WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in Iowa?

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

Brown-headed Cowbirds reach Iowa in the second week of March, are common by early April, peak in the second week of May (46% of checklists), and most are gone by mid 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 15-21mid July

Best: the second week of May. First arrivals the second week of March; most are gone by mid July. Peak: on about 46% of Iowa checklists.

  1. Jannot usually seen
  2. Febrare
  3. Maruncommon
  4. Aprpeak season
  5. Maypeak season
  6. Junpeak season
  7. Julpeak season
  8. Auguncommon
  9. Seprare
  10. Octrare
  11. Novrare
  12. Decnot usually seen

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

0%20%40%60%Jan 1-7: 1.0% of 5551 checklistsJan 8-14: 1.2% of 3498 checklistsJan 15-21: 0.6% of 3803 checklistsJan 22-31: 0.9% of 4807 checklistsFeb 1-7: 0.6% of 3613 checklistsFeb 8-14: 1.2% of 4173 checklistsFeb 15-21: 1.1% of 5796 checklistsFeb 22-28: 1.4% of 4394 checklistsMar 1-7: 3.0% of 5295 checklistsMar 8-14: 4.7% of 5679 checklistsMar 15-21: 6.7% of 5418 checklistsMar 22-31: 12% of 8613 checklistsApr 1-7: 19% of 6970 checklistsApr 8-14: 30% of 7511 checklistsApr 15-21: 34% of 8321 checklistsApr 22-30: 43% of 11976 checklistsMay 1-7: 48% of 11725 checklistsMay 8-14: 46% of 12626 checklistsMay 15-21: 44% of 9372 checklistsMay 22-31: 43% of 10330 checklistsJun 1-7: 40% of 5472 checklistsJun 8-14: 41% of 4568 checklistsJun 15-21: 41% of 3870 checklistsJun 22-30: 38% of 4780 checklistsJul 1-7: 36% of 4201 checklistsJul 8-14: 32% of 3605 checklistsJul 15-21: 27% of 3621 checklistsJul 22-31: 16% of 5093 checklistsAug 1-7: 8.6% of 3931 checklistsAug 8-14: 6.7% of 3488 checklistsAug 15-21: 5.3% of 3802 checklistsAug 22-31: 3.5% of 6293 checklistsSep 1-7: 2.4% of 5328 checklistsSep 8-14: 1.8% of 4622 checklistsSep 15-21: 1.8% of 4367 checklistsSep 22-30: 2.5% of 6086 checklistsOct 1-7: 4.0% of 4708 checklistsOct 8-14: 4.8% of 4791 checklistsOct 15-21: 4.7% of 4429 checklistsOct 22-31: 3.2% of 5633 checklistsNov 1-7: 3.0% of 4090 checklistsNov 8-14: 1.5% of 3416 checklistsNov 15-21: 0.8% of 3398 checklistsNov 22-30: 1.0% of 4679 checklistsDec 1-7: 0.9% of 3442 checklistsDec 8-14: 0.9% of 3322 checklistsDec 15-21: 0.9% of 4619 checklistsDec 22-31: 0.8% of 5113 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 Iowa 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 Iowa

Best counties

Share of checklists at the peak
  1. Clarke CountyBest late April · 22 checklists that week, 459 in all71%
  2. Ringgold CountyBest the second week of May · 34 checklists that week, 1,141 in all68%
  3. Monroe CountyBest early June · 16 checklists that week, 257 in all63%
  4. Cass CountyBest early June · 12 checklists that week, 449 in all63%
  5. Louisa CountyBest the second week of May · 56 checklists that week, 1,639 in all62%
  6. Howard CountyBest early June · 22 checklists that week, 334 in all61%

Best spots

Share of checklists at the peak
  1. Riverview Recreation AreaBlack Hawk County, best late April · 23 checklists that week, 339 in all92%
  2. Saylorville Recreation Area (southeast)Polk County, best late June · 22 checklists that week, 867 in all91%
  3. Mines of Spain State Recreation AreaDubuque County, best the second week of May · 54 checklists that week, 754 in all88%
  4. Trestle to Trestle TrailPolk County, best late May · 20 checklists that week, 434 in all87%
  5. Walnut Woods State ParkPolk County, best the second week of May · 37 checklists that week, 643 in all86%
  6. Robison Wildlife AcresStory County, best the second week of May · 23 checklists that week, 203 in all83%

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

Best time of day

In Iowa, the Brown-headed Cowbird is most often listed on checklists started between 6 and 8 am (33% of those checklists).

0%20%40%60%12 am: too few checklists (96) to judge1 am: too few checklists (99) to judge2 am: too few checklists (67) to judge3 am: too few checklists (82) to judge4 am: too few checklists (201) to judge5 am: too few checklists (1888) to judge6 am: 44% of 11313 checklists7 am: 28% of 25717 checklists8 am: 23% of 29755 checklists9 am: 19% of 28897 checklists10 am: 17% of 26343 checklists11 am: 15% of 21210 checklists12 pm: 13% of 17662 checklists1 pm: 12% of 16595 checklists2 pm: 10% of 16405 checklists3 pm: 11% of 17030 checklists4 pm: 12% of 17378 checklists5 pm: 16% of 14251 checklists6 pm: 18% of 10349 checklists7 pm: 18% of 6233 checklists8 pm: too few checklists (2002) to judge9 pm: too few checklists (392) to judge10 pm: too few checklists (186) to judge11 pm: too few checklists (87) 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
January1.0%1.2%0.6%0.9%
February0.6%1.2%1.1%1.4%
March3.0%4.7%6.7%12%
April19%30%34%43%
May48%46%44%43%
June40%41%41%38%
July36%32%27%16%
August8.6%6.7%5.3%3.5%
September2.4%1.8%1.8%2.5%
October4.0%4.8%4.7%3.2%
November3.0%1.5%0.8%1.0%
December0.9%0.9%0.9%0.8%

Neighboring states

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

Common questions

When do Brown-headed Cowbirds arrive in Iowa?
Brown-headed Cowbirds reach Iowa in the second week of March, are common by early April, peak in the second week of May (46% of checklists), and most are gone by mid July.
When do Brown-headed Cowbirds leave Iowa?
Most Brown-headed Cowbirds have left Iowa by mid July, when reports fall below half their usual rate. Stragglers are still reported until mid August.
When is the best time to see a Brown-headed Cowbird in Iowa?
The Brown-headed Cowbird is reported most often in the second week of May, on about 46% of Iowa checklists.
Where in Iowa are Brown-headed Cowbirds seen most often?
By county, Clarke County, Ringgold County, and Monroe County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).