WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in New Jersey?

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

Brown-headed Cowbirds are in New Jersey all year, but most are here in spring: numbers build from late March, peak in late April (35% of checklists), and fall back by early July. In the quietest week (mid September) they are still on about 3.4% of checklists.

When to look

Numbers buildMar 22-31late March
PeakApr 22-30late April
Fall back byJul 1-7early July
FewestSep 15-21mid September

Best: late April. Here all year; numbers build from late March and fall back by early July. Peak: on about 35% of New Jersey checklists.

  1. Janrare
  2. Febuncommon
  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%10%20%30%40%Jan 1-7: 4.1% of 28167 checklistsJan 8-14: 4.1% of 20583 checklistsJan 15-21: 4.5% of 19739 checklistsJan 22-31: 4.5% of 24586 checklistsFeb 1-7: 4.4% of 17226 checklistsFeb 8-14: 5.4% of 19567 checklistsFeb 15-21: 5.6% of 26553 checklistsFeb 22-28: 6.6% of 19046 checklistsMar 1-7: 9.0% of 17969 checklistsMar 8-14: 12% of 18976 checklistsMar 15-21: 15% of 18581 checklistsMar 22-31: 21% of 27590 checklistsApr 1-7: 26% of 21202 checklistsApr 8-14: 31% of 23609 checklistsApr 15-21: 34% of 25056 checklistsApr 22-30: 36% of 36679 checklistsMay 1-7: 34% of 38378 checklistsMay 8-14: 29% of 44071 checklistsMay 15-21: 30% of 34076 checklistsMay 22-31: 28% of 32558 checklistsJun 1-7: 27% of 18049 checklistsJun 8-14: 26% of 15597 checklistsJun 15-21: 24% of 14894 checklistsJun 22-30: 23% of 17187 checklistsJul 1-7: 21% of 13406 checklistsJul 8-14: 18% of 12550 checklistsJul 15-21: 12% of 12358 checklistsJul 22-31: 8.8% of 18382 checklistsAug 1-7: 8.0% of 13689 checklistsAug 8-14: 7.2% of 14006 checklistsAug 15-21: 6.7% of 14483 checklistsAug 22-31: 6.2% of 23182 checklistsSep 1-7: 5.0% of 17984 checklistsSep 8-14: 3.5% of 18320 checklistsSep 15-21: 3.2% of 20491 checklistsSep 22-30: 3.6% of 26176 checklistsOct 1-7: 4.6% of 24112 checklistsOct 8-14: 4.5% of 24603 checklistsOct 15-21: 4.8% of 24549 checklistsOct 22-31: 5.3% of 26206 checklistsNov 1-7: 4.8% of 17469 checklistsNov 8-14: 4.1% of 15102 checklistsNov 15-21: 4.6% of 14958 checklistsNov 22-30: 3.7% of 19447 checklistsDec 1-7: 3.7% of 14017 checklistsDec 8-14: 3.8% of 14625 checklistsDec 15-21: 4.5% of 16937 checklistsDec 22-31: 3.0% of 26651 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 New Jersey 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 New Jersey

Best counties

Share of checklists at the peak
  1. Burlington CountyBest the second week of May · 1835 checklists that week, 52,358 in all48%
  2. Hunterdon CountyBest mid April · 1392 checklists that week, 57,610 in all48%
  3. Somerset CountyBest late April · 2345 checklists that week, 58,057 in all45%
  4. Mercer CountyBest early May · 1958 checklists that week, 56,614 in all44%
  5. Warren CountyBest late April · 566 checklists that week, 20,077 in all42%
  6. Passaic CountyBest early May · 1398 checklists that week, 24,933 in all42%

Best spots

Share of checklists at the peak
  1. West Essex ParkEssex County, best mid April · 22 checklists that week, 1,556 in all80%
  2. Hoffman ParkHunterdon County, best early May · 22 checklists that week, 512 in all80%
  3. Baldpate MountainMercer County, best mid April · 30 checklists that week, 1,816 in all79%
  4. Palmyra Cove Nature ParkBurlington County, best early May · 268 checklists that week, 6,056 in all78%
  5. Fairview Farm Wildlife PreserveSomerset County, best early June · 36 checklists that week, 1,451 in all78%

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

Best time of day

In New Jersey, the Brown-headed Cowbird is most often listed on checklists started between 6 and 8 am (25% of those checklists).

0%10%20%30%40%12 am: too few checklists (746) to judge1 am: too few checklists (640) to judge2 am: too few checklists (606) to judge3 am: too few checklists (755) to judge4 am: too few checklists (1681) to judge5 am: too few checklists (11258) to judge6 am: 31% of 50447 checklists7 am: 23% of 118312 checklists8 am: 18% of 123273 checklists9 am: 15% of 107783 checklists10 am: 12% of 97537 checklists11 am: 9.9% of 82427 checklists12 pm: 8.8% of 72581 checklists1 pm: 8.1% of 71478 checklists2 pm: 7.9% of 69994 checklists3 pm: 8.2% of 66822 checklists4 pm: 9.1% of 55950 checklists5 pm: 11% of 41360 checklists6 pm: 12% of 28122 checklists7 pm: too few checklists (14727) to judge8 pm: too few checklists (4257) to judge9 pm: too few checklists (1273) to judge10 pm: too few checklists (920) to judge11 pm: too few checklists (693) 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
January4.1%4.1%4.5%4.5%
February4.4%5.4%5.6%6.6%
March9.0%12%15%21%
April26%31%34%36%
May34%29%30%28%
June27%26%24%23%
July21%18%12%8.8%
August8.0%7.2%6.7%6.2%
September5.0%3.5%3.2%3.6%
October4.6%4.5%4.8%5.3%
November4.8%4.1%4.6%3.7%
December3.7%3.8%4.5%3.0%

Neighboring states

  • Delaware: arrives late March, in the same week than New Jersey
  • Connecticut: arrives mid January, 9 weeks earlier than New Jersey
  • Pennsylvania: arrives the second week of February, 6 weeks earlier than New Jersey
  • New York: arrives mid February, 5 weeks earlier than New Jersey

Common questions

When do Brown-headed Cowbirds arrive in New Jersey?
Brown-headed Cowbirds are in New Jersey all year, but most are here in spring: numbers build from late March, peak in late April (35% of checklists), and fall back by early July. In the quietest week (mid September) they are still on about 3.4% of checklists.
When do Brown-headed Cowbirds leave New Jersey?
Brown-headed Cowbirds never fully leave New Jersey: numbers fall back below half their usual rate by early July, but some are still reported in every week, about 3.4% of checklists at the low.
When is the best time to see a Brown-headed Cowbird in New Jersey?
The Brown-headed Cowbird is reported most often in late April, on about 35% of New Jersey checklists.
Where in New Jersey are Brown-headed Cowbirds seen most often?
By county, Burlington County, Hunterdon County, and Somerset County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).