WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in Pennsylvania?

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

Brown-headed Cowbirds reach Pennsylvania in the second week of February, are common by mid March, peak in late April (35% of checklists), and most are gone by the second week of July.

When to look

First arrivalsFeb 8-14the second week of February
Common byMar 15-21mid March
PeakApr 22-30late April
Most gone byJul 8-14the second week of July

Best: late April. First arrivals the second week of February; most are gone by the second week of July. Peak: on about 35% of Pennsylvania checklists.

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

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

0%10%20%30%40%Jan 1-7: 1.6% of 47145 checklistsJan 8-14: 2.0% of 35840 checklistsJan 15-21: 2.0% of 35423 checklistsJan 22-31: 2.0% of 49126 checklistsFeb 1-7: 2.2% of 36366 checklistsFeb 8-14: 3.0% of 42248 checklistsFeb 15-21: 3.4% of 55888 checklistsFeb 22-28: 4.2% of 42050 checklistsMar 1-7: 5.7% of 42709 checklistsMar 8-14: 9.0% of 43985 checklistsMar 15-21: 12% of 44547 checklistsMar 22-31: 18% of 66940 checklistsApr 1-7: 24% of 51147 checklistsApr 8-14: 31% of 54868 checklistsApr 15-21: 34% of 57252 checklistsApr 22-30: 36% of 81727 checklistsMay 1-7: 34% of 76472 checklistsMay 8-14: 31% of 79341 checklistsMay 15-21: 30% of 61724 checklistsMay 22-31: 27% of 70722 checklistsJun 1-7: 26% of 45652 checklistsJun 8-14: 23% of 41810 checklistsJun 15-21: 23% of 38973 checklistsJun 22-30: 21% of 44255 checklistsJul 1-7: 19% of 34822 checklistsJul 8-14: 16% of 31104 checklistsJul 15-21: 11% of 29262 checklistsJul 22-31: 6.5% of 41436 checklistsAug 1-7: 5.5% of 29189 checklistsAug 8-14: 4.9% of 28840 checklistsAug 15-21: 4.4% of 29948 checklistsAug 22-31: 3.7% of 48235 checklistsSep 1-7: 3.0% of 37175 checklistsSep 8-14: 2.5% of 36527 checklistsSep 15-21: 2.4% of 37261 checklistsSep 22-30: 2.9% of 43583 checklistsOct 1-7: 4.1% of 35352 checklistsOct 8-14: 4.1% of 35560 checklistsOct 15-21: 4.3% of 33119 checklistsOct 22-31: 3.3% of 44654 checklistsNov 1-7: 2.8% of 32965 checklistsNov 8-14: 2.0% of 31305 checklistsNov 15-21: 1.7% of 29471 checklistsNov 22-30: 1.2% of 38642 checklistsDec 1-7: 1.3% of 28779 checklistsDec 8-14: 1.3% of 29486 checklistsDec 15-21: 1.5% of 37465 checklistsDec 22-31: 1.4% of 50651 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 Pennsylvania 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 Pennsylvania

Best counties

Share of checklists at the peak
  1. Juniata CountyBest late April · 408 checklists that week, 9,807 in all49%
  2. Northampton CountyBest late April · 2007 checklists that week, 45,480 in all48%
  3. Chester CountyBest mid April · 3942 checklists that week, 144,121 in all47%
  4. Berks CountyBest late April · 2426 checklists that week, 61,798 in all46%
  5. Montour CountyBest early May · 285 checklists that week, 6,082 in all44%
  6. Bucks CountyBest late April · 3125 checklists that week, 95,030 in all43%

Best spots

Share of checklists at the peak
  1. Woodland LakeMonroe County, best early May · 20 checklists that week, 725 in all97%
  2. Kutztown Elementary SchoolBerks County, best the second week of April · 22 checklists that week, 301 in all91%
  3. Owls Nest ParkBucks County, best late April · 24 checklists that week, 708 in all90%
  4. Lower LakeMonroe County, best late April · 36 checklists that week, 1,218 in all90%
  5. Union Twp. Recreation AreaBerks County, best the second week of May · 30 checklists that week, 729 in all85%

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

Best time of day

In Pennsylvania, the Brown-headed Cowbird is most often listed on checklists started between 6 and 8 am (22% of those checklists).

0%10%20%30%12 am: too few checklists (1558) to judge1 am: too few checklists (1136) to judge2 am: too few checklists (839) to judge3 am: too few checklists (842) to judge4 am: too few checklists (2550) to judge5 am: too few checklists (18294) to judge6 am: 27% of 92903 checklists7 am: 20% of 241233 checklists8 am: 17% of 271994 checklists9 am: 14% of 236842 checklists10 am: 11% of 199508 checklists11 am: 9.7% of 161893 checklists12 pm: 9.0% of 143945 checklists1 pm: 8.4% of 134352 checklists2 pm: 7.9% of 129692 checklists3 pm: 8.1% of 126132 checklists4 pm: 9.1% of 115055 checklists5 pm: 12% of 90719 checklists6 pm: 13% of 67534 checklists7 pm: 10% of 43004 checklists8 pm: too few checklists (13966) to judge9 pm: too few checklists (3913) to judge10 pm: too few checklists (1989) to judge11 pm: too few checklists (1146) 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
January1.6%2.0%2.0%2.0%
February2.2%3.0%3.4%4.2%
March5.7%9.0%12%18%
April24%31%34%36%
May34%31%30%27%
June26%23%23%21%
July19%16%11%6.5%
August5.5%4.9%4.4%3.7%
September3.0%2.5%2.4%2.9%
October4.1%4.1%4.3%3.3%
November2.8%2.0%1.7%1.2%
December1.3%1.3%1.5%1.4%

Neighboring states

  • Maryland: arrives late March, 6 weeks later than Pennsylvania
  • District of Columbia: arrives late February, 2 weeks later than Pennsylvania
  • Delaware: arrives late March, 6 weeks later than Pennsylvania
  • New Jersey: arrives late March, 6 weeks later than Pennsylvania

Common questions

When do Brown-headed Cowbirds arrive in Pennsylvania?
Brown-headed Cowbirds reach Pennsylvania in the second week of February, are common by mid March, peak in late April (35% of checklists), and most are gone by the second week of July.
When do Brown-headed Cowbirds leave Pennsylvania?
Most Brown-headed Cowbirds have left Pennsylvania by the second week of July, when reports fall below half their usual rate. Stragglers are still reported until early November.
When is the best time to see a Brown-headed Cowbird in Pennsylvania?
The Brown-headed Cowbird is reported most often in late April, on about 35% of Pennsylvania checklists.
Where in Pennsylvania are Brown-headed Cowbirds seen most often?
By county, Juniata County, Northampton County, and Chester County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).