
±85.8%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, Tree Swallow in California is projected to rise about 13% by 2030 — from 3.2 in 2025 to a central estimate of 3.6 (95% range 2.0–5.2). A 5-year backtest shows a typical error of ±85.8%, with 40% 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 →
20263.51.95.1
20273.51.95.1
20283.51.95.2
20293.61.95.2
20303.62.05.2

Tree Swallow Survey Routes in California

Routes recording Tree Swallow in California.
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 →
Horse Creek7020191970
Glenn6120251975
Auburn4119801969
Coyote4020251974
Oroville3120251972
Ravendale2620251978
El Toro2319841973
Ono1820121984
Honeydew1620171969
Orleans 21620142014
Orland1420221974
Adelaida1420251992
Bodega Bay1420111972
Cordova1319801972
Zamora1220251980
Bass Lake1120181973
Yokohl Val1119971972
Moraga1120251995
Copperopolis1120241979
Likely1020251974
Yreka1020191973
Tres Pinos 21020102007
Glen Ellen 21020232009
Burnt Ranch1020162008
Coalinga920221980

Tree Swallow Population Trend in Other States

alabamaalaskaarizonaarkansascoloradoconnecticutdelawarefloridageorgiaidahoillinoisindianaiowakansaskentuckylouisianamainemarylandmassachusettsmichiganminnesotamississippimissourimontananebraskanevadanew-hampshirenew-jerseynew-mexiconew-yorknorth-carolinanorth-dakotaohiooklahomaoregonpennsylvaniarhode-islandsouth-carolinasouth-dakotatennesseetexasutahvermontvirginiawashingtonwest-virginiawisconsinwyoming

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