WhenToBird

Mergus merganser · Anatidae

When Do Common Mergansers Arrive in Wisconsin?

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

Common Mergansers are winter birds in Wisconsin: the first arrive in mid September, they are common by the second week of November, peak in the second week of March (14% of checklists), and most are gone by mid April.

When to look

First arrivalsSep 15-21mid September
Common byNov 8-14the second week of November
PeakMar 8-14the second week of March
Most gone byApr 15-21mid April

Best: the second week of March. First arrivals mid September; most are gone by mid April. Peak: on about 14% of Wisconsin checklists.

  1. Janpeak season
  2. Febpeak season
  3. Marpeak season
  4. Aprpeak season
  5. Mayuncommon
  6. Junrare
  7. Julnot usually seen
  8. Augnot usually seen
  9. Seprare
  10. Octuncommon
  11. Novcommon
  12. Deccommon

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

0%5%10%15%Jan 1-7: 11% of 27925 checklistsJan 8-14: 10% of 18337 checklistsJan 15-21: 9.3% of 17370 checklistsJan 22-31: 8.7% of 23777 checklistsFeb 1-7: 8.9% of 16656 checklistsFeb 8-14: 7.7% of 18286 checklistsFeb 15-21: 7.2% of 24739 checklistsFeb 22-28: 10% of 20113 checklistsMar 1-7: 13% of 24024 checklistsMar 8-14: 14% of 26722 checklistsMar 15-21: 14% of 27238 checklistsMar 22-31: 13% of 40171 checklistsApr 1-7: 11% of 34961 checklistsApr 8-14: 8.3% of 39128 checklistsApr 15-21: 6.5% of 38887 checklistsApr 22-30: 4.1% of 57231 checklistsMay 1-7: 2.2% of 55115 checklistsMay 8-14: 1.3% of 63098 checklistsMay 15-21: 1.5% of 49821 checklistsMay 22-31: 1.1% of 56796 checklistsJun 1-7: 0.6% of 33621 checklistsJun 8-14: 0.5% of 31282 checklistsJun 15-21: 0.5% of 30186 checklistsJun 22-30: 0.5% of 35128 checklistsJul 1-7: 0.6% of 27088 checklistsJul 8-14: 0.5% of 23293 checklistsJul 15-21: 0.4% of 20933 checklistsJul 22-31: 0.4% of 27595 checklistsAug 1-7: 0.5% of 19147 checklistsAug 8-14: 0.5% of 18286 checklistsAug 15-21: 0.5% of 19195 checklistsAug 22-31: 0.5% of 28229 checklistsSep 1-7: 0.4% of 22408 checklistsSep 8-14: 0.8% of 20857 checklistsSep 15-21: 1.2% of 20707 checklistsSep 22-30: 1.1% of 26469 checklistsOct 1-7: 1.2% of 21069 checklistsOct 8-14: 1.1% of 21011 checklistsOct 15-21: 1.7% of 20006 checklistsOct 22-31: 2.4% of 26362 checklistsNov 1-7: 3.1% of 18928 checklistsNov 8-14: 4.4% of 17052 checklistsNov 15-21: 6.4% of 16291 checklistsNov 22-30: 7.5% of 20564 checklistsDec 1-7: 8.3% of 15354 checklistsDec 8-14: 9.5% of 16294 checklistsDec 15-21: 8.6% of 21430 checklistsDec 22-31: 8.5% of 26314 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 Wisconsin 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 Wisconsin

Best counties

Share of checklists at the peak
  1. Kewaunee CountyBest mid January · 96 checklists that week, 9,747 in all52%
  2. Pierce CountyBest mid January · 72 checklists that week, 7,076 in all36%
  3. Menominee CountyBest late March · 15 checklists that week, 745 in all34%
  4. Ashland CountyBest late October · 464 checklists that week, 16,576 in all30%
  5. Lincoln CountyBest late March · 288 checklists that week, 8,013 in all30%
  6. Marquette CountyBest mid March · 53 checklists that week, 3,706 in all28%

Best spots

Share of checklists at the peak
  1. Lottes Park & Boat LaunchDane County, best late January · 22 checklists that week, 207 in all98%
  2. Mud LakeDane County, best mid February · 57 checklists that week, 552 in all97%
  3. KewauneeKewaunee County, best mid December · 37 checklists that week, 2,215 in all96%
  4. Lower Mud Lake Fishery AreaDane County, best late February · 25 checklists that week, 250 in all95%
  5. Legion Memorial ParkDane County, best late January · 49 checklists that week, 919 in all95%
  6. 6336 ReservoirBrown County, best the second week of December · 24 checklists that week, 519 in all94%

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

Best time of day

In Wisconsin, the Common Merganser is most often listed on checklists started between 1 and 3 pm (6.3% of those checklists).

0%2%4%6%8%12 am: too few checklists (993) to judge1 am: too few checklists (844) to judge2 am: too few checklists (839) to judge3 am: too few checklists (1488) to judge4 am: too few checklists (5344) to judge5 am: 1.5% of 27016 checklists6 am: 2.5% of 83973 checklists7 am: 3.5% of 160717 checklists8 am: 4.3% of 160531 checklists9 am: 5.0% of 136968 checklists10 am: 5.9% of 119110 checklists11 am: 6.1% of 99116 checklists12 pm: 6.2% of 85078 checklists1 pm: 6.3% of 80688 checklists2 pm: 6.3% of 79140 checklists3 pm: 5.7% of 78599 checklists4 pm: 4.6% of 69767 checklists5 pm: 3.4% of 53229 checklists6 pm: 2.7% of 38769 checklists7 pm: too few checklists (24396) to judge8 pm: too few checklists (10155) to judge9 pm: too few checklists (4990) to judge10 pm: too few checklists (2463) to judge11 pm: too few checklists (1281) 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
January11%10%9.3%8.7%
February8.9%7.7%7.2%10%
March13%14%14%13%
April11%8.3%6.5%4.1%
May2.2%1.3%1.5%1.1%
June0.6%0.5%0.5%0.5%
July0.6%0.5%0.4%0.4%
August0.5%0.5%0.5%0.5%
September0.4%0.8%1.2%1.1%
October1.2%1.1%1.7%2.4%
November3.1%4.4%6.4%7.5%
December8.3%9.5%8.6%8.5%

Neighboring states

  • Illinois: arrives the second week of November, 7 weeks later than Wisconsin
  • Iowa: arrives early November, 6 weeks later than Wisconsin
  • Michigan: arrives the second week of September, 1 week earlier than Wisconsin
  • Minnesota: arrives mid July, 8 weeks earlier than Wisconsin

Common questions

When do Common Mergansers arrive in Wisconsin?
Common Mergansers are winter birds in Wisconsin: the first arrive in mid September, they are common by the second week of November, peak in the second week of March (14% of checklists), and most are gone by mid April.
When do Common Mergansers leave Wisconsin?
Most Common Mergansers have left Wisconsin by mid April, when reports fall below half their usual rate. Stragglers are still reported until late May.
When is the best time to see a Common Merganser in Wisconsin?
The Common Merganser is reported most often in the second week of March, on about 14% of Wisconsin checklists.
Where in Wisconsin are Common Mergansers seen most often?
By county, Kewaunee County, Pierce County, and Menominee County have the highest peak reporting rates for the Common Merganser (counties with at least 200 checklists).