Usually for birds whatever lens you have, following sod's law it's invariably too short

most consider 400mm is the minimum for bird photography. At this focal length the Canon 400mm f5.6 prime is considered one of the best in flight lenses, being relatively light with excellent IQ. Zooms of course offer more flexibility and the Canon 100-400 and Sigma 120-400 are the two contenders. Also to consider are the Sigma 150-500 OS and the Sigma 50-500 OS these have the advantage of slightly more range. All these lenses can be found in the £800-£1500 price range.
Moving on to the Canon 500, 600 and 800 primes, here we are talking considerably more money around £5,000 upwards, so not sure if this is beyond your budget, but the Canon 500mm f4 is also an excellent lens.
For myself I use a Sigma 150-500 which I have had for about two years and have been well pleased with it's results as the photo shows. Examples of the other lenses can be found on the forum.
http://SPAM/c3whur/feb/IMG_4316.jpg