This is one neat and easy to do image. Late afternoon gulls like to bathe and will have lots of chances ... all you need is the right wind and sun angle since the birds will turn into the wind before flapping.
You can vary the shutter speed according to distance with closer to the bird being a faster shutter speed. For framing remember the bird is going to extend the wings and jump straight up in the air .... don't frame tight at this point !!! Should come up with a few keepers !!!
Canon 1DsMk2
500 f4.0 IS 1.4X
f 5.6 1/50
ISO 800
Av +1