WhenToBird

Sterna forsteri · Laridae

When Do Forster's Terns Arrive in Illinois?

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

Forster's Terns arrive in Illinois in mid April, peak in early May (1.2% of checklists), and most are gone by late May.

When to look

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

Best: early May. First arrivals mid April; most are gone by late May. Peak: on about 1.2% of Illinois checklists.

  1. Jannot usually seen
  2. Febnot usually seen
  3. Marrare
  4. Aprpeak season
  5. Maypeak season
  6. Jununcommon
  7. Julcommon
  8. Augpeak season
  9. Sepcommon
  10. Octcommon
  11. Novrare
  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 29700 checklistsJan 8-14: 0% of 20189 checklistsJan 15-21: 0% of 19784 checklistsJan 22-31: 0% of 26641 checklistsFeb 1-7: 0% of 20478 checklistsFeb 8-14: 0% of 21967 checklistsFeb 15-21: 0% of 28735 checklistsFeb 22-28: 0% of 24223 checklistsMar 1-7: 0% of 26775 checklistsMar 8-14: 0.1% of 27579 checklistsMar 15-21: 0.1% of 27193 checklistsMar 22-31: 0.1% of 42278 checklistsApr 1-7: 0.2% of 34579 checklistsApr 8-14: 0.3% of 38290 checklistsApr 15-21: 0.5% of 39261 checklistsApr 22-30: 1.1% of 59584 checklistsMay 1-7: 1.6% of 60485 checklistsMay 8-14: 0.9% of 59647 checklistsMay 15-21: 0.7% of 43173 checklistsMay 22-31: 0.4% of 47913 checklistsJun 1-7: 0.3% of 31151 checklistsJun 8-14: 0.2% of 27692 checklistsJun 15-21: 0.2% of 23983 checklistsJun 22-30: 0.2% of 28448 checklistsJul 1-7: 0.3% of 21273 checklistsJul 8-14: 0.4% of 19284 checklistsJul 15-21: 0.6% of 17712 checklistsJul 22-31: 0.7% of 24500 checklistsAug 1-7: 0.6% of 18498 checklistsAug 8-14: 1.4% of 19331 checklistsAug 15-21: 0.8% of 19983 checklistsAug 22-31: 0.9% of 31892 checklistsSep 1-7: 0.9% of 26396 checklistsSep 8-14: 0.8% of 25011 checklistsSep 15-21: 0.8% of 23597 checklistsSep 22-30: 0.8% of 30638 checklistsOct 1-7: 0.4% of 23949 checklistsOct 8-14: 0.3% of 24412 checklistsOct 15-21: 0.3% of 22359 checklistsOct 22-31: 0.2% of 29794 checklistsNov 1-7: 0.1% of 21877 checklistsNov 8-14: 0.1% of 19742 checklistsNov 15-21: 0.1% of 18989 checklistsNov 22-30: 0.1% of 23905 checklistsDec 1-7: 0.1% of 16935 checklistsDec 8-14: 0% of 16687 checklistsDec 15-21: 0.1% of 20931 checklistsDec 22-31: 0% of 29011 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 Illinois 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 Illinois

Best counties

Share of checklists at the peak
  1. Clinton CountyBest mid September · 238 checklists that week, 8,606 in all22%
  2. Franklin CountyBest early September · 45 checklists that week, 1,745 in all19%
  3. Calhoun CountyBest the second week of August · 40 checklists that week, 786 in all17%
  4. Richland CountyBest late April · 31 checklists that week, 580 in all15%
  5. Fayette CountyBest early November · 10 checklists that week, 1,050 in all15%
  6. Jefferson CountyBest early September · 88 checklists that week, 2,716 in all14%

Best spots

Share of checklists at the peak
  1. Carlyle LakeClinton County, best the second week of September · 37 checklists that week, 912 in all63%
  2. Center ParkSangamon County, best early May · 21 checklists that week, 715 in all43%
  3. Crab Orchard National Wildlife RefugeWilliamson County, best early May · 32 checklists that week, 1,040 in all41%
  4. Horshoe Lake State ParkMadison County, best late April · 143 checklists that week, 3,683 in all37%

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

Best time of day

In Illinois, the Forster's Tern is most often listed on checklists started between 6 and 8 am (0.5% of those checklists).

0%0.5%1%12 am: too few checklists (484) to judge1 am: too few checklists (631) to judge2 am: too few checklists (485) to judge3 am: too few checklists (610) to judge4 am: too few checklists (2321) to judge5 am: too few checklists (23536) to judge6 am: 0.7% of 95511 checklists7 am: 0.4% of 164584 checklists8 am: 0.3% of 163654 checklists9 am: 0.4% of 141804 checklists10 am: 0.4% of 122882 checklists11 am: 0.4% of 100640 checklists12 pm: 0.4% of 89894 checklists1 pm: 0.4% of 82729 checklists2 pm: 0.4% of 79801 checklists3 pm: 0.4% of 82169 checklists4 pm: 0.5% of 75124 checklists5 pm: 0.5% of 57708 checklists6 pm: 0.4% of 41843 checklists7 pm: too few checklists (20943) to judge8 pm: too few checklists (6131) to judge9 pm: too few checklists (1677) to judge10 pm: too few checklists (843) to judge11 pm: too few checklists (448) 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%0.1%0.1%0.1%
April0.2%0.3%0.5%1.1%
May1.6%0.9%0.7%0.4%
June0.3%0.2%0.2%0.2%
July0.3%0.4%0.6%0.7%
August0.6%1.4%0.8%0.9%
September0.9%0.8%0.8%0.8%
October0.4%0.3%0.3%0.2%
November0.1%0.1%0.1%0.1%
December0.1%0%0.1%0%

Neighboring states

  • Indiana: arrives mid July, 12 weeks later than Illinois
  • Wisconsin: arrives mid April, in the same week than Illinois
  • Michigan: arrives the second week of April, 1 week earlier than Illinois
  • Iowa: arrives mid April, in the same week than Illinois

Common questions

When do Forster's Terns arrive in Illinois?
Forster's Terns arrive in Illinois in mid April, peak in early May (1.2% of checklists), and most are gone by late May.
When do Forster's Terns leave Illinois?
Most Forster's Terns have left Illinois by late May, when reports fall below half their usual rate.
When is the best time to see a Forster's Tern in Illinois?
The Forster's Tern is reported most often in early May, on about 1.2% of Illinois checklists.
Where in Illinois are Forster's Terns seen most often?
By county, Clinton County, Franklin County, and Calhoun County have the highest peak reporting rates for the Forster's Tern (counties with at least 200 checklists).