WhenToBird

Sterna forsteri · Laridae

When Do Forster's Terns Arrive in Colorado?

Adult Forster's Tern
Photo: Aitor, CC BY 4.0 via iNaturalist

Forster's Terns arrive in Colorado in mid April, peak in early May (1.5% of checklists), and most are gone by early June.

When to look

First arrivalsApr 15-21mid April
Common byApr 15-21mid April
PeakMay 1-7early May
Most gone byJun 1-7early June

Best: early May. First arrivals mid April; most are gone by early June. Peak: on about 1.5% of Colorado checklists.

  1. Jannot usually seen
  2. Febnot usually seen
  3. Marnot usually seen
  4. Aprpeak season
  5. Maypeak season
  6. Jununcommon
  7. Juluncommon
  8. Augpeak season
  9. Seppeak season
  10. Octuncommon
  11. Novnot usually seen
  12. Decnot usually seen

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

0%0.5%1%1.5%2%Jan 1-7: 0% of 32254 checklistsJan 8-14: 0% of 23278 checklistsJan 15-21: 0% of 22933 checklistsJan 22-31: 0% of 30856 checklistsFeb 1-7: 0% of 21464 checklistsFeb 8-14: 0% of 22910 checklistsFeb 15-21: 0% of 28739 checklistsFeb 22-28: 0% of 21833 checklistsMar 1-7: 0% of 23066 checklistsMar 8-14: 0% of 23783 checklistsMar 15-21: 0% of 24797 checklistsMar 22-31: 0% of 38439 checklistsApr 1-7: 0.1% of 31676 checklistsApr 8-14: 0.1% of 35092 checklistsApr 15-21: 0.3% of 36778 checklistsApr 22-30: 1.1% of 50885 checklistsMay 1-7: 2.0% of 44186 checklistsMay 8-14: 1.4% of 49011 checklistsMay 15-21: 0.9% of 40784 checklistsMay 22-31: 0.6% of 53073 checklistsJun 1-7: 0.4% of 32432 checklistsJun 8-14: 0.3% of 29978 checklistsJun 15-21: 0.2% of 29457 checklistsJun 22-30: 0.2% of 36072 checklistsJul 1-7: 0.2% of 28857 checklistsJul 8-14: 0.2% of 27224 checklistsJul 15-21: 0.2% of 27629 checklistsJul 22-31: 0.5% of 37318 checklistsAug 1-7: 0.6% of 25893 checklistsAug 8-14: 0.9% of 24870 checklistsAug 15-21: 0.9% of 24035 checklistsAug 22-31: 1.4% of 35691 checklistsSep 1-7: 1.0% of 27068 checklistsSep 8-14: 1.1% of 26709 checklistsSep 15-21: 0.9% of 26098 checklistsSep 22-30: 0.6% 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: 0.1% of 28319 checklistsNov 1-7: 0% of 21698 checklistsNov 8-14: 0% of 21061 checklistsNov 15-21: 0% of 19855 checklistsNov 22-30: 0% of 25798 checklistsDec 1-7: 0% of 18626 checklistsDec 8-14: 0% of 19004 checklistsDec 15-21: 0% of 23103 checklistsDec 22-31: 0% 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. Washington CountyBest early August · 40 checklists that week, 2,511 in all44%
  2. Sedgwick CountyBest mid August · 25 checklists that week, 1,168 in all24%
  3. Costilla CountyBest early May · 35 checklists that week, 1,323 in all22%
  4. Kiowa CountyBest mid July · 42 checklists that week, 3,854 in all16%
  5. Jackson CountyBest the second week of June · 223 checklists that week, 7,472 in all12%
  6. Bent CountyBest early August · 55 checklists that week, 5,275 in all11%

Best spots

Share of checklists at the peak
  1. Walden ReservoirJackson County, best early June · 30 checklists that week, 546 in all68%
  2. Prewitt Reservoir State Wildlife AreaWashington County, best early August · 30 checklists that week, 957 in all68%
  3. Smith ReservoirCostilla County, best early May · 20 checklists that week, 432 in all52%
  4. Julesburg ReservoirLogan County, best late August · 32 checklists that week, 460 in all42%
  5. Timnath ReservoirLarimer County, best the second week of August · 106 checklists that week, 1,723 in all41%

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

Best time of day

In Colorado, the Forster's Tern is most often listed on checklists started between 5 and 7 pm (0.6% of those checklists).

0%0.5%1%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: 0.5% of 71991 checklists7 am: 0.3% of 159189 checklists8 am: 0.3% of 177206 checklists9 am: 0.4% of 165269 checklists10 am: 0.4% of 141029 checklists11 am: 0.4% of 115283 checklists12 pm: 0.5% of 96068 checklists1 pm: 0.5% of 89512 checklists2 pm: 0.5% of 86319 checklists3 pm: 0.5% of 82496 checklists4 pm: 0.5% of 71648 checklists5 pm: 0.6% of 52445 checklists6 pm: 0.6% 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%0%0%0%
February0%0%0%0%
March0%0%0%0%
April0.1%0.1%0.3%1.1%
May2.0%1.4%0.9%0.6%
June0.4%0.3%0.2%0.2%
July0.2%0.2%0.2%0.5%
August0.6%0.9%0.9%1.4%
September1.0%1.1%0.9%0.6%
October0.4%0.2%0.2%0.1%
November0%0%0%0%
December0%0%0%0%

Neighboring states

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

Common questions

When do Forster's Terns arrive in Colorado?
Forster's Terns arrive in Colorado in mid April, peak in early May (1.5% of checklists), and most are gone by early June.
When do Forster's Terns leave Colorado?
Most Forster's Terns have left Colorado by early June, when reports fall below half their usual rate.
When is the best time to see a Forster's Tern in Colorado?
The Forster's Tern is reported most often in early May, on about 1.5% of Colorado checklists.
Where in Colorado are Forster's Terns seen most often?
By county, Washington County, Sedgwick County, and Costilla County have the highest peak reporting rates for the Forster's Tern (counties with at least 200 checklists).