12 November 2011

A Tail of Two Streamers

A couple of years ago, I noticed a 1st cycle Bonaparte's Gull with what appeared to be longer central tail feathers - longer than the rest of the rectrices. I figured this was just a unique case but kept note of it. With time, I began noticing more and more of these individuals that show this excess length. This slight protrusion sort of resembles the short streamers found on some juvenile jaegers. Here's an example:

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.
For a fraction of a second, one has no choice but to associate this tail pattern with that of Ross's Gull. I tried finding a description or illustration of this phenomenon in the literature, but nothing. How could Dwight, Grant, Howell & Dunn, and Olsen & Larsson all have missed this? It was then brought to my attention that the Crossley ID Guide shows an alternate adult Bonaparte's with this pattern. Whether Crossley intentionally inserted this image in his guide is unknown, but the individual shown shows a pattern much similar to Ross's; had I been doing a "tail quiz", I seriously would identify it as the tail of a Ross's. Also, one can see that on that bird the outer tail feathers are quite worn and this adds to the "wedge-shaped" pattern.

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.
The contrasting black tailband and white undertail coverts are easily delineated. From this photo, it's not very difficult to believe that the, central, undertail coverts could grow out pass the tail. This delineation would not be as clear with an adult (or 2nd cycle) since the rectrices are all white by then:

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.
The undertail coverts clearly project pass the terminal edge of the tail. From above, the straightened tail looks like this:

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.
I'd like to thank those who emailed me with comments on this topic, especially Killian Mullarney. My next question is: "Do Ross's Gulls, to some degree, exhibit a similar pattern"? 

Please feel free to email me your comments and observations: amarayyash@yahoo.com