|1st cycle Bonaparte's Gull. Carlyle, IL. 14 Nov 2010.|
I found it odd that the width of this excess feathering was unlike the rest of the thin, white, terminal edge to the tail. I thought that perhaps this was a trait found on some, but not all, 1st cycle Bonaparte's. I speculated that the central tail feathers grew out longer so as to "maybe" act as coverts for the closed tail from above. Afterall, these tail feathers have to last for almost an entire year.
Then, last week, I found an adult with this tail pattern at Calumet Park along the Chicago lakefront:
|Calumet Park. Chicago, IL. 5 Nov 2011.|
Killian Mullarney from Ireland suggested that what I'm seeing may actually be the longest undertail coverts as he has seen Black-headed Gulls like this before. Incidentally, while out photographing gulls yesterday, I came across this 1st cycle Bonaparte's:
|1st cycle Bonaparte's. Hammond, IN. 11 Nov 2011|
Note how the undertail coverts (white) line up with the edge of the tail, overlapping with the black tailband.
|2nd cycle BOGU. Note how the center of the tail shows a protrusion - this is a result of the longer undertail coverts.|
So, I went back and reviewed more photos of the adult from 5 November 2011:
|All of the rectrices are easily seen here except for the two central feathers. The undertail coverts (utc) are doing exactly what coverts do: they're covering the bottom of the tail.|
|Undertail coverts projecting pass the two central tail feathers. This gives the impression that the central tail feathers are longer than the rest of the rectrices, although this is not the case.|
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