In philosophy, we talk about the complicated semantics of sentences with 'generic' form - that is, instead of "All A are B" or "Some A are B", "As are Bs".
Generic sentences are not universal statements. A generic statement can be true even if it is not universally true, like "Tigers have four legs" or "Mosquitos transmit malaria."
On the other hand, they are not just statements made true by one or a few instances, as shown by a case like "Americans live in New York City".
How to understand the truth-conditions for generics is an open question, but it seems to have something to do with appealing to some kind of intrinsic essential connection between the subject and the predicate.
So "ducks lay eggs" is true, even though only 50% or so of the ducks do that, because it is in some sense 'of their biological nature'.
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