This afternoon, Patricia Lindsay and I identified a juvenile Thayer's Gull at 
Argyle Lake in Babylon village, southwestern Suffolk County, Long Island.

This hatching-year Larus retained full juvenile plumage. At this date this is 
expected for Thayer's and Iceland Gulls, not unusual in Lesser Black-backed 
Gulls, but would be extremely rare in smithsonianus Herring Gulls, all of which 
we saw today had replaced their juvenal scaps and mantle feathers with 
formative feathers. 

The focal bird was smaller, smaller-billed, and rounder-headed than 
smithsonianus. The bill was also darker than on any same-aged Herring Gull seen 
today, generally appearing all black (but showing subtly paler base on good, 
close looks). The folded wingtip and tail appeared black, the dorsal-aspect 
primaries tipped with fine, creamy chevrons. The blackness of these feathers 
was far beyond any dark-end kumlieni Iceland Gull in my experience. In addition 
to the points mentioned above regarding structure, molt, and bill color, this 
bird differed from smithsonianus Herring Gull in showing a pale, Iceland 
Gull-like underwing pattern. This was clearly visible on my best scope views of 
the undersides of the wingtips when perched, and in excellent binocular views 
in flight. Unfortunately, the bird changed position on the rooftop immediately 
after my initial studies of it and before I began photographing it. Thus, the 
photos show only a hint of the extensively pale inner webs of the primaries, 
and incomplete views of the retained juvenile dorsum. Both of these things were 
seen previously, however, and conformed to the descriptions above.

All photos were taken 1:49-1:52 EST, at which point all of the gulls in the 
area went up due to a bread-scattering event on the other side of the lake. I 
had great views of the bird in perfect light as it flew east to west the north 
of us, but Pat was not on it with the Lumix when it flew, so no flight shots. 
We lost track of it as the flocks milled around over the lake and could not 
re-find it.

Photos can be seen here:

https://flic.kr/p/2hPTG9s

Shai Mitra
Bay Shore
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