Perch

I like the rich, blue background and the way the light hits the pylons, but

does this composition work? Is there enough focus on the Gull itself?

The short answer is "Yes" the composition does work. The proportional content is quite well balanced, and there is interest in the textures of the pylons as well as the details of the bird. I think you have done well in capturing the detail in the highlights and shadows in what looks to have been a bright sunny day.

Best Regards

Alf