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

Caspian Tern Population Trend in California

All statesWetland birds

Caspian Tern in California has declined: down 33% on the route-weighted index since 1970.

-33%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 →
54Routes 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 →
55YearsYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

Notable Caspian 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 →

No notable trend signals for Caspian Tern in California. See the full index history below.

Caspian Tern Population Forecast in California

If the recent trend holds, Caspian Tern in California is projected to rise about 222% by 2029 — from 0.21 in 2024 to a central estimate of 0.69 (95% range 0.00–1.4). A 5-year backtest shows a typical error of ±1159.6%, with 20% of held-out values landing inside the 95% band.

+222%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 →
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 →
0.00–1.495% 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 →
±1159.6%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, Caspian Tern in California is projected to rise about 222% by 2029 — from 0.21 in 2024 to a central estimate of 0.69 (95% range 0.00–1.4). A 5-year backtest shows a typical error of ±1159.6%, with 20% 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 →
20250.670.001.4
20260.670.001.4
20270.680.001.4
20280.690.001.4
20290.690.001.4

Caspian Tern Survey Routes in California

Routes recording Caspian 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 →
Iron Gate7920191988
Harbor Lake 21520242014
Dorris1120191972
Adelaida819921992
Colma619721968
Ingalls519991974
Alamo River519861986
Honeydew420111968
Cedarville319981998
Fish Rock220131977
Collegeville219771977
Brawley220122007
Westley220181993
Honey Lake219761973
Foster Park219741974
Moraga220191974
Clear Lk Res220181972
S Catalina220172013
Buena Vista220191993
Nubieber119991989
Eagle Lake120071980
Pennington119781978
Riovista120242024
Shafter119871987
Hughson120102010

Caspian Tern Population Trend in Other States

alabamaalaskadelawarefloridaidahoillinoisindianalouisianamarylandmichiganminnesotamontananevadanorth-carolinanorth-dakotaoregonsouth-carolinatexasutahvermontvirginiawashingtonwisconsinwyoming

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