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

Forster's Tern Population Trend in California

All statesWetland birds

Forster's Tern in California has fallen sharply: down 60% on the route-weighted index since 1970.

-60%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 →
34Routes 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 →
54YearsYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

Notable Forster's Tern 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 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 →

Forster's Tern has fallen sharply in California: down 60% on the route-weighted index since 1970.

Forster's Tern Population Forecast in California

If the recent trend holds, Forster's Tern in California is projected to stay roughly flat through 2029, near 0.69 (95% range 0.00–1.6). A 5-year backtest shows a typical error of ±1365.3%, with 40% of held-out values landing inside the 95% band.

n/aChange 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 →
0.69Projected 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 →
0.00–1.695% 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 →
±1365.3%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, Forster's Tern in California is projected to stay roughly flat through 2029, near 0.69 (95% range 0.00–1.6). A 5-year backtest shows a typical error of ±1365.3%, 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 →
20250.690.001.6
20260.690.001.6
20270.690.001.6
20280.690.001.6
20290.690.001.6

Forster's Tern Survey Routes in California

Routes recording Forster's Tern 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 →
Dorris6820191972
Oroville3020131973
Poway2119781973
Carrizo Pln1519911991
Collegeville819871987
Macdoel720181972
Fairfax619911974
Sattley520161990
Iron Gate420021988
Redding419751975
Copperopolis420011980
Crowder Flat320062002
Crowley Lake219881988
Foster Park219901990
Cedarville220141977
Hollister220031992
Coyote220141974
Ingalls220011974
Nubieber120141980
Eagle Lake120071972
Hughson120241968
Glenn119831982
Cordova119811981
Madeline120242015
Westley120031989

Forster's Tern Population Trend in Other States

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Source: USGS North American Breeding Bird Survey, retrieved 2026-05-22.