WhenToBird

Vireo cassinii · Vireonidae

When Do Cassin's Vireos Arrive in California?

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

Cassin's Vireos arrive in California in the second week of April, peak in early May (3.4% of checklists), and most are gone by early September.

When to look

First arrivalsApr 8-14the second week of April
Common byApr 8-14the second week of April
PeakMay 1-7early May
Most gone bySep 1-7early September

Best: early May. First arrivals the second week of April; most are gone by early September. Peak: on about 3.4% of California checklists.

  1. Jannot usually seen
  2. Febnot usually seen
  3. Marrare
  4. Aprpeak season
  5. Maypeak season
  6. Junpeak season
  7. Julcommon
  8. Auguncommon
  9. Sepuncommon
  10. Octrare
  11. Novnot usually seen
  12. Decnot usually seen

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

0%1%2%3%4%Jan 1-7: 0.1% of 112336 checklistsJan 8-14: 0.1% of 85518 checklistsJan 15-21: 0.1% of 86488 checklistsJan 22-31: 0.1% of 116174 checklistsFeb 1-7: 0.1% of 75171 checklistsFeb 8-14: 0.1% of 84467 checklistsFeb 15-21: 0.1% of 105683 checklistsFeb 22-28: 0.1% of 83396 checklistsMar 1-7: 0.1% of 77470 checklistsMar 8-14: 0.1% of 75180 checklistsMar 15-21: 0.1% of 80770 checklistsMar 22-31: 0.3% of 114116 checklistsApr 1-7: 1.0% of 84057 checklistsApr 8-14: 1.9% of 83692 checklistsApr 15-21: 3.0% of 89242 checklistsApr 22-30: 3.5% of 114289 checklistsMay 1-7: 3.6% of 85728 checklistsMay 8-14: 3.1% of 91984 checklistsMay 15-21: 3.0% of 75478 checklistsMay 22-31: 2.7% of 106250 checklistsJun 1-7: 2.8% of 70051 checklistsJun 8-14: 2.8% of 67485 checklistsJun 15-21: 2.6% of 66269 checklistsJun 22-30: 2.1% of 79172 checklistsJul 1-7: 1.7% of 62877 checklistsJul 8-14: 1.4% of 60328 checklistsJul 15-21: 1.3% of 59705 checklistsJul 22-31: 1.1% of 83796 checklistsAug 1-7: 1.0% of 60215 checklistsAug 8-14: 1.2% of 60172 checklistsAug 15-21: 1.1% of 59877 checklistsAug 22-31: 1.4% of 89107 checklistsSep 1-7: 1.3% of 72000 checklistsSep 8-14: 0.9% of 71228 checklistsSep 15-21: 0.8% of 76808 checklistsSep 22-30: 0.5% of 100204 checklistsOct 1-7: 0.4% of 76571 checklistsOct 8-14: 0.3% of 80874 checklistsOct 15-21: 0.2% of 74581 checklistsOct 22-31: 0.1% of 97498 checklistsNov 1-7: 0.1% of 70851 checklistsNov 8-14: 0.1% of 72431 checklistsNov 15-21: 0.1% of 66767 checklistsNov 22-30: 0.1% of 94737 checklistsDec 1-7: 0.1% of 65393 checklistsDec 8-14: 0.1% of 67449 checklistsDec 15-21: 0.1% of 83421 checklistsDec 22-31: 0.1% of 124993 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 California 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 California

Best counties

Share of checklists at the peak
  1. Mariposa CountyBest the second week of June · 535 checklists that week, 17,385 in all19%
  2. Mendocino CountyBest late April · 1298 checklists that week, 47,934 in all16%
  3. Trinity CountyBest mid May · 175 checklists that week, 9,037 in all16%
  4. Tuolumne CountyBest the second week of May · 492 checklists that week, 21,159 in all15%
  5. Nevada CountyBest the second week of June · 737 checklists that week, 25,896 in all15%
  6. Plumas CountyBest mid August · 338 checklists that week, 24,629 in all15%

Best spots

Share of checklists at the peak
  1. Palmer CreekHumboldt County, best early September · 23 checklists that week, 796 in all81%
  2. Sanborn County Park (The Peak)Santa Clara County, best early June · 24 checklists that week, 398 in all77%
  3. Yosemite National Park (Ackerson Meadow)Tuolumne County, best early May · 27 checklists that week, 409 in all76%
  4. Yosemite National Park (northwest)Mariposa County, best late May · 21 checklists that week, 348 in all67%
  5. Cuyapaipe ReservoirSan Diego County, best mid May · 44 checklists that week, 260 in all65%

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

Best time of day

In California, the Cassin's Vireo is most often listed on checklists started between 6 and 8 am (1.7% of those checklists).

0%1%2%3%12 am: too few checklists (1688) to judge1 am: too few checklists (1451) to judge2 am: too few checklists (1402) to judge3 am: too few checklists (1743) to judge4 am: too few checklists (4647) to judge5 am: too few checklists (32158) to judge6 am: 2.1% of 186053 checklists7 am: 1.6% of 444086 checklists8 am: 1.3% of 535846 checklists9 am: 1.1% of 486061 checklists10 am: 0.9% of 411130 checklists11 am: 0.8% of 325798 checklists12 pm: 0.7% of 269132 checklists1 pm: 0.6% of 244762 checklists2 pm: 0.5% of 235420 checklists3 pm: 0.4% of 232365 checklists4 pm: 0.4% of 208688 checklists5 pm: 0.5% of 146104 checklists6 pm: 0.4% of 96867 checklists7 pm: too few checklists (50942) to judge8 pm: too few checklists (14803) to judge9 pm: too few checklists (5561) to judge10 pm: too few checklists (3546) to judge11 pm: too few checklists (2092) 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
January0.1%0.1%0.1%0.1%
February0.1%0.1%0.1%0.1%
March0.1%0.1%0.1%0.3%
April1.0%1.9%3.0%3.5%
May3.6%3.1%3.0%2.7%
June2.8%2.8%2.6%2.1%
July1.7%1.4%1.3%1.1%
August1.0%1.2%1.1%1.4%
September1.3%0.9%0.8%0.5%
October0.4%0.3%0.2%0.1%
November0.1%0.1%0.1%0.1%
December0.1%0.1%0.1%0.1%

Neighboring states

  • Nevada: arrives the second week of April, in the same week than California
  • Utah: arrives late August, 18 weeks later than California
  • Arizona: arrives early March, 5 weeks earlier than California
  • Oregon: arrives the second week of April, in the same week than California

Common questions

When do Cassin's Vireos arrive in California?
Cassin's Vireos arrive in California in the second week of April, peak in early May (3.4% of checklists), and most are gone by early September.
When do Cassin's Vireos leave California?
Most Cassin's Vireos have left California by early September, when reports fall below half their usual rate.
When is the best time to see a Cassin's Vireo in California?
The Cassin's Vireo is reported most often in early May, on about 3.4% of California checklists.
Where in California are Cassin's Vireos seen most often?
By county, Mariposa County, Mendocino County, and Trinity County have the highest peak reporting rates for the Cassin's Vireo (counties with at least 200 checklists).