Hybrid Caspian Gull

Today was all about searching for my 400th colour-ringed gull at Shawell A5 Lagoons, but my run of finding Caspian Gulls continued. A small female was on the bank between some Great Black-backed Gulls that dwarfed her. 

Adult Female Caspian Gull

A Common Buzzard flushed the gulls from the bank and I lost the female Caspian Gull. Many of the gulls headed for the safety of the water and I turned my attention to them. I quickly came across another Caspian Gull candidate. It looked good, but when it started to preen, instead of seeing the typical pattern of the underside of the longest primary (P10), I saw a mostly black feather.

Presumed Hybrid Caspian Gull (note the extent of the black on the underside of the primaries)

I was not happy with this gull, so I continued to search and found another candidate. This third individual had paler eyes than the hybrid, but otherwise it looked good. Eventually the two gulls met each other and gave me an opportunity to compare them side by side. The good one showed the typical pattern on the underside of P10 - photo of a typical P10 HERE - but the other had too much black on the primaries. The hybrid's bill was the correct colour, but it was perhaps a little short. Without being familiar with the main features I could have easily identified it as a Caspian Gull. The reason it looked like a Caspian Gull structurally may have been because one of its parents was a pure Caspian and the other a hybrid that was closer in looks to Caspian Gull than Herring Gull?

Presumed Hybrid Caspian Gull

The Caspian Gull and the hybrid were actually very similar apart from the amount of black on the longest primaries and the eye colour. Another slight difference was that the hybrid had fine streaks on its head whereas the apparent pure bird had fine streaking on its lower neck. The streaking is not really visible in the photos I took, so it is difficult to actual work out which one it is in some of the photos. The surprising thing was how similar the bills were in colour and size of the gonys spot. I am glad I actually saw these gulls side by side.

The image below is of the good one - I worked it out based on the time I took the photo.

Adult Caspian Gull

By the way I did manage to read a few new colour rings to pass the 400 mark for different colour-ringed gulls seen by me at the site.