WhenToBird

Mergus merganser · Anatidae

When Do Common Mergansers Arrive in Maryland?

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

Common Mergansers are winter birds in Maryland: the first arrive in mid November, they are common by the second week of December, peak in late January (9.2% of checklists), and most are gone by early April.

When to look

First arrivalsNov 15-21mid November
Common byDec 8-14the second week of December
PeakJan 22-31late January
Most gone byApr 1-7early April

Best: late January. First arrivals mid November; most are gone by early April. Peak: on about 9.2% of Maryland checklists.

  1. Janpeak season
  2. Febpeak season
  3. Marpeak season
  4. Aprcommon
  5. Mayrare
  6. Junrare
  7. Julrare
  8. Augrare
  9. Seprare
  10. Octrare
  11. Novuncommon
  12. Deccommon

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

0%5%10%Jan 1-7: 7.5% of 28716 checklistsJan 8-14: 7.7% of 21449 checklistsJan 15-21: 8.8% of 21267 checklistsJan 22-31: 9.1% of 29144 checklistsFeb 1-7: 9.8% of 21347 checklistsFeb 8-14: 8.0% of 22166 checklistsFeb 15-21: 6.7% of 29986 checklistsFeb 22-28: 9.2% of 22427 checklistsMar 1-7: 8.1% of 21389 checklistsMar 8-14: 7.4% of 22192 checklistsMar 15-21: 5.9% of 22823 checklistsMar 22-31: 5.0% of 33720 checklistsApr 1-7: 3.4% of 25513 checklistsApr 8-14: 2.7% of 27718 checklistsApr 15-21: 1.6% of 29328 checklistsApr 22-30: 1.4% of 42595 checklistsMay 1-7: 1.1% of 38532 checklistsMay 8-14: 1.0% of 42722 checklistsMay 15-21: 0.9% of 31936 checklistsMay 22-31: 1.0% of 36903 checklistsJun 1-7: 0.7% of 23191 checklistsJun 8-14: 0.5% of 21853 checklistsJun 15-21: 0.4% of 21000 checklistsJun 22-30: 0.5% of 25133 checklistsJul 1-7: 0.7% of 18715 checklistsJul 8-14: 0.8% of 16376 checklistsJul 15-21: 0.7% of 15793 checklistsJul 22-31: 1.1% of 21969 checklistsAug 1-7: 1.1% of 16013 checklistsAug 8-14: 1.0% of 15249 checklistsAug 15-21: 0.9% of 15674 checklistsAug 22-31: 0.5% of 24925 checklistsSep 1-7: 0.5% of 19808 checklistsSep 8-14: 0.4% of 18513 checklistsSep 15-21: 0.3% of 21901 checklistsSep 22-30: 0.3% of 22901 checklistsOct 1-7: 0.3% of 19111 checklistsOct 8-14: 0.3% of 18482 checklistsOct 15-21: 0.4% of 17380 checklistsOct 22-31: 0.4% of 23992 checklistsNov 1-7: 0.6% of 17915 checklistsNov 8-14: 0.9% of 18905 checklistsNov 15-21: 1.1% of 16259 checklistsNov 22-30: 1.8% of 22852 checklistsDec 1-7: 2.8% of 16108 checklistsDec 8-14: 4.1% of 16512 checklistsDec 15-21: 5.7% of 18952 checklistsDec 22-31: 6.4% of 29728 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 Maryland 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 Maryland

Best counties

Share of checklists at the peak
  1. Cecil CountyBest late January · 386 checklists that week, 18,620 in all26%
  2. Washington CountyBest late January · 776 checklists that week, 24,020 in all26%
  3. Allegany CountyBest late January · 518 checklists that week, 18,322 in all23%
  4. Harford CountyBest early February · 761 checklists that week, 39,311 in all22%
  5. Garrett CountyBest early March · 897 checklists that week, 37,616 in all18%
  6. Baltimore CountyBest the second week of January · 2644 checklists that week, 136,139 in all15%

Best spots

Share of checklists at the peak
  1. Triadelphia Reservoir (Raintree Estates)Howard County, best late February · 20 checklists that week, 532 in all100%
  2. Loch Raven Reservoir (Towson Run)Baltimore County, best early February · 48 checklists that week, 779 in all98%
  3. Quarry LakeBaltimore County, best late January · 39 checklists that week, 915 in all98%
  4. Black Hill Regional ParkMontgomery County, best mid February · 79 checklists that week, 1,746 in all95%

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

Best time of day

In Maryland, the Common Merganser is most often listed on checklists started between 2 and 4 pm (3.7% of those checklists).

0%1%2%3%4%12 am: too few checklists (486) to judge1 am: too few checklists (451) to judge2 am: too few checklists (447) to judge3 am: too few checklists (766) to judge4 am: too few checklists (2076) to judge5 am: too few checklists (11098) to judge6 am: 3.4% of 56604 checklists7 am: 2.8% of 139255 checklists8 am: 2.7% of 148626 checklists9 am: 3.2% of 126820 checklists10 am: 3.4% of 106973 checklists11 am: 3.3% of 87483 checklists12 pm: 3.4% of 74808 checklists1 pm: 3.4% of 69428 checklists2 pm: 3.7% of 67800 checklists3 pm: 3.6% of 67688 checklists4 pm: 3.1% of 60927 checklists5 pm: 2.1% of 43876 checklists6 pm: 1.1% of 32930 checklists7 pm: too few checklists (19051) to judge8 pm: too few checklists (5708) to judge9 pm: too few checklists (1977) to judge10 pm: too few checklists (1170) to judge11 pm: too few checklists (635) 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
January7.5%7.7%8.8%9.1%
February9.8%8.0%6.7%9.2%
March8.1%7.4%5.9%5.0%
April3.4%2.7%1.6%1.4%
May1.1%1.0%0.9%1.0%
June0.7%0.5%0.4%0.5%
July0.7%0.8%0.7%1.1%
August1.1%1.0%0.9%0.5%
September0.5%0.4%0.3%0.3%
October0.3%0.3%0.4%0.4%
November0.6%0.9%1.1%1.8%
December2.8%4.1%5.7%6.4%

Neighboring states

  • District of Columbia: arrives mid November, in the same week than Maryland
  • Delaware: arrives early December, 2 weeks later than Maryland
  • Virginia: arrives early December, 2 weeks later than Maryland
  • Pennsylvania: arrives mid July, 16 weeks earlier than Maryland

Common questions

When do Common Mergansers arrive in Maryland?
Common Mergansers are winter birds in Maryland: the first arrive in mid November, they are common by the second week of December, peak in late January (9.2% of checklists), and most are gone by early April.
When do Common Mergansers leave Maryland?
Most Common Mergansers have left Maryland by early April, when reports fall below half their usual rate. Stragglers are still reported until the second week of May.
When is the best time to see a Common Merganser in Maryland?
The Common Merganser is reported most often in late January, on about 9.2% of Maryland checklists.
Where in Maryland are Common Mergansers seen most often?
By county, Cecil County, Washington County, and Allegany County have the highest peak reporting rates for the Common Merganser (counties with at least 200 checklists).