WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in Wyoming?

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

Brown-headed Cowbirds reach Wyoming in mid April, are common by early May, peak in late May (21% of checklists), and most are gone by mid July.

When to look

First arrivalsApr 15-21mid April
Common byMay 1-7early May
PeakMay 22-31late May
Most gone byJul 15-21mid July

Best: late May. First arrivals mid April; most are gone by mid July. Peak: on about 21% of Wyoming checklists.

  1. Jannot usually seen
  2. Febnot usually seen
  3. Marnot usually seen
  4. Apruncommon
  5. Maypeak season
  6. Junpeak season
  7. Julcommon
  8. Auguncommon
  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% of 2543 checklistsJan 8-14: 0% of 2114 checklistsJan 15-21: 0% of 1984 checklistsJan 22-31: 0% of 2605 checklistsFeb 1-7: 0.2% of 1940 checklistsFeb 8-14: 0.1% of 1954 checklistsFeb 15-21: 0% of 2575 checklistsFeb 22-28: 0% of 1932 checklistsMar 1-7: 0.1% of 2486 checklistsMar 8-14: 0% of 2535 checklistsMar 15-21: 0% of 2726 checklistsMar 22-31: 0.1% of 4349 checklistsApr 1-7: 0.1% of 3300 checklistsApr 8-14: 0.1% of 3500 checklistsApr 15-21: 0.5% of 3701 checklistsApr 22-30: 3.8% of 5887 checklistsMay 1-7: 12% of 5341 checklistsMay 8-14: 19% of 6910 checklistsMay 15-21: 23% of 6884 checklistsMay 22-31: 21% of 10652 checklistsJun 1-7: 19% of 6807 checklistsJun 8-14: 17% of 6714 checklistsJun 15-21: 15% of 6708 checklistsJun 22-30: 15% of 8160 checklistsJul 1-7: 13% of 5755 checklistsJul 8-14: 12% of 5379 checklistsJul 15-21: 9.3% of 5299 checklistsJul 22-31: 4.8% of 6830 checklistsAug 1-7: 3.7% of 4581 checklistsAug 8-14: 3.1% of 4625 checklistsAug 15-21: 2.6% of 4466 checklistsAug 22-31: 2.5% of 6562 checklistsSep 1-7: 1.4% of 4543 checklistsSep 8-14: 1.3% of 4652 checklistsSep 15-21: 0.8% of 4186 checklistsSep 22-30: 0.5% of 5301 checklistsOct 1-7: 0.2% of 3248 checklistsOct 8-14: 0.2% of 3010 checklistsOct 15-21: 0.1% of 2624 checklistsOct 22-31: 0.1% of 3170 checklistsNov 1-7: 0% of 2110 checklistsNov 8-14: 0% of 2153 checklistsNov 15-21: 0% of 1971 checklistsNov 22-30: 0% of 2136 checklistsDec 1-7: 0% of 1646 checklistsDec 8-14: 0.1% of 1609 checklistsDec 15-21: 0% of 1954 checklistsDec 22-31: 0% of 2392 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 Wyoming 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 Wyoming

Best counties

Share of checklists at the peak
  1. Campbell CountyBest mid May · 123 checklists that week, 2,801 in all40%
  2. Platte CountyBest mid May · 83 checklists that week, 1,645 in all40%
  3. Weston CountyBest late May · 250 checklists that week, 2,297 in all32%
  4. Niobrara CountyBest the second week of May · 20 checklists that week, 423 in all32%
  5. Goshen CountyBest late May · 174 checklists that week, 1,920 in all31%
  6. Crook CountyBest late May · 305 checklists that week, 3,915 in all31%

Best spots

Share of checklists at the peak
  1. Optomist ParkAlbany County, best late May · 110 checklists that week, 2,101 in all77%
  2. Camplex ParkCampbell County, best mid May · 33 checklists that week, 659 in all73%
  3. Usbr (southeast)Lincoln County, best late May · 21 checklists that week, 115 in all68%
  4. Excelsior Geyser CraterTeton County, best late June · 21 checklists that week, 278 in all62%
  5. Grand Teton National Park (Jenny Lake)Teton County, best mid June · 29 checklists that week, 424 in all61%

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

Best time of day

In Wyoming, 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 (59) to judge1 am: too few checklists (76) to judge2 am: too few checklists (86) to judge3 am: too few checklists (79) to judge4 am: too few checklists (326) to judge5 am: too few checklists (2271) to judge6 am: 18% of 8808 checklists7 am: 11% of 18458 checklists8 am: 8.5% of 22143 checklists9 am: 6.9% of 21499 checklists10 am: 6.4% of 19425 checklists11 am: 5.6% of 16017 checklists12 pm: 4.7% of 13868 checklists1 pm: 4.4% of 12984 checklists2 pm: 4.0% of 12605 checklists3 pm: 4.1% of 11821 checklists4 pm: 4.5% of 10571 checklists5 pm: 5.4% of 8746 checklists6 pm: 6.4% of 6707 checklists7 pm: 6.3% of 4890 checklists8 pm: too few checklists (2323) to judge9 pm: too few checklists (554) to judge10 pm: too few checklists (134) to judge11 pm: too few checklists (59) 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%0%0%0%
February0.2%0.1%0%0%
March0.1%0%0%0.1%
April0.1%0.1%0.5%3.8%
May12%19%23%21%
June19%17%15%15%
July13%12%9.3%4.8%
August3.7%3.1%2.6%2.5%
September1.4%1.3%0.8%0.5%
October0.2%0.2%0.1%0.1%
November0%0%0%0%
December0%0.1%0%0%

Neighboring states

  • Colorado: arrives the second week of April, 1 week earlier than Wyoming
  • Utah: arrives mid April, in the same week than Wyoming
  • Montana: arrives late April, 1 week later than Wyoming
  • Idaho: arrives mid April, in the same week than Wyoming

Common questions

When do Brown-headed Cowbirds arrive in Wyoming?
Brown-headed Cowbirds reach Wyoming in mid April, are common by early May, peak in late May (21% of checklists), and most are gone by mid July.
When do Brown-headed Cowbirds leave Wyoming?
Most Brown-headed Cowbirds have left Wyoming 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 Wyoming?
The Brown-headed Cowbird is reported most often in late May, on about 21% of Wyoming checklists.
Where in Wyoming are Brown-headed Cowbirds seen most often?
By county, Campbell County, Platte County, and Weston County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).