WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in Wisconsin?

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

Brown-headed Cowbirds reach Wisconsin in the second week of March, are common by early April, peak in the second week of May (40% of checklists), and most are gone by the second week of July.

When to look

First arrivalsMar 8-14the second week of March
Common byApr 1-7early April
PeakMay 8-14the second week of May
Most gone byJul 8-14the second week of July

Best: the second week of May. First arrivals the second week of March; most are gone by the second week of July. Peak: on about 40% of Wisconsin checklists.

  1. Jannot usually seen
  2. Febnot usually seen
  3. Maruncommon
  4. Aprpeak season
  5. Maypeak season
  6. Junpeak season
  7. Julcommon
  8. Augrare
  9. Sepnot usually seen
  10. Octnot usually seen
  11. Novnot usually seen
  12. Decnot usually seen

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

0%20%40%60%Jan 1-7: 0.2% of 27925 checklistsJan 8-14: 0.2% of 18337 checklistsJan 15-21: 0.2% of 17370 checklistsJan 22-31: 0.3% of 23777 checklistsFeb 1-7: 0.3% of 16656 checklistsFeb 8-14: 0.3% of 18286 checklistsFeb 15-21: 0.4% of 24739 checklistsFeb 22-28: 0.9% of 20113 checklistsMar 1-7: 1.9% of 24024 checklistsMar 8-14: 3.2% of 26722 checklistsMar 15-21: 4.9% of 27238 checklistsMar 22-31: 8.3% of 40171 checklistsApr 1-7: 15% of 34961 checklistsApr 8-14: 25% of 39128 checklistsApr 15-21: 30% of 38887 checklistsApr 22-30: 36% of 57231 checklistsMay 1-7: 42% of 55115 checklistsMay 8-14: 41% of 63098 checklistsMay 15-21: 37% of 49821 checklistsMay 22-31: 34% of 56796 checklistsJun 1-7: 33% of 33621 checklistsJun 8-14: 32% of 31282 checklistsJun 15-21: 29% of 30186 checklistsJun 22-30: 27% of 35128 checklistsJul 1-7: 25% of 27088 checklistsJul 8-14: 20% of 23293 checklistsJul 15-21: 14% of 20933 checklistsJul 22-31: 7.3% of 27595 checklistsAug 1-7: 4.5% of 19147 checklistsAug 8-14: 3.5% of 18286 checklistsAug 15-21: 2.7% of 19195 checklistsAug 22-31: 1.9% of 28229 checklistsSep 1-7: 1.0% of 22408 checklistsSep 8-14: 0.8% of 20857 checklistsSep 15-21: 0.7% of 20707 checklistsSep 22-30: 0.9% of 26469 checklistsOct 1-7: 1.3% of 21069 checklistsOct 8-14: 1.4% of 21011 checklistsOct 15-21: 1.1% of 20006 checklistsOct 22-31: 0.8% of 26362 checklistsNov 1-7: 0.6% of 18928 checklistsNov 8-14: 0.3% of 17052 checklistsNov 15-21: 0.3% of 16291 checklistsNov 22-30: 0.1% of 20564 checklistsDec 1-7: 0.2% of 15354 checklistsDec 8-14: 0.3% of 16294 checklistsDec 15-21: 0.3% of 21430 checklistsDec 22-31: 0.2% of 26314 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 Wisconsin 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 Wisconsin

Best counties

Share of checklists at the peak
  1. Lafayette CountyBest early June · 73 checklists that week, 1,911 in all58%
  2. Washburn CountyBest early May · 274 checklists that week, 7,099 in all57%
  3. Taylor CountyBest the second week of May · 104 checklists that week, 4,326 in all56%
  4. Vernon CountyBest the second week of June · 429 checklists that week, 13,255 in all53%
  5. Dunn CountyBest early May · 337 checklists that week, 8,247 in all52%
  6. St. Croix CountyBest the second week of May · 590 checklists that week, 12,346 in all52%

Best spots

Share of checklists at the peak
  1. Lake Park (3)Milwaukee County, best late April · 22 checklists that week, 201 in all96%
  2. Brooklyn Wildlife AreaDane County, best late May · 21 checklists that week, 213 in all92%
  3. Hales Corners ParkMilwaukee County, best late April · 21 checklists that week, 242 in all87%
  4. Bender ParkMilwaukee County, best the second week of June · 23 checklists that week, 1,388 in all87%
  5. Chiwaukee Prairie-Carol Beach Natural AreaKenosha County, best early May · 46 checklists that week, 504 in all86%

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

Best time of day

In Wisconsin, the Brown-headed Cowbird is most often listed on checklists started between 5 and 7 am (32% of those checklists).

0%10%20%30%40%12 am: too few checklists (993) to judge1 am: too few checklists (844) to judge2 am: too few checklists (839) to judge3 am: too few checklists (1488) to judge4 am: too few checklists (5344) to judge5 am: 37% of 27016 checklists6 am: 31% of 83973 checklists7 am: 21% of 160717 checklists8 am: 18% of 160531 checklists9 am: 15% of 136968 checklists10 am: 13% of 119110 checklists11 am: 11% of 99116 checklists12 pm: 10% of 85078 checklists1 pm: 9.4% of 80688 checklists2 pm: 9.0% of 79140 checklists3 pm: 9.1% of 78599 checklists4 pm: 11% of 69767 checklists5 pm: 13% of 53229 checklists6 pm: 14% of 38769 checklists7 pm: too few checklists (24396) to judge8 pm: too few checklists (10155) to judge9 pm: too few checklists (4990) to judge10 pm: too few checklists (2463) to judge11 pm: too few checklists (1281) 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
January0.2%0.2%0.2%0.3%
February0.3%0.3%0.4%0.9%
March1.9%3.2%4.9%8.3%
April15%25%30%36%
May42%41%37%34%
June33%32%29%27%
July25%20%14%7.3%
August4.5%3.5%2.7%1.9%
September1.0%0.8%0.7%0.9%
October1.3%1.4%1.1%0.8%
November0.6%0.3%0.3%0.1%
December0.2%0.3%0.3%0.2%

Neighboring states

  • Illinois: arrives mid February, 3 weeks earlier than Wisconsin
  • Iowa: arrives the second week of March, in the same week than Wisconsin
  • Michigan: arrives early March, 1 week earlier than Wisconsin
  • Minnesota: arrives early April, 3 weeks later than Wisconsin

Common questions

When do Brown-headed Cowbirds arrive in Wisconsin?
Brown-headed Cowbirds reach Wisconsin in the second week of March, are common by early April, peak in the second week of May (40% of checklists), and most are gone by the second week of July.
When do Brown-headed Cowbirds leave Wisconsin?
Most Brown-headed Cowbirds have left Wisconsin by the second week of July, when reports fall below half their usual rate. Stragglers are still reported until the second week of August.
When is the best time to see a Brown-headed Cowbird in Wisconsin?
The Brown-headed Cowbird is reported most often in the second week of May, on about 40% of Wisconsin checklists.
Where in Wisconsin are Brown-headed Cowbirds seen most often?
By county, Lafayette County, Washburn County, and Taylor County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).