You make a good point. Birds, especially, tend to float in water, given that they evolved specifically to have a low density in order to fly (i.e. float in air

You make a good point. Birds, especially, tend to float in water, given that they evolved specifically to have a low density in order to fly (i.e. float in air). A bird left floating for a long time will probably die of starvation or dehydration; but it's much less likely than a mammal to drown when it tires. So there's a better chance that it will still be alive when it washes up on some other beach, if that happens within a couple days.