WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in California?

Adult female Brown-headed Cowbird
Photo: Nicole, CC BY 4.0 via iNaturalist

Brown-headed Cowbirds reach California in mid February, are common by the second week of April, peak in late May (17% of checklists), and most are gone by mid July.

When to look

First arrivalsFeb 15-21mid February
Common byApr 8-14the second week of April
PeakMay 22-31late May
Most gone byJul 15-21mid July

Best: late May. First arrivals mid February; most are gone by mid July. Peak: on about 17% of California checklists.

  1. Janrare
  2. Febrare
  3. Maruncommon
  4. Aprpeak season
  5. Maypeak season
  6. Junpeak season
  7. Julpeak season
  8. Auguncommon
  9. Sepuncommon
  10. Octrare
  11. Novrare
  12. Decrare

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

0%5%10%15%20%Jan 1-7: 1.8% of 112336 checklistsJan 8-14: 1.6% of 85518 checklistsJan 15-21: 1.6% of 86488 checklistsJan 22-31: 1.5% of 116174 checklistsFeb 1-7: 1.4% of 75171 checklistsFeb 8-14: 1.4% of 84467 checklistsFeb 15-21: 1.4% of 105683 checklistsFeb 22-28: 1.6% of 83396 checklistsMar 1-7: 1.7% of 77470 checklistsMar 8-14: 2.2% of 75180 checklistsMar 15-21: 2.9% of 80770 checklistsMar 22-31: 4.4% of 114116 checklistsApr 1-7: 6.8% of 84057 checklistsApr 8-14: 9.7% of 83692 checklistsApr 15-21: 13% of 89242 checklistsApr 22-30: 15% of 114289 checklistsMay 1-7: 17% of 85728 checklistsMay 8-14: 16% of 91984 checklistsMay 15-21: 17% of 75478 checklistsMay 22-31: 17% of 106250 checklistsJun 1-7: 17% of 70051 checklistsJun 8-14: 15% of 67485 checklistsJun 15-21: 15% of 66269 checklistsJun 22-30: 14% of 79172 checklistsJul 1-7: 13% of 62877 checklistsJul 8-14: 12% of 60328 checklistsJul 15-21: 9.1% of 59705 checklistsJul 22-31: 5.6% of 83796 checklistsAug 1-7: 3.9% of 60215 checklistsAug 8-14: 3.6% of 60172 checklistsAug 15-21: 3.3% of 59877 checklistsAug 22-31: 3.1% of 89107 checklistsSep 1-7: 3.1% of 72000 checklistsSep 8-14: 2.8% of 71228 checklistsSep 15-21: 2.6% of 76808 checklistsSep 22-30: 2.3% of 100204 checklistsOct 1-7: 2.4% of 76571 checklistsOct 8-14: 2.4% of 80874 checklistsOct 15-21: 2.5% of 74581 checklistsOct 22-31: 2.3% of 97498 checklistsNov 1-7: 2.0% of 70851 checklistsNov 8-14: 1.8% of 72431 checklistsNov 15-21: 1.7% of 66767 checklistsNov 22-30: 1.5% of 94737 checklistsDec 1-7: 1.5% of 65393 checklistsDec 8-14: 1.6% of 67449 checklistsDec 15-21: 1.6% of 83421 checklistsDec 22-31: 1.5% 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. Kings CountyBest mid May · 89 checklists that week, 3,688 in all41%
  2. Yolo CountyBest early June · 876 checklists that week, 67,424 in all40%
  3. Nevada CountyBest late May · 932 checklists that week, 25,896 in all38%
  4. Merced CountyBest the second week of May · 252 checklists that week, 13,095 in all35%
  5. Colusa CountyBest mid May · 125 checklists that week, 7,653 in all35%
  6. Lake CountyBest the second week of June · 229 checklists that week, 12,989 in all34%

Best spots

Share of checklists at the peak
  1. Sacramento Regional County Sanitation DistrictSacramento County, best the second week of May · 71 checklists that week, 2,065 in all88%
  2. Wild Wings ParkYolo County, best late May · 24 checklists that week, 437 in all88%
  3. Stanton Soccer FieldSanta Clara County, best late May · 22 checklists that week, 630 in all85%

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

Best time of day

In California, the Brown-headed Cowbird is most often listed on checklists started between 6 and 8 am (10% of those checklists).

0%5%10%15%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: 13% of 186053 checklists7 am: 9.5% of 444086 checklists8 am: 7.7% of 535846 checklists9 am: 6.0% of 486061 checklists10 am: 4.8% of 411130 checklists11 am: 3.9% of 325798 checklists12 pm: 3.7% of 269132 checklists1 pm: 3.4% of 244762 checklists2 pm: 3.3% of 235420 checklists3 pm: 3.3% of 232365 checklists4 pm: 3.3% of 208688 checklists5 pm: 4.3% of 146104 checklists6 pm: 4.9% 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
January1.8%1.6%1.6%1.5%
February1.4%1.4%1.4%1.6%
March1.7%2.2%2.9%4.4%
April6.8%9.7%13%15%
May17%16%17%17%
June17%15%15%14%
July13%12%9.1%5.6%
August3.9%3.6%3.3%3.1%
September3.1%2.8%2.6%2.3%
October2.4%2.4%2.5%2.3%
November2.0%1.8%1.7%1.5%
December1.5%1.6%1.6%1.5%

Neighboring states

  • Nevada: arrives late March, 5 weeks later than California
  • Utah: arrives mid April, 8 weeks later than California
  • Arizona: arrives the second week of March, 3 weeks later than California
  • Oregon: arrives early April, 6 weeks later than California

Common questions

When do Brown-headed Cowbirds arrive in California?
Brown-headed Cowbirds reach California in mid February, are common by the second week of April, peak in late May (17% of checklists), and most are gone by mid July.
When do Brown-headed Cowbirds leave California?
Most Brown-headed Cowbirds have left California by mid July, when reports fall below half their usual rate. Stragglers are still reported until early February.
When is the best time to see a Brown-headed Cowbird in California?
The Brown-headed Cowbird is reported most often in late May, on about 17% of California checklists.
Where in California are Brown-headed Cowbirds seen most often?
By county, Kings County, Yolo County, and Nevada County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).