Many years ago I stayed on an island off Wilsons Promontory in Victoria. Nesting on the island were a huge number of mutton birds. Every evening they would all come back from feeding and crisscross the island looking for their nests. It was amazing that so many birds could fly in so many directions at such high speed without colliding. Just as I was pointing this out to the people I was with, two birds clipped wings. We hadn't even stopped laughing before another two birds had a far more serious head-on collision. Both birds must have survived because we couldn't find them on the ground, but they could have been seriously injured.
It turns out that it's not uncommon for other seabirds to have collisions, some of them fatal. Gannets are an interesting seabird that plunge into the water at high speed order to catch fish from close to the surface (although I've heard of divers seeing them 40 meters down!). Gannets can form large flocks above schools of fish. At high densities collisions become more likely as one or more gannets target the same fish. Although collisions are not uncommon, deaths seem to be.
Now there is some cool underwater footage of gannets feeding with sharks and dolphins in Hauraki Gulf, New Zealand. It was taken by Gabriel Capuska, a PhD student at Massey University. It shows a collision between gannets and some interesting other behaviours. For more detail, go to the Massey University story.