WhenToBird

Mergus serrator · Anatidae

When Do Red-breasted Mergansers Arrive in Wisconsin?

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

Red-breasted Mergansers reach Wisconsin in mid October, are common by late October, peak in early April (9.6% of checklists), and most are gone by the second week of May.

When to look

First arrivalsOct 15-21mid October
Common byOct 22-31late October
PeakApr 1-7early April
Most gone byMay 8-14the second week of May

Best: early April. First arrivals mid October; most are gone by the second week of May. Peak: on about 9.6% of Wisconsin checklists.

  1. Jancommon
  2. Febcommon
  3. Marpeak season
  4. Aprpeak season
  5. Maycommon
  6. Junrare
  7. Julnot usually seen
  8. Augnot usually seen
  9. Seprare
  10. Octuncommon
  11. Novpeak season
  12. Decpeak season

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

0%5%10%Jan 1-7: 7.3% of 27925 checklistsJan 8-14: 6.4% of 18337 checklistsJan 15-21: 5.3% of 17370 checklistsJan 22-31: 4.7% of 23777 checklistsFeb 1-7: 4.7% of 16656 checklistsFeb 8-14: 3.6% of 18286 checklistsFeb 15-21: 2.9% of 24739 checklistsFeb 22-28: 4.8% of 20113 checklistsMar 1-7: 6.3% of 24024 checklistsMar 8-14: 6.5% of 26722 checklistsMar 15-21: 6.9% of 27238 checklistsMar 22-31: 9.2% of 40171 checklistsApr 1-7: 9.9% of 34961 checklistsApr 8-14: 9.7% of 39128 checklistsApr 15-21: 9.1% of 38887 checklistsApr 22-30: 7.5% of 57231 checklistsMay 1-7: 5.6% of 55115 checklistsMay 8-14: 3.1% of 63098 checklistsMay 15-21: 2.6% of 49821 checklistsMay 22-31: 1.5% of 56796 checklistsJun 1-7: 0.5% of 33621 checklistsJun 8-14: 0.3% of 31282 checklistsJun 15-21: 0.2% of 30186 checklistsJun 22-30: 0.1% of 35128 checklistsJul 1-7: 0.1% of 27088 checklistsJul 8-14: 0.1% of 23293 checklistsJul 15-21: 0.1% of 20933 checklistsJul 22-31: 0.1% of 27595 checklistsAug 1-7: 0.1% of 19147 checklistsAug 8-14: 0.1% of 18286 checklistsAug 15-21: 0.1% of 19195 checklistsAug 22-31: 0.1% of 28229 checklistsSep 1-7: 0.1% of 22408 checklistsSep 8-14: 0.1% of 20857 checklistsSep 15-21: 0.3% of 20707 checklistsSep 22-30: 0.4% of 26469 checklistsOct 1-7: 0.6% of 21069 checklistsOct 8-14: 0.7% of 21011 checklistsOct 15-21: 1.4% of 20006 checklistsOct 22-31: 2.9% of 26362 checklistsNov 1-7: 7.0% of 18928 checklistsNov 8-14: 7.7% of 17052 checklistsNov 15-21: 8.4% of 16291 checklistsNov 22-30: 10% of 20564 checklistsDec 1-7: 8.7% of 15354 checklistsDec 8-14: 6.7% of 16294 checklistsDec 15-21: 5.8% of 21430 checklistsDec 22-31: 6.1% 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 late November · 108 checklists that week, 9,747 in all57%
  2. Racine CountyBest late November · 313 checklists that week, 18,400 in all40%
  3. Kenosha CountyBest late November · 175 checklists that week, 11,654 in all38%
  4. Manitowoc CountyBest late November · 323 checklists that week, 26,017 in all34%
  5. Sheboygan CountyBest mid November · 353 checklists that week, 25,419 in all32%
  6. Ozaukee CountyBest mid November · 522 checklists that week, 38,294 in all32%

Best spots

Share of checklists at the peak
  1. Root RiverRacine County, best late March · 21 checklists that week, 107 in all100%
  2. O'Donnell ParkMilwaukee County, best mid March · 21 checklists that week, 604 in all99%
  3. Manitowoc Small Boat HarborManitowoc County, best early April · 21 checklists that week, 2,200 in all98%
  4. Coal Dock ParkOzaukee County, best late March · 90 checklists that week, 1,961 in all97%
  5. Lakeshore State ParkMilwaukee County, best mid March · 30 checklists that week, 2,503 in all96%
  6. North Point ParkSheboygan County, best mid April · 24 checklists that week, 1,755 in all96%

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

Best time of day

In Wisconsin, the Red-breasted Merganser is most often listed on checklists started between 11 am and 1 pm (5.2% of those checklists).

0%2%4%6%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: 1.7% of 27016 checklists6 am: 3.1% of 83973 checklists7 am: 3.2% of 160717 checklists8 am: 3.7% of 160531 checklists9 am: 4.3% of 136968 checklists10 am: 4.9% of 119110 checklists11 am: 5.1% of 99116 checklists12 pm: 5.2% of 85078 checklists1 pm: 5.1% of 80688 checklists2 pm: 5.0% of 79140 checklists3 pm: 4.6% of 78599 checklists4 pm: 3.9% of 69767 checklists5 pm: 3.1% of 53229 checklists6 pm: 2.3% 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
January7.3%6.4%5.3%4.7%
February4.7%3.6%2.9%4.8%
March6.3%6.5%6.9%9.2%
April9.9%9.7%9.1%7.5%
May5.6%3.1%2.6%1.5%
June0.5%0.3%0.2%0.1%
July0.1%0.1%0.1%0.1%
August0.1%0.1%0.1%0.1%
September0.1%0.1%0.3%0.4%
October0.6%0.7%1.4%2.9%
November7.0%7.7%8.4%10%
December8.7%6.7%5.8%6.1%

Neighboring states

  • Illinois: arrives late October, 1 week later than Wisconsin
  • Iowa: arrives late February, 17 weeks later than Wisconsin
  • Michigan: arrives late September, 3 weeks earlier than Wisconsin
  • Minnesota: arrives the second week of March, 19 weeks later than Wisconsin

Common questions

When do Red-breasted Mergansers arrive in Wisconsin?
Red-breasted Mergansers reach Wisconsin in mid October, are common by late October, peak in early April (9.6% of checklists), and most are gone by the second week of May.
When do Red-breasted Mergansers leave Wisconsin?
Most Red-breasted Mergansers have left Wisconsin by the second week of May, when reports fall below half their usual rate. Stragglers are still reported until late May.
When is the best time to see a Red-breasted Merganser in Wisconsin?
The Red-breasted Merganser is reported most often in early April, on about 9.6% of Wisconsin checklists.
Where in Wisconsin are Red-breasted Mergansers seen most often?
By county, Kewaunee County, Racine County, and Kenosha County have the highest peak reporting rates for the Red-breasted Merganser (counties with at least 200 checklists).