WhenToBird

Chroicocephalus philadelphia · Laridae

When Do Bonaparte's Gulls Arrive in Pennsylvania?

Adult Bonaparte's Gull
Photo: Kevin Krebs, CC BY 4.0 via iNaturalist

Bonaparte's Gulls arrive in Pennsylvania in the second week of March, peak in the second week of April (3.2% of checklists), and most are gone by the second week of May.

When to look

First arrivalsMar 8-14the second week of March
Common byMar 8-14the second week of March
PeakApr 8-14the second week of April
Most gone byMay 8-14the second week of May

Best: the second week of April. First arrivals the second week of March; most are gone by the second week of May. Peak: on about 3.2% of Pennsylvania checklists.

  1. Janrare
  2. Febrare
  3. Marcommon
  4. Aprpeak season
  5. Mayuncommon
  6. Junnot usually seen
  7. Julnot usually seen
  8. Augrare
  9. Seprare
  10. Octuncommon
  11. Novcommon
  12. Decuncommon

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

0%1%2%3%4%Jan 1-7: 0.2% of 47145 checklistsJan 8-14: 0.2% of 35840 checklistsJan 15-21: 0.1% of 35423 checklistsJan 22-31: 0.1% of 49126 checklistsFeb 1-7: 0.1% of 36366 checklistsFeb 8-14: 0.1% of 42248 checklistsFeb 15-21: 0.1% of 55888 checklistsFeb 22-28: 0.2% of 42050 checklistsMar 1-7: 0.3% of 42709 checklistsMar 8-14: 0.5% of 43985 checklistsMar 15-21: 1.1% of 44547 checklistsMar 22-31: 2.2% of 66940 checklistsApr 1-7: 3.6% of 51147 checklistsApr 8-14: 3.3% of 54868 checklistsApr 15-21: 2.6% of 57252 checklistsApr 22-30: 1.1% of 81727 checklistsMay 1-7: 0.8% of 76472 checklistsMay 8-14: 0.5% of 79341 checklistsMay 15-21: 0.3% of 61724 checklistsMay 22-31: 0.1% of 70722 checklistsJun 1-7: 0.1% of 45652 checklistsJun 8-14: 0.1% of 41810 checklistsJun 15-21: 0.1% of 38973 checklistsJun 22-30: 0.1% of 44255 checklistsJul 1-7: 0.1% of 34822 checklistsJul 8-14: 0.1% of 31104 checklistsJul 15-21: 0.1% of 29262 checklistsJul 22-31: 0.1% of 41436 checklistsAug 1-7: 0.1% of 29189 checklistsAug 8-14: 0.1% of 28840 checklistsAug 15-21: 0.1% of 29948 checklistsAug 22-31: 0.2% of 48235 checklistsSep 1-7: 0.2% of 37175 checklistsSep 8-14: 0.2% of 36527 checklistsSep 15-21: 0.1% of 37261 checklistsSep 22-30: 0.1% of 43583 checklistsOct 1-7: 0.1% of 35352 checklistsOct 8-14: 0.2% of 35560 checklistsOct 15-21: 0.4% of 33119 checklistsOct 22-31: 0.9% of 44654 checklistsNov 1-7: 1.4% of 32965 checklistsNov 8-14: 2.0% of 31305 checklistsNov 15-21: 1.5% of 29471 checklistsNov 22-30: 1.4% of 38642 checklistsDec 1-7: 1.0% of 28779 checklistsDec 8-14: 0.6% of 29486 checklistsDec 15-21: 0.3% of 37465 checklistsDec 22-31: 0.2% of 50651 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 Pennsylvania 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 Pennsylvania

Best counties

Share of checklists at the peak
  1. Erie CountyBest the second week of November · 738 checklists that week, 49,012 in all21%
  2. Montour CountyBest the second week of April · 198 checklists that week, 6,082 in all15%
  3. Mercer CountyBest early April · 553 checklists that week, 17,853 in all15%
  4. Crawford CountyBest mid November · 293 checklists that week, 23,552 in all13%
  5. Dauphin CountyBest late March · 1280 checklists that week, 35,877 in all11%
  6. Clearfield CountyBest the second week of April · 299 checklists that week, 9,768 in all11%

Best spots

Share of checklists at the peak
  1. Strickler RunLancaster County, best late March · 30 checklists that week, 227 in all83%
  2. Ridge PondErie County, best early November · 78 checklists that week, 1,112 in all81%
  3. MarysvilleDauphin County, best late March · 33 checklists that week, 262 in all80%
  4. Pymatuning ReservoirCrawford County, best late March · 21 checklists that week, 223 in all68%
  5. AitchHuntingdon County, best the second week of April · 22 checklists that week, 484 in all63%

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

Best time of day

In Pennsylvania, the Bonaparte's Gull is most often listed on checklists started between 1 and 3 pm (0.8% of those checklists).

0%0.5%1%12 am: too few checklists (1558) to judge1 am: too few checklists (1136) to judge2 am: too few checklists (839) to judge3 am: too few checklists (842) to judge4 am: too few checklists (2550) to judge5 am: too few checklists (18294) to judge6 am: 0.7% of 92903 checklists7 am: 0.7% of 241233 checklists8 am: 0.5% of 271994 checklists9 am: 0.6% of 236842 checklists10 am: 0.6% of 199508 checklists11 am: 0.7% of 161893 checklists12 pm: 0.7% of 143945 checklists1 pm: 0.8% of 134352 checklists2 pm: 0.8% of 129692 checklists3 pm: 0.8% of 126132 checklists4 pm: 0.7% of 115055 checklists5 pm: 0.7% of 90719 checklists6 pm: 0.7% of 67534 checklists7 pm: 0.4% of 43004 checklists8 pm: too few checklists (13966) to judge9 pm: too few checklists (3913) to judge10 pm: too few checklists (1989) to judge11 pm: too few checklists (1146) 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%0.2%0.1%0.1%
February0.1%0.1%0.1%0.2%
March0.3%0.5%1.1%2.2%
April3.6%3.3%2.6%1.1%
May0.8%0.5%0.3%0.1%
June0.1%0.1%0.1%0.1%
July0.1%0.1%0.1%0.1%
August0.1%0.1%0.1%0.2%
September0.2%0.2%0.1%0.1%
October0.1%0.2%0.4%0.9%
November1.4%2.0%1.5%1.4%
December1.0%0.6%0.3%0.2%

Neighboring states

  • Maryland: arrives late October, 18 weeks earlier than Pennsylvania
  • District of Columbia: arrives mid March, 1 week later than Pennsylvania
  • Delaware: arrives early November, 17 weeks earlier than Pennsylvania
  • New Jersey: arrives late February, 2 weeks earlier than Pennsylvania

Common questions

When do Bonaparte's Gulls arrive in Pennsylvania?
Bonaparte's Gulls arrive in Pennsylvania in the second week of March, peak in the second week of April (3.2% of checklists), and most are gone by the second week of May.
When do Bonaparte's Gulls leave Pennsylvania?
Most Bonaparte's Gulls have left Pennsylvania by the second week of May, when reports fall below half their usual rate.
When is the best time to see a Bonaparte's Gull in Pennsylvania?
The Bonaparte's Gull is reported most often in the second week of April, on about 3.2% of Pennsylvania checklists.
Where in Pennsylvania are Bonaparte's Gulls seen most often?
By county, Erie County, Montour County, and Mercer County have the highest peak reporting rates for the Bonaparte's Gull (counties with at least 200 checklists).