WhenToBird

Mergus merganser · Anatidae

When Do Common Mergansers Arrive in Colorado?

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

Common Mergansers are winter birds in Colorado: the first arrive in early September, they are common by early November, peak in mid January (14% of checklists), and most are gone by late April.

When to look

First arrivalsSep 1-7early September
Common byNov 1-7early November
PeakJan 15-21mid January
Most gone byApr 22-30late April

Best: mid January. First arrivals early September; most are gone by late April. Peak: on about 14% of Colorado checklists.

  1. Janpeak season
  2. Febpeak season
  3. Marpeak season
  4. Aprcommon
  5. Mayuncommon
  6. Jununcommon
  7. Julrare
  8. Auguncommon
  9. Sepuncommon
  10. Octuncommon
  11. Novpeak season
  12. Decpeak season

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

0%5%10%15%Jan 1-7: 14% of 32254 checklistsJan 8-14: 15% of 23278 checklistsJan 15-21: 15% of 22933 checklistsJan 22-31: 14% of 30856 checklistsFeb 1-7: 13% of 21464 checklistsFeb 8-14: 11% of 22910 checklistsFeb 15-21: 9.9% of 28739 checklistsFeb 22-28: 14% of 21833 checklistsMar 1-7: 14% of 23066 checklistsMar 8-14: 13% of 23783 checklistsMar 15-21: 13% of 24797 checklistsMar 22-31: 12% of 38439 checklistsApr 1-7: 11% of 31676 checklistsApr 8-14: 9.5% of 35092 checklistsApr 15-21: 8.4% of 36778 checklistsApr 22-30: 6.5% of 50885 checklistsMay 1-7: 4.7% of 44186 checklistsMay 8-14: 4.1% of 49011 checklistsMay 15-21: 3.7% of 40784 checklistsMay 22-31: 3.2% of 53073 checklistsJun 1-7: 3.0% of 32432 checklistsJun 8-14: 2.1% of 29978 checklistsJun 15-21: 1.9% of 29457 checklistsJun 22-30: 1.9% of 36072 checklistsJul 1-7: 1.7% of 28857 checklistsJul 8-14: 1.9% of 27224 checklistsJul 15-21: 1.7% of 27629 checklistsJul 22-31: 1.7% of 37318 checklistsAug 1-7: 1.6% of 25893 checklistsAug 8-14: 2.0% of 24870 checklistsAug 15-21: 2.2% of 24035 checklistsAug 22-31: 2.5% of 35691 checklistsSep 1-7: 2.6% of 27068 checklistsSep 8-14: 3.3% of 26709 checklistsSep 15-21: 3.3% of 26098 checklistsSep 22-30: 3.3% of 31550 checklistsOct 1-7: 3.2% of 24110 checklistsOct 8-14: 3.3% of 23352 checklistsOct 15-21: 3.9% of 21895 checklistsOct 22-31: 5.6% of 28319 checklistsNov 1-7: 7.7% of 21698 checklistsNov 8-14: 10% of 21061 checklistsNov 15-21: 10% of 19855 checklistsNov 22-30: 11% of 25798 checklistsDec 1-7: 12% of 18626 checklistsDec 8-14: 11% of 19004 checklistsDec 15-21: 10% of 23103 checklistsDec 22-31: 12% of 30802 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 Colorado 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 Colorado

Best counties

Share of checklists at the peak
  1. Delta CountyBest early March · 108 checklists that week, 6,509 in all52%
  2. Costilla CountyBest late February · 14 checklists that week, 1,323 in all51%
  3. Ouray CountyBest the second week of December · 92 checklists that week, 10,572 in all48%
  4. Mineral CountyBest the second week of April · 18 checklists that week, 1,750 in all44%
  5. Kiowa CountyBest mid January · 28 checklists that week, 3,854 in all42%
  6. Huerfano CountyBest the second week of December · 17 checklists that week, 3,008 in all40%

Best spots

Share of checklists at the peak
  1. Great Western ReservoirBroomfield County, best late December · 27 checklists that week, 628 in all98%
  2. Ridgway State ParkOuray County, best late December · 37 checklists that week, 1,406 in all97%
  3. Platte Canyon Reservoir (Brush Creek)Douglas County, best mid March · 25 checklists that week, 1,284 in all97%
  4. Totten LakeMontezuma County, best late January · 22 checklists that week, 994 in all95%

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

Best time of day

In Colorado, the Common Merganser is most often listed on checklists started between 1 and 3 pm (8.2% of those checklists).

0%5%10%12 am: too few checklists (512) to judge1 am: too few checklists (468) to judge2 am: too few checklists (513) to judge3 am: too few checklists (656) to judge4 am: too few checklists (1918) to judge5 am: too few checklists (16045) to judge6 am: 4.0% of 71991 checklists7 am: 5.4% of 159189 checklists8 am: 6.9% of 177206 checklists9 am: 7.7% of 165269 checklists10 am: 7.7% of 141029 checklists11 am: 7.8% of 115283 checklists12 pm: 7.6% of 96068 checklists1 pm: 8.1% of 89512 checklists2 pm: 8.3% of 86319 checklists3 pm: 8.0% of 82496 checklists4 pm: 6.7% of 71648 checklists5 pm: 4.9% of 52445 checklists6 pm: 3.5% of 40199 checklists7 pm: too few checklists (24246) to judge8 pm: too few checklists (7986) to judge9 pm: too few checklists (2004) to judge10 pm: too few checklists (888) to judge11 pm: too few checklists (450) 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
January14%15%15%14%
February13%11%9.9%14%
March14%13%13%12%
April11%9.5%8.4%6.5%
May4.7%4.1%3.7%3.2%
June3.0%2.1%1.9%1.9%
July1.7%1.9%1.7%1.7%
August1.6%2.0%2.2%2.5%
September2.6%3.3%3.3%3.3%
October3.2%3.3%3.9%5.6%
November7.7%10%10%11%
December12%11%10%12%

Neighboring states

  • Wyoming: arrives mid March, 22 weeks earlier than Colorado
  • New Mexico: arrives early November, 8 weeks later than Colorado
  • Utah: arrives mid September, 2 weeks later than Colorado
  • Nebraska: arrives early November, 8 weeks later than Colorado

Common questions

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