WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in Colorado?

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

Brown-headed Cowbirds reach Colorado in the second week of April, are common by late April, peak in mid May (25% of checklists), and most are gone by the second week of July.

When to look

First arrivalsApr 8-14the second week of April
Common byApr 22-30late April
PeakMay 15-21mid May
Most gone byJul 8-14the second week of July

Best: mid May. First arrivals the second week of April; most are gone by the second week of July. Peak: on about 25% of Colorado checklists.

  1. Jannot usually seen
  2. Febnot usually seen
  3. Marnot usually seen
  4. Aprcommon
  5. Maypeak season
  6. Junpeak season
  7. Julcommon
  8. Augrare
  9. Seprare
  10. Octnot usually seen
  11. Novnot usually seen
  12. Decnot usually seen

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

0%10%20%30%Jan 1-7: 0.1% of 32254 checklistsJan 8-14: 0.1% of 23278 checklistsJan 15-21: 0.1% of 22933 checklistsJan 22-31: 0.1% of 30856 checklistsFeb 1-7: 0.1% of 21464 checklistsFeb 8-14: 0.1% of 22910 checklistsFeb 15-21: 0.1% of 28739 checklistsFeb 22-28: 0.1% of 21833 checklistsMar 1-7: 0.1% of 23066 checklistsMar 8-14: 0.1% of 23783 checklistsMar 15-21: 0.2% of 24797 checklistsMar 22-31: 0.3% of 38439 checklistsApr 1-7: 0.7% of 31676 checklistsApr 8-14: 1.7% of 35092 checklistsApr 15-21: 3.5% of 36778 checklistsApr 22-30: 11% of 50885 checklistsMay 1-7: 21% of 44186 checklistsMay 8-14: 25% of 49011 checklistsMay 15-21: 27% of 40784 checklistsMay 22-31: 25% of 53073 checklistsJun 1-7: 21% of 32432 checklistsJun 8-14: 20% of 29978 checklistsJun 15-21: 17% of 29457 checklistsJun 22-30: 16% of 36072 checklistsJul 1-7: 16% of 28857 checklistsJul 8-14: 13% of 27224 checklistsJul 15-21: 9.9% of 27629 checklistsJul 22-31: 4.8% of 37318 checklistsAug 1-7: 2.9% of 25893 checklistsAug 8-14: 2.6% of 24870 checklistsAug 15-21: 2.3% of 24035 checklistsAug 22-31: 1.6% of 35691 checklistsSep 1-7: 1.0% of 27068 checklistsSep 8-14: 0.9% of 26709 checklistsSep 15-21: 0.7% of 26098 checklistsSep 22-30: 0.5% of 31550 checklistsOct 1-7: 0.4% of 24110 checklistsOct 8-14: 0.2% of 23352 checklistsOct 15-21: 0.2% of 21895 checklistsOct 22-31: 0.2% of 28319 checklistsNov 1-7: 0.1% of 21698 checklistsNov 8-14: 0.1% of 21061 checklistsNov 15-21: 0.1% of 19855 checklistsNov 22-30: 0.1% of 25798 checklistsDec 1-7: 0.1% of 18626 checklistsDec 8-14: 0.1% of 19004 checklistsDec 15-21: 0.1% of 23103 checklistsDec 22-31: 0.1% of 30802 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 Colorado 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 Colorado

Best counties

Share of checklists at the peak
  1. Phillips CountyBest the second week of May · 55 checklists that week, 1,052 in all48%
  2. Logan CountyBest early June · 177 checklists that week, 4,117 in all47%
  3. Sedgwick CountyBest early June · 38 checklists that week, 1,168 in all46%
  4. Washington CountyBest mid May · 96 checklists that week, 2,511 in all45%
  5. Yuma CountyBest the second week of May · 121 checklists that week, 3,844 in all44%
  6. Kit Carson CountyBest the second week of May · 111 checklists that week, 2,633 in all41%

Best spots

Share of checklists at the peak
  1. Fountain Creek Regional Park (south)El Paso County, best the second week of May · 28 checklists that week, 116 in all82%
  2. Northgate Open SpaceEl Paso County, best late May · 22 checklists that week, 403 in all81%
  3. Tamarack Ranch State Wildlife AreaLogan County, best late June · 25 checklists that week, 568 in all79%
  4. Lake PuebloPueblo County, best early June · 30 checklists that week, 1,695 in all77%

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

Best time of day

In Colorado, the Brown-headed Cowbird is most often listed on checklists started between 6 and 8 am (13% of those checklists).

0%5%10%15%20%12 am: too few checklists (512) to judge1 am: too few checklists (468) to judge2 am: too few checklists (513) to judge3 am: too few checklists (656) to judge4 am: too few checklists (1918) to judge5 am: too few checklists (16045) to judge6 am: 16% of 71991 checklists7 am: 11% of 159189 checklists8 am: 8.8% of 177206 checklists9 am: 7.0% of 165269 checklists10 am: 5.4% of 141029 checklists11 am: 4.5% of 115283 checklists12 pm: 3.8% of 96068 checklists1 pm: 3.4% of 89512 checklists2 pm: 3.0% of 86319 checklists3 pm: 3.1% of 82496 checklists4 pm: 3.5% of 71648 checklists5 pm: 4.6% of 52445 checklists6 pm: 5.6% of 40199 checklists7 pm: too few checklists (24246) to judge8 pm: too few checklists (7986) to judge9 pm: too few checklists (2004) to judge10 pm: too few checklists (888) to judge11 pm: too few checklists (450) 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
January0.1%0.1%0.1%0.1%
February0.1%0.1%0.1%0.1%
March0.1%0.1%0.2%0.3%
April0.7%1.7%3.5%11%
May21%25%27%25%
June21%20%17%16%
July16%13%9.9%4.8%
August2.9%2.6%2.3%1.6%
September1.0%0.9%0.7%0.5%
October0.4%0.2%0.2%0.2%
November0.1%0.1%0.1%0.1%
December0.1%0.1%0.1%0.1%

Neighboring states

  • Wyoming: arrives mid April, 1 week later than Colorado
  • New Mexico: arrives the second week of April, in the same week than Colorado
  • Utah: arrives mid April, 1 week later than Colorado
  • Nebraska: arrives mid March, 3 weeks earlier than Colorado

Common questions

When do Brown-headed Cowbirds arrive in Colorado?
Brown-headed Cowbirds reach Colorado in the second week of April, are common by late April, peak in mid May (25% of checklists), and most are gone by the second week of July.
When do Brown-headed Cowbirds leave Colorado?
Most Brown-headed Cowbirds have left Colorado by the second week of 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 Colorado?
The Brown-headed Cowbird is reported most often in mid May, on about 25% of Colorado checklists.
Where in Colorado are Brown-headed Cowbirds seen most often?
By county, Phillips County, Logan County, and Sedgwick County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).