WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in Delaware?

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

Brown-headed Cowbirds are in Delaware 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 (early December) they are still on about 2.5% of checklists.

When to look

Numbers buildMar 22-31late March
PeakApr 22-30late April
Fall back byJul 1-7early July
FewestDec 1-7early December

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

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

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

0%10%20%30%40%Jan 1-7: 3.8% of 5901 checklistsJan 8-14: 3.4% of 4091 checklistsJan 15-21: 4.6% of 3841 checklistsJan 22-31: 3.9% of 5084 checklistsFeb 1-7: 5.0% of 3705 checklistsFeb 8-14: 6.7% of 3991 checklistsFeb 15-21: 6.7% of 6056 checklistsFeb 22-28: 6.7% of 4036 checklistsMar 1-7: 8.5% of 3822 checklistsMar 8-14: 10% of 3873 checklistsMar 15-21: 14% of 3856 checklistsMar 22-31: 21% of 5702 checklistsApr 1-7: 26% of 4104 checklistsApr 8-14: 29% of 4456 checklistsApr 15-21: 34% of 4871 checklistsApr 22-30: 35% of 7249 checklistsMay 1-7: 34% of 6806 checklistsMay 8-14: 31% of 7229 checklistsMay 15-21: 27% of 6466 checklistsMay 22-31: 26% of 7437 checklistsJun 1-7: 26% of 3596 checklistsJun 8-14: 25% of 2847 checklistsJun 15-21: 23% of 2901 checklistsJun 22-30: 22% of 3644 checklistsJul 1-7: 20% of 3142 checklistsJul 8-14: 17% of 2767 checklistsJul 15-21: 12% of 2882 checklistsJul 22-31: 10% of 4175 checklistsAug 1-7: 7.8% of 3482 checklistsAug 8-14: 7.6% of 3477 checklistsAug 15-21: 7.4% of 3386 checklistsAug 22-31: 6.0% of 5028 checklistsSep 1-7: 5.0% of 3631 checklistsSep 8-14: 4.1% of 3507 checklistsSep 15-21: 3.3% of 3404 checklistsSep 22-30: 3.7% of 4252 checklistsOct 1-7: 6.4% of 3563 checklistsOct 8-14: 5.5% of 3440 checklistsOct 15-21: 6.5% of 3282 checklistsOct 22-31: 6.0% of 4025 checklistsNov 1-7: 4.7% of 3030 checklistsNov 8-14: 3.7% of 3141 checklistsNov 15-21: 3.8% of 2858 checklistsNov 22-30: 2.1% of 4104 checklistsDec 1-7: 2.8% of 2472 checklistsDec 8-14: 3.0% of 2415 checklistsDec 15-21: 3.0% of 3306 checklistsDec 22-31: 2.3% of 5527 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 Delaware 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 Delaware

Best counties

Share of checklists at the peak
  1. New Castle CountyBest late April · 2922 checklists that week, 72,140 in all41%
  2. Sussex CountyBest late April · 3082 checklists that week, 91,027 in all32%
  3. Kent CountyBest late April · 1245 checklists that week, 36,693 in all24%

Best spots

Share of checklists at the peak
  1. Craigs PondSussex County, best the second week of April · 24 checklists that week, 378 in all82%
  2. White Clay Creek State Park (Lamborn Run)New Castle County, best mid April · 40 checklists that week, 444 in all81%
  3. Redden State ForestSussex County, best late April · 125 checklists that week, 822 in all81%
  4. Auburn HeightsNew Castle County, best late April · 29 checklists that week, 678 in all77%
  5. WoodlawnNew Castle County, best late April · 55 checklists that week, 1,011 in all76%
  6. Redden State Forest (Prong Number One)Sussex County, best mid April · 28 checklists that week, 260 in all75%

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

Best time of day

In Delaware, the Brown-headed Cowbird is most often listed on checklists started between 6 and 8 am (27% of those checklists).

0%10%20%30%40%12 am: too few checklists (129) to judge1 am: too few checklists (122) to judge2 am: too few checklists (102) to judge3 am: too few checklists (114) to judge4 am: too few checklists (210) to judge5 am: too few checklists (1334) to judge6 am: 30% of 8055 checklists7 am: 25% of 21367 checklists8 am: 20% of 25648 checklists9 am: 14% of 22669 checklists10 am: 11% of 20679 checklists11 am: 9.1% of 17516 checklists12 pm: 8.3% of 15361 checklists1 pm: 7.4% of 14319 checklists2 pm: 7.6% of 13726 checklists3 pm: 7.9% of 12793 checklists4 pm: 9.1% of 10392 checklists5 pm: 11% of 6859 checklists6 pm: 12% of 4542 checklists7 pm: too few checklists (2633) to judge8 pm: too few checklists (831) to judge9 pm: too few checklists (211) to judge10 pm: too few checklists (150) to judge11 pm: too few checklists (98) 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
January3.8%3.4%4.6%3.9%
February5.0%6.7%6.7%6.7%
March8.5%10%14%21%
April26%29%34%35%
May34%31%27%26%
June26%25%23%22%
July20%17%12%10%
August7.8%7.6%7.4%6.0%
September5.0%4.1%3.3%3.7%
October6.4%5.5%6.5%6.0%
November4.7%3.7%3.8%2.1%
December2.8%3.0%3.0%2.3%

Neighboring states

  • Maryland: arrives late March, in the same week than Delaware
  • New Jersey: arrives late March, in the same week than Delaware
  • District of Columbia: arrives late February, 4 weeks earlier than Delaware
  • Pennsylvania: arrives the second week of February, 6 weeks earlier than Delaware

Common questions

When do Brown-headed Cowbirds arrive in Delaware?
Brown-headed Cowbirds are in Delaware 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 (early December) they are still on about 2.5% of checklists.
When do Brown-headed Cowbirds leave Delaware?
Brown-headed Cowbirds never fully leave Delaware: numbers fall back below half their usual rate by early July, but some are still reported in every week, about 2.5% of checklists at the low.
When is the best time to see a Brown-headed Cowbird in Delaware?
The Brown-headed Cowbird is reported most often in late April, on about 35% of Delaware checklists.
Where in Delaware are Brown-headed Cowbirds seen most often?
By county, New Castle County, Sussex County, and Kent County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).