WhenToBird

Mergus merganser · Anatidae

When Do Common Mergansers Arrive in Wyoming?

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

Common Mergansers are in Wyoming all year, but most are here in spring: numbers build from mid March, peak in the second week of April (15% of checklists), and fall back by late May. In the quietest week (late January) they are still on about 3.9% of checklists.

When to look

Numbers buildMar 15-21mid March
PeakApr 8-14the second week of April
Fall back byMay 22-31late May
FewestJan 22-31late January

Best: the second week of April. Here all year; numbers build from mid March and fall back by late May. Peak: on about 15% of Wyoming checklists.

  1. Januncommon
  2. Febuncommon
  3. Marpeak season
  4. Aprpeak season
  5. Maypeak season
  6. Juncommon
  7. Julcommon
  8. Augcommon
  9. Sepcommon
  10. Octcommon
  11. Novcommon
  12. Deccommon

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

0%5%10%15%20%Jan 1-7: 4.9% of 2543 checklistsJan 8-14: 4.4% of 2114 checklistsJan 15-21: 4.0% of 1984 checklistsJan 22-31: 3.5% of 2605 checklistsFeb 1-7: 4.3% of 1940 checklistsFeb 8-14: 4.6% of 1954 checklistsFeb 15-21: 4.7% of 2575 checklistsFeb 22-28: 4.8% of 1932 checklistsMar 1-7: 7.9% of 2486 checklistsMar 8-14: 9.7% of 2535 checklistsMar 15-21: 10% of 2726 checklistsMar 22-31: 13% of 4349 checklistsApr 1-7: 14% of 3300 checklistsApr 8-14: 16% of 3500 checklistsApr 15-21: 15% of 3701 checklistsApr 22-30: 14% of 5887 checklistsMay 1-7: 13% of 5341 checklistsMay 8-14: 11% of 6910 checklistsMay 15-21: 9.5% of 6884 checklistsMay 22-31: 9.6% of 10652 checklistsJun 1-7: 9.8% of 6807 checklistsJun 8-14: 8.8% of 6714 checklistsJun 15-21: 7.3% of 6708 checklistsJun 22-30: 7.2% of 8160 checklistsJul 1-7: 5.5% of 5755 checklistsJul 8-14: 5.8% of 5379 checklistsJul 15-21: 5.9% of 5299 checklistsJul 22-31: 5.5% of 6830 checklistsAug 1-7: 6.0% of 4581 checklistsAug 8-14: 5.2% of 4625 checklistsAug 15-21: 6.4% of 4466 checklistsAug 22-31: 6.8% of 6562 checklistsSep 1-7: 6.9% of 4543 checklistsSep 8-14: 8.5% of 4652 checklistsSep 15-21: 8.7% of 4186 checklistsSep 22-30: 9.8% of 5301 checklistsOct 1-7: 8.1% of 3248 checklistsOct 8-14: 8.0% of 3010 checklistsOct 15-21: 8.2% of 2624 checklistsOct 22-31: 7.9% of 3170 checklistsNov 1-7: 10% of 2110 checklistsNov 8-14: 10% of 2153 checklistsNov 15-21: 11% of 1971 checklistsNov 22-30: 7.1% of 2136 checklistsDec 1-7: 6.1% of 1646 checklistsDec 8-14: 5.0% of 1609 checklistsDec 15-21: 4.0% of 1954 checklistsDec 22-31: 4.2% of 2392 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 Wyoming 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 Wyoming

Best counties

Share of checklists at the peak
  1. Platte CountyBest mid November · 34 checklists that week, 1,645 in all41%
  2. Converse CountyBest early April · 84 checklists that week, 3,460 in all30%
  3. Hot Springs CountyBest the second week of February · 22 checklists that week, 1,345 in all30%
  4. Weston CountyBest the second week of April · 51 checklists that week, 2,297 in all30%
  5. Natrona CountyBest mid November · 53 checklists that week, 5,810 in all29%
  6. Sweetwater CountyBest mid April · 150 checklists that week, 5,064 in all27%

Best spots

Share of checklists at the peak
  1. Grand Teton National Park (Spring Creek)Teton County, best mid September · 24 checklists that week, 724 in all87%
  2. Wyoming Hereford Ranch Reservoir Number OneLaramie County, best early March · 26 checklists that week, 1,242 in all84%
  3. Lake Hattie ReservoirAlbany County, best mid September · 22 checklists that week, 476 in all79%
  4. LeHardys RapidsPark County, best the second week of September · 63 checklists that week, 1,146 in all78%
  5. National Elk RefugeTeton County, best late January · 25 checklists that week, 1,797 in all75%
  6. Usbr (southeast)Lincoln County, best late May · 21 checklists that week, 115 in all74%

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

Best time of day

In Wyoming, the Common Merganser is most often listed on checklists started between 12 and 2 pm (9.4% of those checklists).

0%5%10%12 am: too few checklists (59) to judge1 am: too few checklists (76) to judge2 am: too few checklists (86) to judge3 am: too few checklists (79) to judge4 am: too few checklists (326) to judge5 am: too few checklists (2271) to judge6 am: 6.8% of 8808 checklists7 am: 7.0% of 18458 checklists8 am: 7.6% of 22143 checklists9 am: 8.4% of 21499 checklists10 am: 9.1% of 19425 checklists11 am: 8.9% of 16017 checklists12 pm: 9.1% of 13868 checklists1 pm: 9.6% of 12984 checklists2 pm: 9.0% of 12605 checklists3 pm: 8.6% of 11821 checklists4 pm: 8.6% of 10571 checklists5 pm: 8.5% of 8746 checklists6 pm: 8.5% of 6707 checklists7 pm: 7.8% of 4890 checklists8 pm: too few checklists (2323) to judge9 pm: too few checklists (554) to judge10 pm: too few checklists (134) to judge11 pm: too few checklists (59) 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
January4.9%4.4%4.0%3.5%
February4.3%4.6%4.7%4.8%
March7.9%9.7%10%13%
April14%16%15%14%
May13%11%9.5%9.6%
June9.8%8.8%7.3%7.2%
July5.5%5.8%5.9%5.5%
August6.0%5.2%6.4%6.8%
September6.9%8.5%8.7%9.8%
October8.1%8.0%8.2%7.9%
November10%10%11%7.1%
December6.1%5.0%4.0%4.2%

Neighboring states

  • Colorado: arrives early September, 22 weeks later than Wyoming
  • Utah: arrives mid September, 24 weeks earlier than Wyoming
  • Montana: arrives the second week of March, 1 week earlier than Wyoming
  • Idaho: arrives mid October, 20 weeks earlier than Wyoming

Common questions

When do Common Mergansers arrive in Wyoming?
Common Mergansers are in Wyoming all year, but most are here in spring: numbers build from mid March, peak in the second week of April (15% of checklists), and fall back by late May. In the quietest week (late January) they are still on about 3.9% of checklists.
When do Common Mergansers leave Wyoming?
Common Mergansers never fully leave Wyoming: numbers fall back below half their usual rate by late May, but some are still reported in every week, about 3.9% of checklists at the low.
When is the best time to see a Common Merganser in Wyoming?
The Common Merganser is reported most often in the second week of April, on about 15% of Wyoming checklists.
Where in Wyoming are Common Mergansers seen most often?
By county, Platte County, Converse County, and Hot Springs County have the highest peak reporting rates for the Common Merganser (counties with at least 200 checklists).