WhenToBird

Mergus serrator · Anatidae

When Do Red-breasted Mergansers Arrive in Colorado?

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

Red-breasted Mergansers are winter birds in Colorado: they arrive in late October, peak in the second week of November (2.7% of checklists), and most are gone by mid April.

When to look

First arrivalsOct 22-31late October
Common byOct 22-31late October
PeakNov 8-14the second week of November
Most gone byApr 15-21mid April

Best: the second week of November. First arrivals late October; most are gone by mid April. Peak: on about 2.7% of Colorado checklists.

  1. Janpeak season
  2. Febcommon
  3. Marcommon
  4. Aprcommon
  5. Mayrare
  6. Junnot usually seen
  7. Julnot usually seen
  8. Augnot usually seen
  9. Sepnot usually seen
  10. Octcommon
  11. Novpeak season
  12. Decpeak season

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

0%1%2%3%4%Jan 1-7: 2.5% of 32254 checklistsJan 8-14: 2.2% of 23278 checklistsJan 15-21: 2.3% of 22933 checklistsJan 22-31: 1.8% of 30856 checklistsFeb 1-7: 1.6% of 21464 checklistsFeb 8-14: 1.4% of 22910 checklistsFeb 15-21: 1.4% of 28739 checklistsFeb 22-28: 1.7% of 21833 checklistsMar 1-7: 1.4% of 23066 checklistsMar 8-14: 1.5% of 23783 checklistsMar 15-21: 1.2% of 24797 checklistsMar 22-31: 1.5% of 38439 checklistsApr 1-7: 1.5% of 31676 checklistsApr 8-14: 1.6% of 35092 checklistsApr 15-21: 1.4% of 36778 checklistsApr 22-30: 0.8% of 50885 checklistsMay 1-7: 0.2% of 44186 checklistsMay 8-14: 0.1% of 49011 checklistsMay 15-21: 0.1% of 40784 checklistsMay 22-31: 0.1% of 53073 checklistsJun 1-7: 0.1% of 32432 checklistsJun 8-14: 0.1% of 29978 checklistsJun 15-21: 0.1% of 29457 checklistsJun 22-30: 0.1% of 36072 checklistsJul 1-7: 0.1% of 28857 checklistsJul 8-14: 0.1% of 27224 checklistsJul 15-21: 0.1% of 27629 checklistsJul 22-31: 0.1% of 37318 checklistsAug 1-7: 0% of 25893 checklistsAug 8-14: 0% of 24870 checklistsAug 15-21: 0.1% of 24035 checklistsAug 22-31: 0.1% of 35691 checklistsSep 1-7: 0.1% of 27068 checklistsSep 8-14: 0% of 26709 checklistsSep 15-21: 0.1% of 26098 checklistsSep 22-30: 0.1% of 31550 checklistsOct 1-7: 0.1% of 24110 checklistsOct 8-14: 0.1% of 23352 checklistsOct 15-21: 0.2% of 21895 checklistsOct 22-31: 1.0% of 28319 checklistsNov 1-7: 2.3% of 21698 checklistsNov 8-14: 3.4% of 21061 checklistsNov 15-21: 2.4% of 19855 checklistsNov 22-30: 2.3% of 25798 checklistsDec 1-7: 2.1% of 18626 checklistsDec 8-14: 1.5% of 19004 checklistsDec 15-21: 1.6% of 23103 checklistsDec 22-31: 1.6% 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. Kiowa CountyBest the second week of November · 58 checklists that week, 3,854 in all25%
  2. Crowley CountyBest the second week of December · 11 checklists that week, 1,883 in all17%
  3. Bent CountyBest early January · 50 checklists that week, 5,275 in all14%
  4. Huerfano CountyBest mid November · 37 checklists that week, 3,008 in all13%
  5. Sedgwick CountyBest the second week of November · 14 checklists that week, 1,168 in all13%
  6. Custer CountyBest late November · 12 checklists that week, 3,017 in all13%

Best spots

Share of checklists at the peak
  1. Leggett and Hillcrest ReservoirBoulder County, best mid December · 40 checklists that week, 436 in all76%
  2. Hillcrest ReservoirBoulder County, best the second week of March · 26 checklists that week, 1,078 in all76%
  3. Duke LakeJefferson County, best the second week of January · 26 checklists that week, 267 in all66%

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

Best time of day

In Colorado, the Red-breasted Merganser is most often listed on checklists started between 1 and 3 pm (1.2% of those checklists).

0%0.5%1%1.5%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: 0.4% of 71991 checklists7 am: 0.5% of 159189 checklists8 am: 0.8% of 177206 checklists9 am: 1.0% of 165269 checklists10 am: 1.0% of 141029 checklists11 am: 1.1% of 115283 checklists12 pm: 1.1% of 96068 checklists1 pm: 1.2% of 89512 checklists2 pm: 1.2% of 86319 checklists3 pm: 1.1% of 82496 checklists4 pm: 0.8% of 71648 checklists5 pm: 0.4% of 52445 checklists6 pm: 0.3% 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
January2.5%2.2%2.3%1.8%
February1.6%1.4%1.4%1.7%
March1.4%1.5%1.2%1.5%
April1.5%1.6%1.4%0.8%
May0.2%0.1%0.1%0.1%
June0.1%0.1%0.1%0.1%
July0.1%0.1%0.1%0.1%
August0%0%0.1%0.1%
September0.1%0%0.1%0.1%
October0.1%0.1%0.2%1.0%
November2.3%3.4%2.4%2.3%
December2.1%1.5%1.6%1.6%

Neighboring states

  • Wyoming: arrives early October, 3 weeks earlier than Colorado
  • Utah: arrives late February, 16 weeks later than Colorado
  • Nebraska: arrives early November, 1 week later than Colorado
  • South Dakota: arrives the second week of November, 2 weeks later than Colorado

Common questions

When do Red-breasted Mergansers arrive in Colorado?
Red-breasted Mergansers are winter birds in Colorado: they arrive in late October, peak in the second week of November (2.7% of checklists), and most are gone by mid April.
When do Red-breasted Mergansers leave Colorado?
Most Red-breasted Mergansers have left Colorado by mid April, when reports fall below half their usual rate.
When is the best time to see a Red-breasted Merganser in Colorado?
The Red-breasted Merganser is reported most often in the second week of November, on about 2.7% of Colorado checklists.
Where in Colorado are Red-breasted Mergansers seen most often?
By county, Kiowa County, Crowley County, and Bent County have the highest peak reporting rates for the Red-breasted Merganser (counties with at least 200 checklists).