WhenToBird

Mergus serrator · Anatidae

When Do Red-breasted Mergansers Arrive in Idaho?

Adult male Red-breasted Merganser
Photo: Peter Chen 2.0, CC BY 4.0 via iNaturalist

Red-breasted Mergansers arrive in Idaho in mid October, peak in the second week of April (3.3% of checklists), and most are gone by the second week of May.

When to look

First arrivalsOct 15-21mid October
Common byOct 15-21mid October
PeakApr 8-14the second week of April
Most gone byMay 8-14the second week of May

Best: the second week of April. First arrivals mid October; most are gone by the second week of May. Peak: on about 3.3% of Idaho checklists.

  1. Jancommon
  2. Febuncommon
  3. Marcommon
  4. Aprpeak season
  5. Maycommon
  6. Junrare
  7. Julnot usually seen
  8. Augnot usually seen
  9. Sepnot usually seen
  10. Octcommon
  11. Novcommon
  12. Deccommon

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

0%1%2%3%4%Jan 1-7: 1.2% of 9357 checklistsJan 8-14: 1.7% of 6408 checklistsJan 15-21: 1.4% of 6358 checklistsJan 22-31: 1.4% of 8601 checklistsFeb 1-7: 0.9% of 5611 checklistsFeb 8-14: 0.9% of 6090 checklistsFeb 15-21: 1.0% of 7973 checklistsFeb 22-28: 1.3% of 5866 checklistsMar 1-7: 0.9% of 6144 checklistsMar 8-14: 1.3% of 6456 checklistsMar 15-21: 1.7% of 6786 checklistsMar 22-31: 2.3% of 9539 checklistsApr 1-7: 3.0% of 7202 checklistsApr 8-14: 3.2% of 7608 checklistsApr 15-21: 3.6% of 8069 checklistsApr 22-30: 2.7% of 10973 checklistsMay 1-7: 1.6% of 9481 checklistsMay 8-14: 0.8% of 11318 checklistsMay 15-21: 0.7% of 9141 checklistsMay 22-31: 0.3% of 13984 checklistsJun 1-7: 0.1% of 8897 checklistsJun 8-14: 0.1% of 8504 checklistsJun 15-21: 0.1% of 8078 checklistsJun 22-30: 0.1% of 9665 checklistsJul 1-7: 0.1% of 7144 checklistsJul 8-14: 0.1% of 6557 checklistsJul 15-21: 0.1% of 6430 checklistsJul 22-31: 0.1% of 8696 checklistsAug 1-7: 0.1% of 5896 checklistsAug 8-14: 0.1% of 5643 checklistsAug 15-21: 0.1% of 6151 checklistsAug 22-31: 0.1% of 7863 checklistsSep 1-7: 0.1% of 5769 checklistsSep 8-14: 0% of 5646 checklistsSep 15-21: 0.1% of 5460 checklistsSep 22-30: 0.1% of 6363 checklistsOct 1-7: 0.1% of 4980 checklistsOct 8-14: 0.3% of 5096 checklistsOct 15-21: 0.3% of 4878 checklistsOct 22-31: 1.8% of 6426 checklistsNov 1-7: 2.0% of 4822 checklistsNov 8-14: 2.9% of 4999 checklistsNov 15-21: 2.1% of 4601 checklistsNov 22-30: 1.6% of 6027 checklistsDec 1-7: 1.7% of 4180 checklistsDec 8-14: 1.0% of 4775 checklistsDec 15-21: 1.0% of 6522 checklistsDec 22-31: 1.1% 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. Power CountyBest mid April · 55 checklists that week, 2,278 in all41%
  2. Jefferson CountyBest mid April · 188 checklists that week, 5,319 in all31%
  3. Minidoka CountyBest the second week of April · 32 checklists that week, 1,365 in all28%
  4. Bonneville CountyBest the second week of April · 160 checklists that week, 6,998 in all27%
  5. Franklin CountyBest late March · 16 checklists that week, 651 in all25%
  6. Bear Lake CountyBest early April · 23 checklists that week, 1,581 in all21%

Best spots

Share of checklists at the peak
  1. Market Lake Wildlife Management AreaJefferson County, best mid April · 37 checklists that week, 697 in all68%
  2. Edson Fichter Nature AreaBannock County, best the second week of April · 20 checklists that week, 512 in all43%

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

Best time of day

In Idaho, the Red-breasted Merganser is most often listed on checklists started between 2 and 4 pm (1.5% of those checklists).

0%0.5%1%1.5%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: 0.5% of 9122 checklists7 am: 0.7% of 22204 checklists8 am: 0.7% of 38672 checklists9 am: 1.0% of 38930 checklists10 am: 1.0% of 37338 checklists11 am: 1.1% of 33508 checklists12 pm: 1.2% of 28544 checklists1 pm: 1.1% of 25774 checklists2 pm: 1.5% of 24296 checklists3 pm: 1.5% of 21784 checklists4 pm: 1.3% of 19051 checklists5 pm: 0.9% of 14150 checklists6 pm: 1.0% of 10014 checklists7 pm: 0.7% 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
January1.2%1.7%1.4%1.4%
February0.9%0.9%1.0%1.3%
March0.9%1.3%1.7%2.3%
April3.0%3.2%3.6%2.7%
May1.6%0.8%0.7%0.3%
June0.1%0.1%0.1%0.1%
July0.1%0.1%0.1%0.1%
August0.1%0.1%0.1%0.1%
September0.1%0%0.1%0.1%
October0.1%0.3%0.3%1.8%
November2.0%2.9%2.1%1.6%
December1.7%1.0%1.0%1.1%

Neighboring states

  • Montana: arrives late February, 17 weeks later than Idaho
  • Utah: arrives late February, 17 weeks later than Idaho
  • Oregon: arrives late October, 1 week later than Idaho
  • Washington: arrives the second week of October, 1 week earlier than Idaho

Common questions

When do Red-breasted Mergansers arrive in Idaho?
Red-breasted Mergansers arrive in Idaho in mid October, peak in the second week of April (3.3% of checklists), and most are gone by the second week of May.
When do Red-breasted Mergansers leave Idaho?
Most Red-breasted Mergansers have left Idaho by the second week of May, when reports fall below half their usual rate.
When is the best time to see a Red-breasted Merganser in Idaho?
The Red-breasted Merganser is reported most often in the second week of April, on about 3.3% of Idaho checklists.
Where in Idaho are Red-breasted Mergansers seen most often?
By county, Power County, Jefferson County, and Minidoka County have the highest peak reporting rates for the Red-breasted Merganser (counties with at least 200 checklists).