Thanks for the thorough reply. As a long time student of gulls, I am aware
of the concepts you delineated about timing and odd individuals that don't fit
the norms.
As for how did it (they) get here, stranger things have happened, namely the
Gray-hooded gull of a few years back. Ship-assisted? Who knows.
Bob Lewis
On Thursday, June 14, 2018, 9:08:12 AM EDT, Joseph DiCostanzo
<[email protected]> wrote:
Bob,
Molt, (and I am including change in soft part colors such as bill and leg color
under the term “molt”) is hormonally controlled. There is also considerable
variation between individuals in the timing of molt. The timing and sequence of
molt is no where as neat and fixed as many references might lead you to
believe. Having worked extensively with a banded population, where the age of
individuals is known from their banding histories, I know how much variation
there is. I have seen birds with extensive white foreheads (or heavy speckling)
and extensive carpal bars that I knew from their bands were two years old. I
have also seen two year olds that were indistinguishable from normal adults. I
have also seen that I knew were three, or four years old (or older based on
their bands) that showed speckled foreheads and traces of carpal bars. I have
seen adults in August at the breeding colony on Great Gull Island that were
already in full winter plumage (black bill, dark legs, carpal bar). Individuals
that don’t match the expected are unusual, but they do occur.
As for the lack of a carpal bar on these dark billed, dark legged individuals,
you are assuming that feather molt (carpal bar) and soft part colors (legs and
bill) must be in total lock-step with each other. There is no reason they
couldn’t be out of sync in some instances. As for primary length, this is being
based on photos, not actual measurements of the bird in hand. How is the
primary length being assessed? I am guessing versus tail length. How does
anyone know the tail length?
These dark billed, dark legged Common Terns get reported every year at this
season, just at the time young Common Terns are coming back from South America.
Where are these supposed longipennis birds coming from? The race breeds in
Siberia and winters in the Indian Ocean east to Australia. If longipennis were
going to occur here on the East Coast, the fall seems a more likely time than
the spring migration. It seems far more likely that these birds are aberrant
hirundo retaining aspects of their winter/non-breeding plumage than that there
is an annual movement of Siberian based longipennis birds through Long Island.
Joe DiCostanzo
Sent from my iPad
On Jun 14, 2018, at 7:59 AM, Robert Lewis <[email protected]> wrote:
Good points Joe.
Let's go with the null hypothesis: it's an unusual Common tern (hirundo).
Apparently a Common tern in September can get gray blotches on the underparts.
During the winter they have a black bill, black legs, and a black cap with
white forehead, as first summer birds do now. If it is four - eight months late
in molt (or four months early), maybe that would explain some features. But
then there should be a prominent dark carpal bar. Instead, there is none. Also,
how to explain the very long primaries? And the fact that the back is a bit
darker gray than neighboring Commons, both adult and immature?
I found a number of images of longipennis on the web. Here are some screen
shots:
Index of /lewis/birds/other_long
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Index of /lewis/birds/other_long
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Bob Lewis
On Wednesday, June 13, 2018, 9:01:37 PM EDT, Joseph DiCostanzo
<[email protected]> wrote:
One thing that has to be kept in mind about dark billed and/or dark legged
Common Terns seen on Long Island in the spring is that our Common Terns (Sterna
hirundo hirundo) have dark bills and dark legs in winter (both the young birds
and adults). The possibility that these birds are just S. hirundo hirundo
retaining aspects of their winter plumage must be considered. Indeed, given the
breeding and wintering ranges of S. hirundo longipennis, S. hirundo hirundo
with retained winter characters seems a more likely possibility.
Joe DiCostanzo
Sent from my iPad
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