WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in Illinois?

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

Brown-headed Cowbirds reach Illinois in mid February, are common by late March, peak in early May (45% of checklists), and most are gone by mid July.

When to look

First arrivalsFeb 15-21mid February
Common byMar 22-31late March
PeakMay 1-7early May
Most gone byJul 15-21mid July

Best: early May. First arrivals mid February; most are gone by mid July. Peak: on about 45% of Illinois checklists.

  1. Janrare
  2. Febrare
  3. Marcommon
  4. Aprpeak season
  5. Maypeak season
  6. Junpeak season
  7. Julcommon
  8. Auguncommon
  9. Seprare
  10. Octrare
  11. Novrare
  12. Decrare

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

0%20%40%60%Jan 1-7: 1.9% of 29700 checklistsJan 8-14: 2.7% of 20189 checklistsJan 15-21: 2.7% of 19784 checklistsJan 22-31: 2.7% of 26641 checklistsFeb 1-7: 2.8% of 20478 checklistsFeb 8-14: 3.7% of 21967 checklistsFeb 15-21: 3.8% of 28735 checklistsFeb 22-28: 4.2% of 24223 checklistsMar 1-7: 6.1% of 26775 checklistsMar 8-14: 9.4% of 27579 checklistsMar 15-21: 14% of 27193 checklistsMar 22-31: 21% of 42278 checklistsApr 1-7: 29% of 34579 checklistsApr 8-14: 38% of 38290 checklistsApr 15-21: 42% of 39261 checklistsApr 22-30: 45% of 59584 checklistsMay 1-7: 45% of 60485 checklistsMay 8-14: 45% of 59647 checklistsMay 15-21: 44% of 43173 checklistsMay 22-31: 43% of 47913 checklistsJun 1-7: 37% of 31151 checklistsJun 8-14: 36% of 27692 checklistsJun 15-21: 37% of 23983 checklistsJun 22-30: 34% of 28448 checklistsJul 1-7: 36% of 21273 checklistsJul 8-14: 30% of 19284 checklistsJul 15-21: 23% of 17712 checklistsJul 22-31: 14% of 24500 checklistsAug 1-7: 8.8% of 18498 checklistsAug 8-14: 7.6% of 19331 checklistsAug 15-21: 6.8% of 19983 checklistsAug 22-31: 5.1% of 31892 checklistsSep 1-7: 3.4% of 26396 checklistsSep 8-14: 2.5% of 25011 checklistsSep 15-21: 2.4% of 23597 checklistsSep 22-30: 2.9% of 30638 checklistsOct 1-7: 4.4% of 23949 checklistsOct 8-14: 5.3% of 24412 checklistsOct 15-21: 4.3% of 22359 checklistsOct 22-31: 3.3% of 29794 checklistsNov 1-7: 2.6% of 21877 checklistsNov 8-14: 2.2% of 19742 checklistsNov 15-21: 1.6% of 18989 checklistsNov 22-30: 1.3% of 23905 checklistsDec 1-7: 1.6% of 16935 checklistsDec 8-14: 1.3% of 16687 checklistsDec 15-21: 1.5% of 20931 checklistsDec 22-31: 2.0% of 29011 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 Illinois 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 Illinois

Best counties

Share of checklists at the peak
  1. Edgar CountyBest early May · 31 checklists that week, 914 in all69%
  2. St. Clair CountyBest the second week of June · 226 checklists that week, 10,340 in all68%
  3. Effingham CountyBest late April · 42 checklists that week, 1,014 in all68%
  4. Monroe CountyBest mid April · 78 checklists that week, 2,547 in all67%
  5. Schuyler CountyBest late May · 14 checklists that week, 617 in all67%
  6. Whiteside CountyBest early June · 82 checklists that week, 4,111 in all66%

Best spots

Share of checklists at the peak
  1. Deer Lake (Lincolnshire)Will County, best the second week of May · 21 checklists that week, 474 in all94%
  2. Winston VillageWill County, best early May · 20 checklists that week, 556 in all92%
  3. Tawny Oaks Field StationPeoria County, best mid April · 28 checklists that week, 990 in all90%
  4. Waterfall Glen (Canyon Creek Cascade)DuPage County, best early June · 27 checklists that week, 539 in all90%
  5. Salt Creek ParkDuPage County, best mid May · 21 checklists that week, 565 in all89%

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

Best time of day

In Illinois, the Brown-headed Cowbird is most often listed on checklists started between 6 and 8 am (30% of those checklists).

0%10%20%30%40%12 am: too few checklists (484) to judge1 am: too few checklists (631) to judge2 am: too few checklists (485) to judge3 am: too few checklists (610) to judge4 am: too few checklists (2321) to judge5 am: too few checklists (23536) to judge6 am: 33% of 95511 checklists7 am: 28% of 164584 checklists8 am: 23% of 163654 checklists9 am: 20% of 141804 checklists10 am: 17% of 122882 checklists11 am: 15% of 100640 checklists12 pm: 14% of 89894 checklists1 pm: 13% of 82729 checklists2 pm: 12% of 79801 checklists3 pm: 12% of 82169 checklists4 pm: 13% of 75124 checklists5 pm: 16% of 57708 checklists6 pm: 16% of 41843 checklists7 pm: too few checklists (20943) to judge8 pm: too few checklists (6131) to judge9 pm: too few checklists (1677) to judge10 pm: too few checklists (843) to judge11 pm: too few checklists (448) 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
January1.9%2.7%2.7%2.7%
February2.8%3.7%3.8%4.2%
March6.1%9.4%14%21%
April29%38%42%45%
May45%45%44%43%
June37%36%37%34%
July36%30%23%14%
August8.8%7.6%6.8%5.1%
September3.4%2.5%2.4%2.9%
October4.4%5.3%4.3%3.3%
November2.6%2.2%1.6%1.3%
December1.6%1.3%1.5%2.0%

Neighboring states

  • Indiana: arrives mid February, in the same week than Illinois
  • Wisconsin: arrives the second week of March, 3 weeks later than Illinois
  • Michigan: arrives early March, 2 weeks later than Illinois
  • Iowa: arrives the second week of March, 3 weeks later than Illinois

Common questions

When do Brown-headed Cowbirds arrive in Illinois?
Brown-headed Cowbirds reach Illinois in mid February, are common by late March, peak in early May (45% of checklists), and most are gone by mid July.
When do Brown-headed Cowbirds leave Illinois?
Most Brown-headed Cowbirds have left Illinois by mid July, when reports fall below half their usual rate. Stragglers are still reported until early September.
When is the best time to see a Brown-headed Cowbird in Illinois?
The Brown-headed Cowbird is reported most often in early May, on about 45% of Illinois checklists.
Where in Illinois are Brown-headed Cowbirds seen most often?
By county, Edgar County, St. Clair County, and Effingham County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).