BirdAnalyst
Species · BCR 32 · BBS 2026 Release · 1966–2025

Forster's Tern In Coastal California

All regionsCoastal CaliforniaWetland birds

Forster's Tern in Coastal California has fallen sharply: down 72% on the route-weighted index since 1970.

-72%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 →
18Routes In RegionSurvey routesHow many standard-protocol BBS routes contributed counts. More routes means a steadier, better-sampled index; very thin coverage is suppressed.Full methodology →
52YearsYears surveyedNumber of years with at least one standard-protocol run in this geography. A longer record makes the trend more reliable.Full methodology →

Notable SignalsNotable 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 Coastal California: down 72% on the route-weighted index since 1970.

Forster's Tern In Coastal California Population Forecast

If the recent trend holds, Forster's Tern in Coastal California is projected to rise about 85% by 2030 — from 0.10 in 2025 to a central estimate of 0.19 (95% range 0.00–0.52). A 5-year backtest shows a typical error of ±253.6%, with 100% of held-out values landing inside the 95% band.

+85%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 →
0.19Projected 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 →
0.00–0.5295% 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 →
±253.6%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, Forster's Tern in Coastal California is projected to rise about 85% by 2030 — from 0.10 in 2025 to a central estimate of 0.19 (95% range 0.00–0.52). A 5-year backtest shows a typical error of ±253.6%, with 100% 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 →
20260.200.000.52
20270.200.000.52
20280.200.000.52
20290.200.000.52
20300.190.000.52

Routes In Coastal California

Routes recording Forster's Tern in Coastal 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 →
OrovilleCALIFORNIA302013
PowayCALIFORNIA211978
Carrizo PlnCALIFORNIA151991
CollegevilleCALIFORNIA81987
FairfaxCALIFORNIA61991
ReddingCALIFORNIA41975
CopperopolisCALIFORNIA42001
MorenoCALIFORNIA32025
Foster ParkCALIFORNIA21990
HollisterCALIFORNIA22003
CoyoteCALIFORNIA22014
HughsonCALIFORNIA12025
San YsidroCALIFORNIA12003
SangerCALIFORNIA12013
GlennCALIFORNIA11983
CordovaCALIFORNIA11981
WestleyCALIFORNIA12003
Staten Is.CALIFORNIA12014

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