WhenToBird

Molothrus ater · Icteridae

When Do Brown-headed Cowbirds Arrive in Idaho?

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

Brown-headed Cowbirds reach Idaho in mid April, are common by early May, peak in late May (28% 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 28% of Idaho 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: 0.2% of 9357 checklistsJan 8-14: 0.1% of 6408 checklistsJan 15-21: 0.1% of 6358 checklistsJan 22-31: 0.2% of 8601 checklistsFeb 1-7: 0.1% of 5611 checklistsFeb 8-14: 0.1% of 6090 checklistsFeb 15-21: 0.1% of 7973 checklistsFeb 22-28: 0.1% of 5866 checklistsMar 1-7: 0.2% of 6144 checklistsMar 8-14: 0.1% of 6456 checklistsMar 15-21: 0.1% of 6786 checklistsMar 22-31: 0.3% of 9539 checklistsApr 1-7: 0.4% of 7202 checklistsApr 8-14: 1.0% of 7608 checklistsApr 15-21: 2.5% of 8069 checklistsApr 22-30: 6.8% of 10973 checklistsMay 1-7: 17% of 9481 checklistsMay 8-14: 24% of 11318 checklistsMay 15-21: 29% of 9141 checklistsMay 22-31: 28% of 13984 checklistsJun 1-7: 25% of 8897 checklistsJun 8-14: 24% of 8504 checklistsJun 15-21: 23% of 8078 checklistsJun 22-30: 20% of 9665 checklistsJul 1-7: 19% of 7144 checklistsJul 8-14: 16% of 6557 checklistsJul 15-21: 12% of 6430 checklistsJul 22-31: 5.7% of 8696 checklistsAug 1-7: 4.2% of 5896 checklistsAug 8-14: 3.8% of 5643 checklistsAug 15-21: 3.1% of 6151 checklistsAug 22-31: 2.3% of 7863 checklistsSep 1-7: 1.3% of 5769 checklistsSep 8-14: 0.8% of 5646 checklistsSep 15-21: 0.7% of 5460 checklistsSep 22-30: 0.5% of 6363 checklistsOct 1-7: 0.2% of 4980 checklistsOct 8-14: 0.2% of 5096 checklistsOct 15-21: 0.1% of 4878 checklistsOct 22-31: 0.2% of 6426 checklistsNov 1-7: 0.1% of 4822 checklistsNov 8-14: 0.2% of 4999 checklistsNov 15-21: 0.2% of 4601 checklistsNov 22-30: 0.1% of 6027 checklistsDec 1-7: 0.1% of 4180 checklistsDec 8-14: 0.1% of 4775 checklistsDec 15-21: 0.2% of 6522 checklistsDec 22-31: 0.3% of 7792 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 Idaho 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 Idaho

Best counties

Share of checklists at the peak
  1. Adams CountyBest late May · 213 checklists that week, 5,826 in all60%
  2. Nez Perce CountyBest the second week of June · 175 checklists that week, 10,708 in all57%
  3. Minidoka CountyBest the second week of May · 28 checklists that week, 1,365 in all46%
  4. Jefferson CountyBest late May · 542 checklists that week, 5,319 in all40%
  5. Shoshone CountyBest the second week of May · 79 checklists that week, 2,886 in all39%
  6. Power CountyBest mid June · 46 checklists that week, 2,278 in all36%

Best spots

Share of checklists at the peak
  1. Hulls Gulch Reserve (Highlands)Ada County, best late May · 21 checklists that week, 394 in all85%
  2. AvimorAda County, best mid May · 21 checklists that week, 219 in all80%
  3. Eagle Island State ParkAda County, best the second week of June · 33 checklists that week, 1,077 in all78%
  4. Greenbelt SeAda County, best late May · 20 checklists that week, 582 in all76%

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

Best time of day

In Idaho, the Brown-headed Cowbird is most often listed on checklists started between 6 and 8 am (18% of those checklists).

0%10%20%30%12 am: too few checklists (180) to judge1 am: too few checklists (176) to judge2 am: too few checklists (169) to judge3 am: too few checklists (164) to judge4 am: too few checklists (450) to judge5 am: too few checklists (2270) to judge6 am: 25% of 9122 checklists7 am: 15% of 22204 checklists8 am: 12% of 38672 checklists9 am: 8.4% of 38930 checklists10 am: 6.6% of 37338 checklists11 am: 5.4% of 33508 checklists12 pm: 4.3% of 28544 checklists1 pm: 3.6% of 25774 checklists2 pm: 3.4% of 24296 checklists3 pm: 3.2% of 21784 checklists4 pm: 3.8% of 19051 checklists5 pm: 5.1% of 14150 checklists6 pm: 6.3% of 10014 checklists7 pm: 6.8% of 7495 checklists8 pm: too few checklists (4352) to judge9 pm: too few checklists (1449) to judge10 pm: too few checklists (525) to judge11 pm: too few checklists (208) 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
January0.2%0.1%0.1%0.2%
February0.1%0.1%0.1%0.1%
March0.2%0.1%0.1%0.3%
April0.4%1.0%2.5%6.8%
May17%24%29%28%
June25%24%23%20%
July19%16%12%5.7%
August4.2%3.8%3.1%2.3%
September1.3%0.8%0.7%0.5%
October0.2%0.2%0.1%0.2%
November0.1%0.2%0.2%0.1%
December0.1%0.1%0.2%0.3%

Neighboring states

  • Montana: arrives late April, 1 week later than Idaho
  • Utah: arrives mid April, in the same week than Idaho
  • Oregon: arrives early April, 2 weeks earlier than Idaho
  • Washington: arrives the second week of April, 1 week earlier than Idaho

Common questions

When do Brown-headed Cowbirds arrive in Idaho?
Brown-headed Cowbirds reach Idaho in mid April, are common by early May, peak in late May (28% of checklists), and most are gone by mid July.
When do Brown-headed Cowbirds leave Idaho?
Most Brown-headed Cowbirds have left Idaho by mid July, when reports fall below half their usual rate. Stragglers are still reported until late August.
When is the best time to see a Brown-headed Cowbird in Idaho?
The Brown-headed Cowbird is reported most often in late May, on about 28% of Idaho checklists.
Where in Idaho are Brown-headed Cowbirds seen most often?
By county, Adams County, Nez Perce County, and Minidoka County have the highest peak reporting rates for the Brown-headed Cowbird (counties with at least 200 checklists).