WhenToBird

Vireo cassinii · Vireonidae

When Do Cassin's Vireos Arrive in Colorado?

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

Cassin's Vireos mostly pass through Colorado: they arrive in late August, peak in the second week of September (1.5% of checklists), and most are gone by late September.

When to look

First arrivalsAug 22-31late August
Common byAug 22-31late August
PeakSep 8-14the second week of September
Most gone bySep 22-30late September

Best: the second week of September. First arrivals late August; most are gone by late September. Peak: on about 1.5% of Colorado checklists.

  1. Jannot usually seen
  2. Febnot usually seen
  3. Marnot usually seen
  4. Aprrare
  5. Mayrare
  6. Junnot usually seen
  7. Julnot usually seen
  8. Augcommon
  9. Seppeak season
  10. Octuncommon
  11. Novnot usually seen
  12. Decnot usually seen

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

0%0.5%1%1.5%2%Jan 1-7: 0% of 32254 checklistsJan 8-14: 0% of 23278 checklistsJan 15-21: 0% of 22933 checklistsJan 22-31: 0% of 30856 checklistsFeb 1-7: 0% of 21464 checklistsFeb 8-14: 0% of 22910 checklistsFeb 15-21: 0% of 28739 checklistsFeb 22-28: 0% of 21833 checklistsMar 1-7: 0% of 23066 checklistsMar 8-14: 0% of 23783 checklistsMar 15-21: 0% of 24797 checklistsMar 22-31: 0% of 38439 checklistsApr 1-7: 0% of 31676 checklistsApr 8-14: 0% of 35092 checklistsApr 15-21: 0.1% of 36778 checklistsApr 22-30: 0.1% of 50885 checklistsMay 1-7: 0.2% of 44186 checklistsMay 8-14: 0.2% 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% of 36072 checklistsJul 1-7: 0% of 28857 checklistsJul 8-14: 0% of 27224 checklistsJul 15-21: 0% of 27629 checklistsJul 22-31: 0% of 37318 checklistsAug 1-7: 0% of 25893 checklistsAug 8-14: 0.1% of 24870 checklistsAug 15-21: 0.2% of 24035 checklistsAug 22-31: 0.8% of 35691 checklistsSep 1-7: 1.5% of 27068 checklistsSep 8-14: 1.7% of 26709 checklistsSep 15-21: 1.1% of 26098 checklistsSep 22-30: 0.6% of 31550 checklistsOct 1-7: 0.2% of 24110 checklistsOct 8-14: 0.1% of 23352 checklistsOct 15-21: 0.1% of 21895 checklistsOct 22-31: 0.1% of 28319 checklistsNov 1-7: 0% of 21698 checklistsNov 8-14: 0% of 21061 checklistsNov 15-21: 0% of 19855 checklistsNov 22-30: 0% of 25798 checklistsDec 1-7: 0% of 18626 checklistsDec 8-14: 0% of 19004 checklistsDec 15-21: 0% of 23103 checklistsDec 22-31: 0% 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. Lincoln CountyBest early September · 88 checklists that week, 2,707 in all8.6%
  2. Baca CountyBest the second week of September · 40 checklists that week, 3,209 in all7.6%
  3. Kit Carson CountyBest early September · 57 checklists that week, 2,633 in all7.2%
  4. Moffat CountyBest the second week of September · 121 checklists that week, 5,700 in all6.6%
  5. Yuma CountyBest early September · 114 checklists that week, 3,844 in all6.3%
  6. Kiowa CountyBest early September · 70 checklists that week, 3,854 in all6.0%

Best spots

Share of checklists at the peak
  1. Crow Valley ParkWeld County, best the second week of September · 54 checklists that week, 2,041 in all24%
  2. Chico BasinPueblo County, best early September · 38 checklists that week, 963 in all23%
  3. Colorado State Land Board State Land Board (southwest)El Paso County, best early September · 56 checklists that week, 908 in all22%
  4. Flagler Reservoir State Wildlife AreaKit Carson County, best late August · 21 checklists that week, 602 in all20%
  5. Colorado State Land Board State Land BoardEl Paso County, best early September · 68 checklists that week, 1,363 in all19%

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

Best time of day

In Colorado, the Cassin's Vireo is most often listed on checklists started between 6 and 8 am (0.3% of those checklists).

0%0.5%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: 0.3% of 71991 checklists7 am: 0.3% of 159189 checklists8 am: 0.2% of 177206 checklists9 am: 0.2% of 165269 checklists10 am: 0.1% of 141029 checklists11 am: 0.1% of 115283 checklists12 pm: 0.1% of 96068 checklists1 pm: 0.1% of 89512 checklists2 pm: 0.1% of 86319 checklists3 pm: 0.1% of 82496 checklists4 pm: 0.1% of 71648 checklists5 pm: 0.1% of 52445 checklists6 pm: 0.1% 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
January0%0%0%0%
February0%0%0%0%
March0%0%0%0%
April0%0%0.1%0.1%
May0.2%0.2%0.1%0.1%
June0.1%0.1%0.1%0%
July0%0%0%0%
August0%0.1%0.2%0.8%
September1.5%1.7%1.1%0.6%
October0.2%0.1%0.1%0.1%
November0%0%0%0%
December0%0%0%0%

Neighboring states

  • New Mexico: arrives mid August, 1 week earlier than Colorado
  • Utah: arrives late August, in the same week than Colorado
  • Arizona: arrives early March, 23 weeks earlier than Colorado
  • Montana: arrives late April, 16 weeks earlier than Colorado

Common questions

When do Cassin's Vireos arrive in Colorado?
Cassin's Vireos mostly pass through Colorado: they arrive in late August, peak in the second week of September (1.5% of checklists), and most are gone by late September.
When do Cassin's Vireos leave Colorado?
Most Cassin's Vireos have left Colorado by late September, when reports fall below half their usual rate.
When is the best time to see a Cassin's Vireo in Colorado?
The Cassin's Vireo is reported most often in the second week of September, on about 1.5% of Colorado checklists.
Where in Colorado are Cassin's Vireos seen most often?
By county, Lincoln County, Baca County, and Kit Carson County have the highest peak reporting rates for the Cassin's Vireo (counties with at least 200 checklists).