WhenToBird

Mergus merganser · Anatidae

When Do Common Mergansers Arrive in Delaware?

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

Common Mergansers are winter birds in Delaware: the first arrive in early December, they are common by the second week of December, peak in mid February (7.1% of checklists), and most are gone by early April.

When to look

First arrivalsDec 1-7early December
Common byDec 8-14the second week of December
PeakFeb 15-21mid February
Most gone byApr 1-7early April

Best: mid February. First arrivals early December; most are gone by early April. Peak: on about 7.1% of Delaware checklists.

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

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

0%2%4%6%8%Jan 1-7: 2.6% of 5901 checklistsJan 8-14: 3.8% of 4091 checklistsJan 15-21: 4.1% of 3841 checklistsJan 22-31: 5.7% of 5084 checklistsFeb 1-7: 6.4% of 3705 checklistsFeb 8-14: 7.2% of 3991 checklistsFeb 15-21: 6.6% of 6056 checklistsFeb 22-28: 7.7% of 4036 checklistsMar 1-7: 7.0% of 3822 checklistsMar 8-14: 6.3% of 3873 checklistsMar 15-21: 6.6% of 3856 checklistsMar 22-31: 5.6% of 5702 checklistsApr 1-7: 2.8% of 4104 checklistsApr 8-14: 2.0% of 4456 checklistsApr 15-21: 1.1% of 4871 checklistsApr 22-30: 0.6% of 7249 checklistsMay 1-7: 0.3% of 6806 checklistsMay 8-14: 0.2% of 7229 checklistsMay 15-21: 0.2% of 6466 checklistsMay 22-31: 0.1% of 7437 checklistsJun 1-7: 0.1% of 3596 checklistsJun 8-14: 0.1% of 2847 checklistsJun 15-21: 0.1% of 2901 checklistsJun 22-30: 0% of 3644 checklistsJul 1-7: 0% of 3142 checklistsJul 8-14: 0% of 2767 checklistsJul 15-21: 0% of 2882 checklistsJul 22-31: 0% of 4175 checklistsAug 1-7: 0% of 3482 checklistsAug 8-14: 0.1% of 3477 checklistsAug 15-21: 0% of 3386 checklistsAug 22-31: 0% of 5028 checklistsSep 1-7: 0% of 3631 checklistsSep 8-14: 0.1% of 3507 checklistsSep 15-21: 0% of 3404 checklistsSep 22-30: 0.1% of 4252 checklistsOct 1-7: 0.1% of 3563 checklistsOct 8-14: 0.1% of 3440 checklistsOct 15-21: 0.1% of 3282 checklistsOct 22-31: 0.1% of 4025 checklistsNov 1-7: 0.1% of 3030 checklistsNov 8-14: 0.2% of 3141 checklistsNov 15-21: 0.4% of 2858 checklistsNov 22-30: 0.8% of 4104 checklistsDec 1-7: 1.3% of 2472 checklistsDec 8-14: 1.7% of 2415 checklistsDec 15-21: 4.1% of 3306 checklistsDec 22-31: 3.2% of 5527 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 Delaware 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 Delaware

Best counties

Share of checklists at the peak
  1. New Castle CountyBest mid February · 2488 checklists that week, 72,140 in all11%
  2. Kent CountyBest late February · 640 checklists that week, 36,693 in all10%
  3. Sussex CountyBest early March · 1800 checklists that week, 91,027 in all3.4%

Best spots

Share of checklists at the peak
  1. Newark ReservoirNew Castle County, best the second week of February · 32 checklists that week, 1,314 in all98%
  2. Little Creek Wildlife Area (Taylors Gut)Kent County, best early March · 21 checklists that week, 421 in all60%
  3. Augustine Wildlife AreaNew Castle County, best the second week of February · 59 checklists that week, 2,762 in all57%
  4. C&D Canal Conservation AreaNew Castle County, best mid February · 21 checklists that week, 303 in all52%
  5. Oak PondSussex County, best late February · 23 checklists that week, 1,265 in all52%
  6. WoodlawnNew Castle County, best late March · 24 checklists that week, 1,011 in all50%

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

Best time of day

In Delaware, the Common Merganser is most often listed on checklists started between 3 and 5 pm (2.5% of those checklists).

0%1%2%3%12 am: too few checklists (129) to judge1 am: too few checklists (122) to judge2 am: too few checklists (102) to judge3 am: too few checklists (114) to judge4 am: too few checklists (210) to judge5 am: too few checklists (1334) to judge6 am: 1.4% of 8055 checklists7 am: 2.0% of 21367 checklists8 am: 2.1% of 25648 checklists9 am: 2.0% of 22669 checklists10 am: 2.0% of 20679 checklists11 am: 1.8% of 17516 checklists12 pm: 2.1% of 15361 checklists1 pm: 2.1% of 14319 checklists2 pm: 2.2% of 13726 checklists3 pm: 2.5% of 12793 checklists4 pm: 2.4% of 10392 checklists5 pm: 1.4% of 6859 checklists6 pm: 0.7% of 4542 checklists7 pm: too few checklists (2633) to judge8 pm: too few checklists (831) to judge9 pm: too few checklists (211) to judge10 pm: too few checklists (150) to judge11 pm: too few checklists (98) 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
January2.6%3.8%4.1%5.7%
February6.4%7.2%6.6%7.7%
March7.0%6.3%6.6%5.6%
April2.8%2.0%1.1%0.6%
May0.3%0.2%0.2%0.1%
June0.1%0.1%0.1%0%
July0%0%0%0%
August0%0.1%0%0%
September0%0.1%0%0.1%
October0.1%0.1%0.1%0.1%
November0.1%0.2%0.4%0.8%
December1.3%1.7%4.1%3.2%

Neighboring states

  • Maryland: arrives mid November, 2 weeks earlier than Delaware
  • New Jersey: arrives early November, 4 weeks earlier than Delaware
  • District of Columbia: arrives mid November, 2 weeks earlier than Delaware
  • Pennsylvania: arrives mid July, 18 weeks earlier than Delaware

Common questions

When do Common Mergansers arrive in Delaware?
Common Mergansers are winter birds in Delaware: the first arrive in early December, they are common by the second week of December, peak in mid February (7.1% of checklists), and most are gone by early April.
When do Common Mergansers leave Delaware?
Most Common Mergansers have left Delaware by early April, when reports fall below half their usual rate. Stragglers are still reported until mid April.
When is the best time to see a Common Merganser in Delaware?
The Common Merganser is reported most often in mid February, on about 7.1% of Delaware checklists.
Where in Delaware are Common Mergansers seen most often?
By county, New Castle County, Kent County, and Sussex County have the highest peak reporting rates for the Common Merganser (counties with at least 200 checklists).