WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in Arizona?

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

Brown-headed Cowbirds reach Arizona in the second week of March, are common by late April, peak in late May (25% of checklists), and most are gone by early August.

When to look

First arrivalsMar 8-14the second week of March
Common byApr 22-30late April
PeakMay 22-31late May
Most gone byAug 1-7early August

Best: late May. First arrivals the second week of March; most are gone by early August. Peak: on about 25% of Arizona checklists.

  1. Janrare
  2. Febrare
  3. Maruncommon
  4. Aprcommon
  5. Maypeak season
  6. Junpeak season
  7. Julpeak season
  8. Augcommon
  9. Sepuncommon
  10. Octuncommon
  11. Novrare
  12. Decrare

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

0%10%20%30%Jan 1-7: 1.8% of 34758 checklistsJan 8-14: 1.8% of 29835 checklistsJan 15-21: 1.7% of 29279 checklistsJan 22-31: 1.7% of 39716 checklistsFeb 1-7: 1.8% of 27299 checklistsFeb 8-14: 1.7% of 28703 checklistsFeb 15-21: 1.8% of 35357 checklistsFeb 22-28: 1.9% of 29889 checklistsMar 1-7: 2.3% of 29932 checklistsMar 8-14: 2.5% of 32982 checklistsMar 15-21: 3.0% of 34731 checklistsMar 22-31: 4.0% of 50213 checklistsApr 1-7: 5.6% of 35788 checklistsApr 8-14: 7.1% of 38087 checklistsApr 15-21: 10% of 41520 checklistsApr 22-30: 16% of 55218 checklistsMay 1-7: 23% of 39688 checklistsMay 8-14: 25% of 39398 checklistsMay 15-21: 26% of 29562 checklistsMay 22-31: 25% of 36774 checklistsJun 1-7: 25% of 21313 checklistsJun 8-14: 23% of 19129 checklistsJun 15-21: 24% of 17922 checklistsJun 22-30: 25% of 21188 checklistsJul 1-7: 25% of 17325 checklistsJul 8-14: 24% of 16463 checklistsJul 15-21: 23% of 18642 checklistsJul 22-31: 19% of 31215 checklistsAug 1-7: 13% of 28163 checklistsAug 8-14: 9.2% of 31329 checklistsAug 15-21: 7.1% of 23878 checklistsAug 22-31: 6.1% of 28508 checklistsSep 1-7: 5.5% of 20878 checklistsSep 8-14: 4.6% of 19671 checklistsSep 15-21: 4.0% of 19859 checklistsSep 22-30: 3.8% of 25179 checklistsOct 1-7: 4.1% of 19420 checklistsOct 8-14: 4.1% of 21925 checklistsOct 15-21: 3.9% of 20995 checklistsOct 22-31: 3.4% of 29258 checklistsNov 1-7: 2.8% of 21269 checklistsNov 8-14: 2.5% of 22753 checklistsNov 15-21: 2.3% of 21982 checklistsNov 22-30: 2.0% of 29475 checklistsDec 1-7: 1.9% of 20810 checklistsDec 8-14: 1.9% of 22229 checklistsDec 15-21: 1.8% of 25148 checklistsDec 22-31: 1.6% of 39567 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 Arizona 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 Arizona

Best counties

Share of checklists at the peak
  1. Santa Cruz CountyBest early July · 1896 checklists that week, 162,726 in all43%
  2. La Paz CountyBest mid May · 124 checklists that week, 7,279 in all36%
  3. Maricopa CountyBest the second week of May · 5935 checklists that week, 269,896 in all35%
  4. Yavapai CountyBest the second week of July · 1053 checklists that week, 83,606 in all33%
  5. Yuma CountyBest the second week of July · 124 checklists that week, 14,687 in all33%
  6. Mohave CountyBest the second week of May · 593 checklists that week, 18,083 in all28%

Best spots

Share of checklists at the peak
  1. Silver CreekCochise County, best the second week of May · 214 checklists that week, 3,612 in all86%
  2. ParadiseCochise County, best mid June · 44 checklists that week, 2,350 in all84%
  3. Moon Valley ParkMaricopa County, best late April · 102 checklists that week, 1,481 in all84%
  4. Initial PointMaricopa County, best mid June · 34 checklists that week, 1,660 in all82%
  5. Monterey ParkMaricopa County, best early April · 23 checklists that week, 175 in all81%

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

Best time of day

In Arizona, the Brown-headed Cowbird is most often listed on checklists started between 5 and 7 am (25% of those checklists).

0%10%20%30%40%12 am: too few checklists (488) to judge1 am: too few checklists (503) to judge2 am: too few checklists (489) to judge3 am: too few checklists (735) to judge4 am: too few checklists (4115) to judge5 am: 33% of 38350 checklists6 am: 23% of 118786 checklists7 am: 13% of 183415 checklists8 am: 8.3% of 187346 checklists9 am: 6.3% of 151342 checklists10 am: 5.1% of 124522 checklists11 am: 4.5% of 97088 checklists12 pm: 4.2% of 78110 checklists1 pm: 4.2% of 73059 checklists2 pm: 4.3% of 68912 checklists3 pm: 4.6% of 68060 checklists4 pm: 5.0% of 69387 checklists5 pm: 6.2% of 52951 checklists6 pm: 6.7% of 31061 checklists7 pm: too few checklists (16076) to judge8 pm: too few checklists (5352) to judge9 pm: too few checklists (2369) to judge10 pm: too few checklists (1100) to judge11 pm: too few checklists (604) 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%1.8%1.7%1.7%
February1.8%1.7%1.8%1.9%
March2.3%2.5%3.0%4.0%
April5.6%7.1%10%16%
May23%25%26%25%
June25%23%24%25%
July25%24%23%19%
August13%9.2%7.1%6.1%
September5.5%4.6%4.0%3.8%
October4.1%4.1%3.9%3.4%
November2.8%2.5%2.3%2.0%
December1.9%1.9%1.8%1.6%

Neighboring states

  • New Mexico: arrives the second week of April, 4 weeks later than Arizona
  • Nevada: arrives late March, 2 weeks later than Arizona
  • Utah: arrives mid April, 5 weeks later than Arizona
  • California: arrives mid February, 3 weeks earlier than Arizona

Common questions

When do Brown-headed Cowbirds arrive in Arizona?
Brown-headed Cowbirds reach Arizona in the second week of March, are common by late April, peak in late May (25% of checklists), and most are gone by early August.
When do Brown-headed Cowbirds leave Arizona?
Most Brown-headed Cowbirds have left Arizona by early August, when reports fall below half their usual rate. Stragglers are still reported until the second week of November.
When is the best time to see a Brown-headed Cowbird in Arizona?
The Brown-headed Cowbird is reported most often in late May, on about 25% of Arizona checklists.
Where in Arizona are Brown-headed Cowbirds seen most often?
By county, Santa Cruz County, La Paz County, and Maricopa County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).