WhenToBird

Vireo cassinii · Vireonidae

When Do Cassin's Vireos Arrive in Montana?

Adult Cassin's Vireo
Photo: David Anderson, CC BY 4.0 via iNaturalist

Cassin's Vireos reach Montana in late April, are common by early May, peak in late June (4.8% of checklists), and most are gone by the second week of September.

When to look

First arrivalsApr 22-30late April
Common byMay 1-7early May
PeakJun 22-30late June
Most gone bySep 8-14the second week of September

Best: late June. First arrivals late April; most are gone by the second week of September. Peak: on about 4.8% of Montana checklists.

  1. Jannot usually seen
  2. Febnot usually seen
  3. Marnot usually seen
  4. Apruncommon
  5. Maypeak season
  6. Junpeak season
  7. Julpeak season
  8. Augcommon
  9. Sepcommon
  10. Octrare
  11. Novnot usually seen
  12. Decnot usually seen

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

0%2%4%6%Jan 1-7: 0% of 7987 checklistsJan 8-14: 0% of 5561 checklistsJan 15-21: 0% of 5613 checklistsJan 22-31: 0% of 7881 checklistsFeb 1-7: 0% of 5018 checklistsFeb 8-14: 0% of 5598 checklistsFeb 15-21: 0% of 7247 checklistsFeb 22-28: 0% of 5531 checklistsMar 1-7: 0% of 6110 checklistsMar 8-14: 0% of 6623 checklistsMar 15-21: 0% of 7687 checklistsMar 22-31: 0% of 11095 checklistsApr 1-7: 0% of 7774 checklistsApr 8-14: 0% of 8785 checklistsApr 15-21: 0% of 9684 checklistsApr 22-30: 0.6% of 13805 checklistsMay 1-7: 2.5% of 11847 checklistsMay 8-14: 3.6% of 13547 checklistsMay 15-21: 3.8% of 13019 checklistsMay 22-31: 4.4% of 20456 checklistsJun 1-7: 4.6% of 12993 checklistsJun 8-14: 4.7% of 11594 checklistsJun 15-21: 4.9% of 11159 checklistsJun 22-30: 4.5% of 13786 checklistsJul 1-7: 5.0% of 9584 checklistsJul 8-14: 4.0% of 9078 checklistsJul 15-21: 3.7% of 7905 checklistsJul 22-31: 3.0% of 10906 checklistsAug 1-7: 2.3% of 7335 checklistsAug 8-14: 2.2% of 6868 checklistsAug 15-21: 2.5% of 6866 checklistsAug 22-31: 3.0% of 10076 checklistsSep 1-7: 2.7% of 7670 checklistsSep 8-14: 1.7% of 7490 checklistsSep 15-21: 1.1% of 6804 checklistsSep 22-30: 0.8% of 8016 checklistsOct 1-7: 0.2% of 5996 checklistsOct 8-14: 0.1% of 6156 checklistsOct 15-21: 0% of 5742 checklistsOct 22-31: 0% of 7790 checklistsNov 1-7: 0% of 5353 checklistsNov 8-14: 0% of 5475 checklistsNov 15-21: 0% of 4603 checklistsNov 22-30: 0% of 6067 checklistsDec 1-7: 0% of 4362 checklistsDec 8-14: 0% of 4467 checklistsDec 15-21: 0% of 5328 checklistsDec 22-31: 0% 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.

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Where in Montana

Best counties

Share of checklists at the peak
  1. Mineral CountyBest early June · 65 checklists that week, 2,163 in all22%
  2. Lincoln CountyBest early June · 219 checklists that week, 5,543 in all17%
  3. Sanders CountyBest late August · 99 checklists that week, 3,767 in all16%
  4. Flathead CountyBest the second week of June · 1114 checklists that week, 40,185 in all14%
  5. Missoula CountyBest early June · 1741 checklists that week, 64,368 in all13%
  6. Lake CountyBest the second week of June · 425 checklists that week, 19,478 in all9.4%

Best spots

Share of checklists at the peak
  1. Rattlesnake National Recreation AreaMissoula County, best late May · 27 checklists that week, 298 in all68%
  2. Pattee Canyon Ski Tour National Recreation TrailMissoula County, best the second week of June · 26 checklists that week, 550 in all67%
  3. Greer GulchJefferson County, best late May · 24 checklists that week, 197 in all55%
  4. Blue Mountain Nature National Recreation TrailMissoula County, best early June · 20 checklists that week, 177 in all55%
  5. Lewis and Clark Caverns State Park (Dolomite Knob)Jefferson County, best late May · 20 checklists that week, 308 in all44%

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

Best time of day

In Montana, the Cassin's Vireo is most often listed on checklists started between 6 and 8 am (3.5% of those checklists).

0%2%4%6%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: 4.2% of 12719 checklists7 am: 3.2% of 31202 checklists8 am: 2.6% of 47646 checklists9 am: 2.4% of 44624 checklists10 am: 2.1% of 40704 checklists11 am: 1.6% of 33825 checklists12 pm: 1.4% of 29407 checklists1 pm: 0.9% of 27959 checklists2 pm: 0.9% of 26928 checklists3 pm: 0.8% of 25313 checklists4 pm: 0.8% of 22151 checklists5 pm: 1.1% of 16884 checklists6 pm: 1.1% of 13780 checklists7 pm: 1.4% 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
January0%0%0%0%
February0%0%0%0%
March0%0%0%0%
April0%0%0%0.6%
May2.5%3.6%3.8%4.4%
June4.6%4.7%4.9%4.5%
July5.0%4.0%3.7%3.0%
August2.3%2.2%2.5%3.0%
September2.7%1.7%1.1%0.8%
October0.2%0.1%0%0%
November0%0%0%0%
December0%0%0%0%

Neighboring states

  • Idaho: arrives late April, in the same week than Montana
  • Utah: arrives late August, 16 weeks later than Montana
  • Washington: arrives mid April, 1 week earlier than Montana
  • Oregon: arrives the second week of April, 2 weeks earlier than Montana

Common questions

When do Cassin's Vireos arrive in Montana?
Cassin's Vireos reach Montana in late April, are common by early May, peak in late June (4.8% of checklists), and most are gone by the second week of September.
When do Cassin's Vireos leave Montana?
Most Cassin's Vireos have left Montana by the second week of September, when reports fall below half their usual rate. Stragglers are still reported until mid September.
When is the best time to see a Cassin's Vireo in Montana?
The Cassin's Vireo is reported most often in late June, on about 4.8% of Montana checklists.
Where in Montana are Cassin's Vireos seen most often?
By county, Mineral County, Lincoln County, and Sanders County have the highest peak reporting rates for the Cassin's Vireo (counties with at least 200 checklists).