WhenToBird

Mergus merganser · Anatidae

When Do Common Mergansers Arrive in Idaho?

Adult female Common Merganser
Photo: Christoph Moning, CC BY 4.0 via iNaturalist

Common Mergansers are in Idaho all year, but most are here in winter: numbers build from mid October, peak in mid March (12% of checklists), and fall back by late April. In the quietest week (early July) they are still on about 2.6% of checklists.

When to look

Numbers buildOct 15-21mid October
PeakMar 15-21mid March
Fall back byApr 22-30late April
FewestJul 1-7early July

Best: mid March. Here all year; numbers build from mid October and fall back by late April. Peak: on about 12% of Idaho checklists.

  1. Janpeak season
  2. Febpeak season
  3. Marpeak season
  4. Aprpeak season
  5. Maycommon
  6. Jununcommon
  7. Juluncommon
  8. Auguncommon
  9. Sepcommon
  10. Octcommon
  11. Novpeak season
  12. Decpeak season

Darker months: more checklists report it. Outlined: peak month.

0%5%10%15%Jan 1-7: 12% of 9357 checklistsJan 8-14: 11% of 6408 checklistsJan 15-21: 11% of 6358 checklistsJan 22-31: 13% of 8601 checklistsFeb 1-7: 11% of 5611 checklistsFeb 8-14: 9.8% of 6090 checklistsFeb 15-21: 10% of 7973 checklistsFeb 22-28: 12% of 5866 checklistsMar 1-7: 12% of 6144 checklistsMar 8-14: 12% of 6456 checklistsMar 15-21: 12% of 6786 checklistsMar 22-31: 12% of 9539 checklistsApr 1-7: 11% of 7202 checklistsApr 8-14: 9.8% of 7608 checklistsApr 15-21: 9.1% of 8069 checklistsApr 22-30: 8.1% of 10973 checklistsMay 1-7: 6.6% of 9481 checklistsMay 8-14: 5.1% of 11318 checklistsMay 15-21: 3.7% of 9141 checklistsMay 22-31: 3.7% of 13984 checklistsJun 1-7: 3.0% of 8897 checklistsJun 8-14: 3.0% of 8504 checklistsJun 15-21: 2.9% of 8078 checklistsJun 22-30: 2.7% of 9665 checklistsJul 1-7: 2.8% of 7144 checklistsJul 8-14: 2.4% of 6557 checklistsJul 15-21: 2.9% of 6430 checklistsJul 22-31: 2.9% of 8696 checklistsAug 1-7: 2.6% of 5896 checklistsAug 8-14: 3.3% of 5643 checklistsAug 15-21: 3.1% of 6151 checklistsAug 22-31: 3.7% of 7863 checklistsSep 1-7: 3.9% of 5769 checklistsSep 8-14: 5.3% of 5646 checklistsSep 15-21: 5.5% of 5460 checklistsSep 22-30: 6.0% of 6363 checklistsOct 1-7: 6.2% of 4980 checklistsOct 8-14: 6.2% of 5096 checklistsOct 15-21: 6.8% of 4878 checklistsOct 22-31: 9.4% of 6426 checklistsNov 1-7: 10% of 4822 checklistsNov 8-14: 11% of 4999 checklistsNov 15-21: 9.2% of 4601 checklistsNov 22-30: 11% of 6027 checklistsDec 1-7: 11% of 4180 checklistsDec 8-14: 11% of 4775 checklistsDec 15-21: 10% of 6522 checklistsDec 22-31: 11% of 7792 checklistsJanFebMarAprMayJunJulAugSepOctNovDec
Share of complete checklists reporting it, by week (4 per month). Grey bars: fewer than 10 checklists, not counted.

Frequency is the share of complete checklists in Idaho that listed the bird, 2016–2025. It is not a head count, and it follows where and when people go birding. How to read it.

JanFebMarAprMayJunJulAugSepOctNovDec
today
Second week of November

Each county is shaded by the share of complete checklists there that listed the bird, blended with the weeks either side. Darker is more often. It follows where people watch birds, not how many birds there are. How to read it.

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Where in Idaho

Best counties

Share of checklists at the peak
  1. Lewis CountyBest late October · 12 checklists that week, 603 in all59%
  2. Jerome CountyBest late March · 17 checklists that week, 804 in all58%
  3. Power CountyBest mid March · 32 checklists that week, 2,278 in all46%
  4. Minidoka CountyBest early March · 25 checklists that week, 1,365 in all33%
  5. Bingham CountyBest the second week of January · 10 checklists that week, 1,427 in all33%
  6. Fremont CountyBest early October · 65 checklists that week, 5,201 in all30%

Best spots

Share of checklists at the peak
  1. Spring Valley Reservoir Access SiteLatah County, best the second week of November · 31 checklists that week, 536 in all97%
  2. Greenbelt SeAda County, best late January · 21 checklists that week, 582 in all91%
  3. Borden LakeOwyhee County, best mid October · 20 checklists that week, 451 in all89%
  4. Sandy Point BeachAda County, best late April · 24 checklists that week, 763 in all88%
  5. Boise Cascade LakeAda County, best late December · 54 checklists that week, 1,168 in all87%

Busy spots report more of everything; compare the checklist counts before you drive anywhere.

Best time of day

In Idaho, the Common Merganser is most often listed on checklists started between 2 and 4 pm (9.3% of those checklists).

0%5%10%12 am: too few checklists (180) to judge1 am: too few checklists (176) to judge2 am: too few checklists (169) to judge3 am: too few checklists (164) to judge4 am: too few checklists (450) to judge5 am: too few checklists (2270) to judge6 am: 4.8% of 9122 checklists7 am: 5.4% of 22204 checklists8 am: 6.2% of 38672 checklists9 am: 7.7% of 38930 checklists10 am: 8.2% of 37338 checklists11 am: 8.2% of 33508 checklists12 pm: 8.5% of 28544 checklists1 pm: 8.8% of 25774 checklists2 pm: 9.1% of 24296 checklists3 pm: 9.4% of 21784 checklists4 pm: 7.3% of 19051 checklists5 pm: 6.0% of 14150 checklists6 pm: 5.2% of 10014 checklists7 pm: 4.0% of 7495 checklists8 pm: too few checklists (4352) to judge9 pm: too few checklists (1449) to judge10 pm: too few checklists (525) to judge11 pm: too few checklists (208) to judge12a3a6a9a12p3p6p9p
Share of checklists started in each local hour that reported it. It reflects when people go birding as well as when birds are active. Grey hours have too few checklists (under 2% of the state’s, or under 200) to judge.
Week-by-week table
MonthDays 1-78-1415-2122-end
January12%11%11%13%
February11%9.8%10%12%
March12%12%12%12%
April11%9.8%9.1%8.1%
May6.6%5.1%3.7%3.7%
June3.0%3.0%2.9%2.7%
July2.8%2.4%2.9%2.9%
August2.6%3.3%3.1%3.7%
September3.9%5.3%5.5%6.0%
October6.2%6.2%6.8%9.4%
November10%11%9.2%11%
December11%11%10%11%

Neighboring states

  • Montana: arrives the second week of March, 19 weeks later than Idaho
  • Utah: arrives mid September, 4 weeks earlier than Idaho
  • Oregon: arrives mid August, 8 weeks earlier than Idaho
  • Washington: arrives early November, 2 weeks later than Idaho

Common questions

When do Common Mergansers arrive in Idaho?
Common Mergansers are in Idaho all year, but most are here in winter: numbers build from mid October, peak in mid March (12% of checklists), and fall back by late April. In the quietest week (early July) they are still on about 2.6% of checklists.
When do Common Mergansers leave Idaho?
Common Mergansers never fully leave Idaho: numbers fall back below half their usual rate by late April, but some are still reported in every week, about 2.6% of checklists at the low.
When is the best time to see a Common Merganser in Idaho?
The Common Merganser is reported most often in mid March, on about 12% of Idaho checklists.
Where in Idaho are Common Mergansers seen most often?
By county, Lewis County, Jerome County, and Power County have the highest peak reporting rates for the Common Merganser (counties with at least 200 checklists).