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

Tree Swallow Population Trend in California

All statesAerial insectivores

Tree Swallow in California has risen sharply: up 63% on the route-weighted index since 1970.

+63%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 →
173Routes 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 →
56YearsYears 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 risen sharply in California: up 63% 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 26% by 2029 — from 2.8 in 2024 to a central estimate of 3.6 (95% range 1.9–5.2). A 5-year backtest shows a typical error of ±84.7%, with 40% of held-out values landing inside the 95% band.

+26%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 →
3.6Projected 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 →
1.9–5.295% 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 →
±84.7%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 26% by 2029 — from 2.8 in 2024 to a central estimate of 3.6 (95% range 1.9–5.2). A 5-year backtest shows a typical error of ±84.7%, with 40% of held-out values landing inside the 95% band.19682029
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 →
20253.51.85.1
20263.51.95.1
20273.51.95.2
20283.61.95.2
20293.61.95.2

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 →
Oroville8920241972
Horse Creek7020191970
Glenn7020231975
Auburn4119801969
Moraga3620241995
El Toro2319841973
Coyote2020241974
Ono1820121984
Honeydew1620171969
Orleans 21620142014
Orland1420221974
Bodega Bay1420111972
Cordova1319801972
Crescent Cty1120241972
Bass Lake1120181973
Yokohl Val1119971972
Copperopolis1120241979
Yreka1020191973
Tres Pinos 21020102007
Glen Ellen 21020232009
Burnt Ranch1020162008
Coalinga920221980
Zamora920241980
Hackamore920102001
Holmes820221969

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 2026-05-22.