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An abundant species in the Great Lakes, breeding just to our north in the boreal forest. There are several "migrations" into and out of our area. Adults generally arrive in March and April, molting some body/head feathers into breeding plumage before continuing north in April and May. First year birds arrive a bit later in April and May, sometimes with large numbers summering in our area to molt. Fall migration of adults is early, starting in late June and continuing into mid August. They stay on the lakes for several months as they feed and complete a full-body molt into "winter" plumage. Juveniles arrive by late July and continue into September. They also undertake a smaller body molt. Large numbers stage into December, when numbers decrease thereafter. Particularity cold weather and/or ice will push most remaining birds away by late January; however, some individuals may still be found on the southern Lakes/Rivers (and slightly more will stay during mild winters).
Juvenile / First Cycle
June
July
August
The extent of brown feathering is somewhat variable between individuals.
Advanced young Bonaparte's Gulls may be quickly losing their brown feathering by the end of August.
September
October
November
December
January
February
March
April
May
Rare "dark morph" first cycle Bonaparte's Gulls, with extensively black upperwings, are sometimes recorded. They can occur in any month, but for whatever reason, appear to be somewhat more common in spring and early summer.
Second Cycle
The change from first to second cycle depends on the timing of the start of the inner primary molt. May/June is used here as an arbitrary cutoff.
June
July
Infrequent first summer Bonaparte's Gulls can obtain fully black heads, similar to adults.
August
Bonaparte's Gulls are considered "two year" gulls - so by the end of August, our "second cycle" birds are rapidly obtaining an adult like plumage. A keen eye may detect subtle signs of their "second year" age class throughout the winter by the presence of extra black markings in the wing coverts, primaries and (rarely) tail.
September
A few examples of these possible "second cycle" individuals are shown below. Some further food for thought:
It may be impossible to know for sure that they are, in fact, second cycle individuals.
Some second cycle birds may be indistinguishable from adults.
Many of these features may be very difficult to see in the field (i.e., without a camera) in all but the most extreme examples.
October
November
December
January
February
March
April
It is assumed that second year birds would obtain a black head in spring, similar to "full" adults.
May
Adult
Full adult Bonaparte's Gulls are presented below from June through May. True "third cycle" individuals would still be recognizable until they finish their summer molt.
June
Full adult Bonaparte's Gulls have generally left the Great Lakes by early June, with isolated individuals remaining locally, south of their breeding grounds.
As one of our earliest migrants, adults returning from their breeding grounds can start returning to the Great Lakes by late June.
July
With the Great Lakes acting as a "staging" or "molting" ground for Bonaparte's Gulls, larger numbers start to arrive in early July and begin their molt soon thereafter (note primary molt below).
Rarely, adult Bonaparte's Gulls can be found in "non-breeding" plumage in summer/July.
August
Initiation of molt may be tied to an individuals arrival on their staging grounds. Therefore, a wide array of molt progress is visible locally through the Great Lakes - with early birds nearly complete, to late individuals still sporting black heads mid- to late-month.
September
By late September, molt into "winter" plumage will be complete for most Bonaparte's Gulls locally. Note how the outer primaries are still growing in the right hand bird above, while the primary molt is complete for the individual beside it.
October
November
December
January
February
March
Adult Bonaparte's Gulls begin their molt into breeding plumage, replacing head & body feathers (i.e., no "flight" feathers), in mid- to late-March. This molt is rapid, with variable progress between individuals.
April
Most adult Bonaparte's Gulls will have obtained full "breeding plumage" by late month.
May
Adult Bonaparte's Gulls often appear particularly pristine just before leaving our area for their breeding grounds. Returning migrants by late June / early July have usually lost this "lusture".
Some "adults" that remain in the Great Lakes later into May (or June) may be sick or injured, leading to a less polished appearance.
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