I live in coastal new Jersey and Northern Harriers are quite common. I see them daily cruising over the wetlands. I understand that they eat mice and voles and things like that. My question is this. When there are very high tides the entire meadows are submerged. How is it that all the prey animals aren’t drowned when there is a foot of water over everything? Some sort of air pockets in in underground chambers? Or are they dining on crustaceans and things like that?
I used to see the same thing in So Cal and wondered the exact same thing you are asking. Our fields would flood and that is when the harriers would show up. They would cruise low over the flooded fields but I never could tell for sure what they ere catching. I suspected blackbirds might have been on their menu. As the blackbirds would build nests in the wild mustard that would grow in the temporary swamps.
According to this article, Northern Harriers (the current name for Marsh Hawk) took exclusively birds when foraging over a salt marsh in South Carolina. They took small mammals and other prey at a freshwater marsh in Florida.
Based on this, I would say that when they are cruising over tidal marshes they are looking mostly for birds, which won’t be bothered by the tides.
Based on bird watcher’s websites, it looks like the Northern Harrier would be interested in hunting small marsh predators (including other birds) rather than crab or mussels. However, birds, rabbits, weasels, lizards, mice, snakes and many more creatures do live in salt marshes. When the tides get high, they just move upland, although I believe some species have adapted burrows that leave them dry pockets even when the marshes flood. Except in very unusual circumstances, there’s probably some relative high ground nearby. And many species can simply wait out high tide anyway.