WhenToBird

Mergus merganser · Anatidae

When Do Common Mergansers Arrive in Montana?

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

Common Mergansers are in Montana all year, but most are here in spring: numbers build from the second week of March, peak in early April (14% of checklists), and fall back by late April. In the quietest week (late July) they are still on about 4.4% of checklists.

When to look

Numbers buildMar 8-14the second week of March
PeakApr 1-7early April
Fall back byApr 22-30late April
FewestJul 22-31late July

Best: early April. Here all year; numbers build from the second week of March and fall back by late April. Peak: on about 14% of Montana checklists.

  1. Jancommon
  2. Febcommon
  3. Marpeak season
  4. Aprpeak season
  5. Maycommon
  6. Jununcommon
  7. Juluncommon
  8. Augcommon
  9. Sepcommon
  10. Octcommon
  11. Novpeak season
  12. Deccommon

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

0%5%10%15%Jan 1-7: 5.9% of 7987 checklistsJan 8-14: 5.3% of 5561 checklistsJan 15-21: 6.1% of 5613 checklistsJan 22-31: 6.9% of 7881 checklistsFeb 1-7: 6.3% of 5018 checklistsFeb 8-14: 5.8% of 5598 checklistsFeb 15-21: 5.9% of 7247 checklistsFeb 22-28: 6.8% of 5531 checklistsMar 1-7: 9.4% of 6110 checklistsMar 8-14: 10% of 6623 checklistsMar 15-21: 13% of 7687 checklistsMar 22-31: 14% of 11095 checklistsApr 1-7: 14% of 7774 checklistsApr 8-14: 14% of 8785 checklistsApr 15-21: 14% of 9684 checklistsApr 22-30: 11% of 13805 checklistsMay 1-7: 8.3% of 11847 checklistsMay 8-14: 7.2% of 13547 checklistsMay 15-21: 5.7% of 13019 checklistsMay 22-31: 5.1% of 20456 checklistsJun 1-7: 4.5% of 12993 checklistsJun 8-14: 5.1% of 11594 checklistsJun 15-21: 5.0% of 11159 checklistsJun 22-30: 5.0% of 13786 checklistsJul 1-7: 4.4% of 9584 checklistsJul 8-14: 4.6% of 9078 checklistsJul 15-21: 4.3% of 7905 checklistsJul 22-31: 4.5% of 10906 checklistsAug 1-7: 4.4% of 7335 checklistsAug 8-14: 5.4% of 6868 checklistsAug 15-21: 5.5% of 6866 checklistsAug 22-31: 5.5% of 10076 checklistsSep 1-7: 6.5% of 7670 checklistsSep 8-14: 7.1% of 7490 checklistsSep 15-21: 8.0% of 6804 checklistsSep 22-30: 8.0% of 8016 checklistsOct 1-7: 8.2% of 5996 checklistsOct 8-14: 9.1% of 6156 checklistsOct 15-21: 9.7% of 5742 checklistsOct 22-31: 9.4% of 7790 checklistsNov 1-7: 11% of 5353 checklistsNov 8-14: 11% of 5475 checklistsNov 15-21: 9.6% of 4603 checklistsNov 22-30: 9.0% of 6067 checklistsDec 1-7: 7.3% of 4362 checklistsDec 8-14: 6.7% of 4467 checklistsDec 15-21: 6.5% of 5328 checklistsDec 22-31: 5.9% of 6672 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 Montana 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.

Embed this map on your site

Where in Montana

Best counties

Share of checklists at the peak
  1. Mineral CountyBest the second week of January · 11 checklists that week, 2,163 in all51%
  2. McCone CountyBest early January · 11 checklists that week, 537 in all44%
  3. Madison CountyBest the second week of April · 143 checklists that week, 7,008 in all41%
  4. Wheatland CountyBest late March · 11 checklists that week, 1,384 in all38%
  5. Granite CountyBest the second week of April · 65 checklists that week, 1,737 in all35%
  6. Big Horn CountyBest late February · 15 checklists that week, 1,279 in all34%

Best spots

Share of checklists at the peak
  1. Swiftcurrent FallsGlacier County, best the second week of July · 23 checklists that week, 177 in all87%
  2. Helena Valley Reservoir Fishing Access SiteLewis and Clark County, best mid November · 38 checklists that week, 1,628 in all86%
  3. Ducharme Fishing Access SiteLake County, best early January · 22 checklists that week, 354 in all82%
  4. Foy LakeFlathead County, best mid October · 22 checklists that week, 328 in all76%

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

Best time of day

In Montana, the Common Merganser is most often listed on checklists started between 5 and 7 pm (8.4% of those checklists).

0%5%10%12 am: too few checklists (183) to judge1 am: too few checklists (169) to judge2 am: too few checklists (163) to judge3 am: too few checklists (212) to judge4 am: too few checklists (503) to judge5 am: too few checklists (4022) to judge6 am: 7.8% of 12719 checklists7 am: 6.3% of 31202 checklists8 am: 7.0% of 47646 checklists9 am: 8.0% of 44624 checklists10 am: 7.6% of 40704 checklists11 am: 8.2% of 33825 checklists12 pm: 7.5% of 29407 checklists1 pm: 7.8% of 27959 checklists2 pm: 8.0% of 26928 checklists3 pm: 7.4% of 25313 checklists4 pm: 7.2% of 22151 checklists5 pm: 7.5% of 16884 checklists6 pm: 9.6% of 13780 checklists7 pm: 5.1% of 9926 checklists8 pm: too few checklists (5765) to judge9 pm: too few checklists (2099) to judge10 pm: too few checklists (595) to judge11 pm: too few checklists (230) 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
January5.9%5.3%6.1%6.9%
February6.3%5.8%5.9%6.8%
March9.4%10%13%14%
April14%14%14%11%
May8.3%7.2%5.7%5.1%
June4.5%5.1%5.0%5.0%
July4.4%4.6%4.3%4.5%
August4.4%5.4%5.5%5.5%
September6.5%7.1%8.0%8.0%
October8.2%9.1%9.7%9.4%
November11%11%9.6%9.0%
December7.3%6.7%6.5%5.9%

Neighboring states

  • Idaho: arrives mid October, 19 weeks earlier than Montana
  • Wyoming: arrives mid March, 1 week later than Montana
  • Utah: arrives mid September, 23 weeks earlier than Montana
  • Washington: arrives early November, 17 weeks earlier than Montana

Common questions

When do Common Mergansers arrive in Montana?
Common Mergansers are in Montana all year, but most are here in spring: numbers build from the second week of March, peak in early April (14% of checklists), and fall back by late April. In the quietest week (late July) they are still on about 4.4% of checklists.
When do Common Mergansers leave Montana?
Common Mergansers never fully leave Montana: numbers fall back below half their usual rate by late April, but some are still reported in every week, about 4.4% of checklists at the low.
When is the best time to see a Common Merganser in Montana?
The Common Merganser is reported most often in early April, on about 14% of Montana checklists.
Where in Montana are Common Mergansers seen most often?
By county, Mineral County, McCone County, and Madison County have the highest peak reporting rates for the Common Merganser (counties with at least 200 checklists).