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 <rfer...@yahoo.com> 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
> 
> Index of /lewis/birds/other_long
> 
> Bob Lewis
> 
> 
> 
> On Wednesday, June 13, 2018, 9:01:37 PM EDT, Joseph DiCostanzo 
> <jdic...@nyc.rr.com> 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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