WhenToBird

Mergus merganser · Anatidae

When Do Common Mergansers Arrive in Illinois?

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

Common Mergansers are winter birds in Illinois: the first arrive in the second week of November, they are common by the second week of December, peak in early March (14% of checklists), and most are gone by late March.

When to look

First arrivalsNov 8-14the second week of November
Common byDec 8-14the second week of December
PeakMar 1-7early March
Most gone byMar 22-31late March

Best: early March. First arrivals the second week of November; most are gone by late March. Peak: on about 14% of Illinois checklists.

  1. Janpeak season
  2. Febpeak season
  3. Marpeak season
  4. Apruncommon
  5. Maynot usually seen
  6. Junnot usually seen
  7. Julnot usually seen
  8. Augnot usually seen
  9. Sepnot usually seen
  10. Octnot usually seen
  11. Novuncommon
  12. Deccommon

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

0%5%10%15%20%Jan 1-7: 11% of 29700 checklistsJan 8-14: 11% of 20189 checklistsJan 15-21: 12% of 19784 checklistsJan 22-31: 13% of 26641 checklistsFeb 1-7: 14% of 20478 checklistsFeb 8-14: 12% of 21967 checklistsFeb 15-21: 12% of 28735 checklistsFeb 22-28: 16% of 24223 checklistsMar 1-7: 14% of 26775 checklistsMar 8-14: 12% of 27579 checklistsMar 15-21: 9.2% of 27193 checklistsMar 22-31: 6.7% of 42278 checklistsApr 1-7: 3.9% of 34579 checklistsApr 8-14: 1.9% of 38290 checklistsApr 15-21: 1.1% of 39261 checklistsApr 22-30: 0.4% of 59584 checklistsMay 1-7: 0.1% of 60485 checklistsMay 8-14: 0.1% of 59647 checklistsMay 15-21: 0.1% of 43173 checklistsMay 22-31: 0.1% of 47913 checklistsJun 1-7: 0.1% of 31151 checklistsJun 8-14: 0.1% of 27692 checklistsJun 15-21: 0.1% of 23983 checklistsJun 22-30: 0.1% of 28448 checklistsJul 1-7: 0.1% of 21273 checklistsJul 8-14: 0.1% of 19284 checklistsJul 15-21: 0.1% of 17712 checklistsJul 22-31: 0.1% of 24500 checklistsAug 1-7: 0.1% of 18498 checklistsAug 8-14: 0.1% of 19331 checklistsAug 15-21: 0.1% of 19983 checklistsAug 22-31: 0.1% of 31892 checklistsSep 1-7: 0.1% of 26396 checklistsSep 8-14: 0.1% of 25011 checklistsSep 15-21: 0.1% of 23597 checklistsSep 22-30: 0.1% of 30638 checklistsOct 1-7: 0.1% of 23949 checklistsOct 8-14: 0.1% of 24412 checklistsOct 15-21: 0.2% of 22359 checklistsOct 22-31: 0.1% of 29794 checklistsNov 1-7: 0.5% of 21877 checklistsNov 8-14: 1.5% of 19742 checklistsNov 15-21: 2.9% of 18989 checklistsNov 22-30: 3.9% of 23905 checklistsDec 1-7: 5.7% of 16935 checklistsDec 8-14: 6.7% of 16687 checklistsDec 15-21: 7.2% of 20931 checklistsDec 22-31: 9.1% of 29011 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 Illinois 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 Illinois

Best counties

Share of checklists at the peak
  1. Putnam CountyBest the second week of January · 18 checklists that week, 3,333 in all57%
  2. Rock Island CountyBest early February · 264 checklists that week, 12,910 in all31%
  3. Fulton CountyBest mid February · 91 checklists that week, 5,877 in all31%
  4. Kendall CountyBest late January · 601 checklists that week, 21,004 in all30%
  5. Whiteside CountyBest early March · 83 checklists that week, 4,111 in all30%
  6. Carroll CountyBest the second week of March · 95 checklists that week, 4,152 in all27%

Best spots

Share of checklists at the peak
  1. Rock Run Rookery Preserve 1Will County, best the second week of February · 36 checklists that week, 902 in all94%
  2. Ben Butterworth Memorial ParkwayRock Island County, best mid January · 25 checklists that week, 700 in all93%
  3. Saganashkee SloughCook County, best late February · 55 checklists that week, 1,986 in all92%
  4. Midlothian Creek Detention ReservoirCook County, best late February · 20 checklists that week, 279 in all89%
  5. Lake RenwickWill County, best mid February · 21 checklists that week, 582 in all89%

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

Best time of day

In Illinois, the Common Merganser is most often listed on checklists started between 1 and 3 pm (4.9% of those checklists).

0%2%4%6%12 am: too few checklists (484) to judge1 am: too few checklists (631) to judge2 am: too few checklists (485) to judge3 am: too few checklists (610) to judge4 am: too few checklists (2321) to judge5 am: too few checklists (23536) to judge6 am: 1.5% of 95511 checklists7 am: 2.9% of 164584 checklists8 am: 3.4% of 163654 checklists9 am: 4.0% of 141804 checklists10 am: 4.4% of 122882 checklists11 am: 4.8% of 100640 checklists12 pm: 4.6% of 89894 checklists1 pm: 4.9% of 82729 checklists2 pm: 5.0% of 79801 checklists3 pm: 4.5% of 82169 checklists4 pm: 3.1% of 75124 checklists5 pm: 1.6% of 57708 checklists6 pm: 0.8% of 41843 checklists7 pm: too few checklists (20943) to judge8 pm: too few checklists (6131) to judge9 pm: too few checklists (1677) to judge10 pm: too few checklists (843) to judge11 pm: too few checklists (448) 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%11%12%13%
February14%12%12%16%
March14%12%9.2%6.7%
April3.9%1.9%1.1%0.4%
May0.1%0.1%0.1%0.1%
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%0.1%0.1%0.1%
October0.1%0.1%0.2%0.1%
November0.5%1.5%2.9%3.9%
December5.7%6.7%7.2%9.1%

Neighboring states

  • Indiana: arrives late November, 2 weeks later than Illinois
  • Wisconsin: arrives mid September, 7 weeks earlier than Illinois
  • Michigan: arrives the second week of September, 8 weeks earlier than Illinois
  • Iowa: arrives early November, 1 week earlier than Illinois

Common questions

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