WhenToBird

Hydroprogne caspia · Laridae

When Do Caspian Terns Arrive in Iowa?

Adult Caspian Tern
Photo: Daughter Dad, CC BY 4.0 via iNaturalist

Caspian Terns arrive in Iowa in late July, peak in early September (4.9% of checklists), and most are gone by early October.

When to look

First arrivalsJul 22-31late July
Common byJul 22-31late July
PeakSep 1-7early September
Most gone byOct 1-7early October

Best: early September. First arrivals late July; most are gone by early October. Peak: on about 4.9% of Iowa checklists.

  1. Jannot usually seen
  2. Febnot usually seen
  3. Marnot usually seen
  4. Apruncommon
  5. Maycommon
  6. Junrare
  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%2%4%6%Jan 1-7: 0% of 5551 checklistsJan 8-14: 0% of 3498 checklistsJan 15-21: 0% of 3803 checklistsJan 22-31: 0% of 4807 checklistsFeb 1-7: 0% of 3613 checklistsFeb 8-14: 0% of 4173 checklistsFeb 15-21: 0% of 5796 checklistsFeb 22-28: 0% of 4394 checklistsMar 1-7: 0% of 5295 checklistsMar 8-14: 0% of 5679 checklistsMar 15-21: 0% of 5418 checklistsMar 22-31: 0% of 8613 checklistsApr 1-7: 0.1% of 6970 checklistsApr 8-14: 0.1% of 7511 checklistsApr 15-21: 0.2% of 8321 checklistsApr 22-30: 0.9% of 11976 checklistsMay 1-7: 2.1% of 11725 checklistsMay 8-14: 2.2% of 12626 checklistsMay 15-21: 2.1% of 9372 checklistsMay 22-31: 0.9% of 10330 checklistsJun 1-7: 0.3% of 5472 checklistsJun 8-14: 0.3% of 4568 checklistsJun 15-21: 0.3% of 3870 checklistsJun 22-30: 0.6% of 4780 checklistsJul 1-7: 0.5% of 4201 checklistsJul 8-14: 0.6% of 3605 checklistsJul 15-21: 0.5% of 3621 checklistsJul 22-31: 1.1% of 5093 checklistsAug 1-7: 1.2% of 3931 checklistsAug 8-14: 2.4% of 3488 checklistsAug 15-21: 3.0% of 3802 checklistsAug 22-31: 4.8% of 6293 checklistsSep 1-7: 5.7% of 5328 checklistsSep 8-14: 4.3% of 4622 checklistsSep 15-21: 3.6% of 4367 checklistsSep 22-30: 1.8% of 6086 checklistsOct 1-7: 0.8% of 4708 checklistsOct 8-14: 0.1% of 4791 checklistsOct 15-21: 0% of 4429 checklistsOct 22-31: 0% of 5633 checklistsNov 1-7: 0.1% of 4090 checklistsNov 8-14: 0% of 3416 checklistsNov 15-21: 0% of 3398 checklistsNov 22-30: 0% of 4679 checklistsDec 1-7: 0% of 3442 checklistsDec 8-14: 0% of 3322 checklistsDec 15-21: 0% of 4619 checklistsDec 22-31: 0% of 5113 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 Iowa 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 Iowa

Best counties

Share of checklists at the peak
  1. Marion CountyBest the second week of September · 58 checklists that week, 3,389 in all26%
  2. Polk CountyBest early September · 931 checklists that week, 37,521 in all16%
  3. Buena Vista CountyBest early September · 74 checklists that week, 2,846 in all12%
  4. Appanoose CountyBest early September · 14 checklists that week, 820 in all12%
  5. Clayton CountyBest the second week of September · 19 checklists that week, 1,181 in all8.6%
  6. Lee CountyBest the second week of August · 14 checklists that week, 1,488 in all8.1%

Best spots

Share of checklists at the peak
  1. Saylorville Recreation Area (northwest)Polk County, best the second week of May · 52 checklists that week, 1,546 in all63%
  2. Saylorville Recreation Area (Rock Creek)Polk County, best early September · 38 checklists that week, 1,401 in all54%
  3. Saylorville Recreation Area (Big Creek)Polk County, best mid May · 49 checklists that week, 999 in all37%
  4. Saylorville Recreation Area (Corydon (historical))Polk County, best the second week of September · 79 checklists that week, 1,186 in all36%
  5. Jester ParkPolk County, best late August · 24 checklists that week, 735 in all34%
  6. Storm LakeBuena Vista County, best early September · 22 checklists that week, 836 in all33%

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

Best time of day

In Iowa, the Caspian Tern is most often listed on checklists started between 6 and 8 pm (1.4% of those checklists).

0%0.5%1%1.5%2%12 am: too few checklists (96) to judge1 am: too few checklists (99) to judge2 am: too few checklists (67) to judge3 am: too few checklists (82) to judge4 am: too few checklists (201) to judge5 am: too few checklists (1888) to judge6 am: 1.1% of 11313 checklists7 am: 1.1% of 25717 checklists8 am: 1.0% of 29755 checklists9 am: 0.9% of 28897 checklists10 am: 0.8% of 26343 checklists11 am: 1.1% of 21210 checklists12 pm: 1.0% of 17662 checklists1 pm: 0.8% of 16595 checklists2 pm: 0.7% of 16405 checklists3 pm: 0.8% of 17030 checklists4 pm: 0.7% of 17378 checklists5 pm: 1.2% of 14251 checklists6 pm: 1.5% of 10349 checklists7 pm: 1.1% of 6233 checklists8 pm: too few checklists (2002) to judge9 pm: too few checklists (392) to judge10 pm: too few checklists (186) to judge11 pm: too few checklists (87) 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%0.2%0.9%
May2.1%2.2%2.1%0.9%
June0.3%0.3%0.3%0.6%
July0.5%0.6%0.5%1.1%
August1.2%2.4%3.0%4.8%
September5.7%4.3%3.6%1.8%
October0.8%0.1%0%0%
November0.1%0%0%0%
December0%0%0%0%

Neighboring states

  • Missouri: arrives the second week of July, 2 weeks earlier than Iowa
  • Wisconsin: arrives the second week of April, 14 weeks earlier than Iowa
  • Minnesota: arrives late April, 12 weeks earlier than Iowa
  • Illinois: arrives late March, 16 weeks earlier than Iowa

Common questions

When do Caspian Terns arrive in Iowa?
Caspian Terns arrive in Iowa in late July, peak in early September (4.9% of checklists), and most are gone by early October.
When do Caspian Terns leave Iowa?
Most Caspian Terns have left Iowa by early October, when reports fall below half their usual rate.
When is the best time to see a Caspian Tern in Iowa?
The Caspian Tern is reported most often in early September, on about 4.9% of Iowa checklists.
Where in Iowa are Caspian Terns seen most often?
By county, Marion County, Polk County, and Buena Vista County have the highest peak reporting rates for the Caspian Tern (counties with at least 200 checklists).