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

Common Merganser Population Trend in California

All statesWaterfowl

Common Merganser in California has fallen sharply: down 53% on the route-weighted index since 1970.

-53%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 →
68Routes 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 Common Merganser 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 →

Common Merganser has fallen sharply in California: down 53% on the route-weighted index since 1970.

Common Merganser Population Forecast in California

If the recent trend holds, Common Merganser in California is projected to stay roughly flat through 2029, near 0.47 (95% range 0.14–0.80). A 5-year backtest shows a typical error of ±638%, with 60% 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 →
0.47Projected 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 →
0.14–0.8095% 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 →
±638%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, Common Merganser in California is projected to stay roughly flat through 2029, near 0.47 (95% range 0.14–0.80). A 5-year backtest shows a typical error of ±638%, with 60% 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 →
20250.460.120.79
20260.460.130.79
20270.460.130.79
20280.460.130.80
20290.470.140.80

Common Merganser Survey Routes in California

Routes recording Common Merganser 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 Gate2420191988
Alderpoint1820061975
Horse Creek1520191970
Longvale720231972
Genesee620231985
Junction Cty520121972
Copperopolis520211989
Riverton420171980
Bodega Bay420211981
Eagle Lake320231969
Likely319991985
Chilcoot320231998
Shasta Lake320071973
Woodfords320221986
Holmes220221968
Redwood N.P.220061996
Hiouchi220032003
Whiskeytown220072002
Tomhead220052005
Meadow Valley220232018
Little Truckee220172013
Lockwood220241984
Clear Lk Res220042004
Korbel219811979
Ono220122002

Common Merganser Population Trend in Other States

alaskaarizonacoloradoconnecticutidahomainemarylandmassachusettsmichiganminnesotamontananevadanew-hampshirenew-jerseynew-mexiconew-yorknorth-dakotaohiooregonpennsylvaniasouth-dakotautahvermontvirginiawashingtonwest-virginiawisconsinwyoming

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