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

Black Tern Population Trend in California

All statesWetland birds

Black Tern in California has collapsed: down 77% on the route-weighted index since 1973.

-77%Since 1973TrendPercent 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 →
19Routes 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 →
50YearsYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

Notable Black 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 →

Black Tern has collapsed in California: down 77% on the route-weighted index since 1973.

Black Tern Population Forecast in California

If the recent trend holds, Black Tern in California is projected to stay roughly flat through 2027, near 0.11 (95% range 0.00–0.31). A 5-year backtest shows a typical error of ±315.9%, with 100% of held-out values landing inside the 95% band.

n/aChange by 2027TrendPercent 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 →
0.11Projected 2027 indexProjected indexThe central forecast of the abundance index if the recent trend continues. A projection of the current trajectory, not a prediction.Full methodology →
0.00–0.3195% 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 →
±315.9%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, Black Tern in California is projected to stay roughly flat through 2027, near 0.11 (95% range 0.00–0.31). A 5-year backtest shows a typical error of ±315.9%, with 100% of held-out values landing inside the 95% band.19712027
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 →
20230.120.000.31
20240.120.000.31
20250.110.000.31
20260.110.000.31
20270.110.000.31

Black Tern Survey Routes in California

Routes recording Black 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 →
Ora Loma4319731973
Clear Lk Res1320041980
Sattley519761976
Dorris520171976
Crowder Flat320192002
Madeline320221997
Crowley Lake319821982
Glenn220211971
Macdoel220051972
Tranquillity219951972
Ravendale219791973
Orland219961977
Nubieber120001973
Eagle Lake119741973
Hughson119911991
Likely120081972
Goumaz120041997
Honey Lake119941974
Ingalls119881982

Black Tern Population Trend in Other States

coloradoidahoillinoisindianaiowakansasmainemichiganminnesotamissourimontananebraskanevadanew-yorknorth-dakotaohiooregonsouth-dakotatexasutahwashingtonwisconsinwyoming

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