Dawdler:
Birds on a land based airfield can be discouraged by keeping the grass at the rght length. They don't seem to like six inch high grass as they can't see predators arriving. I don't believe the same precautions would be available over wetlands.
Wetland birds also tend to be bigger and heavier than the main inland varieties. Hit a sparrow, starling etc you will probably get away with it, I wouldn't have the same confidence after hitting a Greylag or Canadian goose.
Birds like estuary wetlands, as you say. But this airport will not be a wetland, it will be a concrete slab in the coastal North Sea. There will be no wetlands, no reed beds, no shallow waters - nothing of interest to most sea birds.
Apart from birds migrating from shore to shore, which can be dealt with, there should be no greater bird threat on an isolated island, than at Heathrow. As an aside, the area for this new island is predominantly saline, and most geese (apart from the brent) do not like saline water.
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