Passage Summary: There is a specific bird that is the most common resident of a marsh, but it is almost never seen, even by experts who are looking for it.

Reasoning: The marsh hen is the most populous bird in the area, yet it is observed less frequently than any other species, including those that are rarer and smaller.

Analysis: The paradox here lies in the inverse relationship between population density and visibility. Usually, the more common a creature is, the more often you see it. To resolve this, we need to find a reason why a high population wouldn't lead to high visibility—perhaps the birds are nocturnal, perfectly camouflaged, or live in parts of the marsh humans can't reach. Since this is an 'EXCEPT' question, look for the one answer choice that fails to explain why such a common bird remains so elusive.