If something flies and builds a nest, it usually lays eggs; if a living thing has four legs, it gives birth (with a few exceptions) to live young. We say that these features are:

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The features mentioned (flying/building a nest and laying eggs vs. having four legs and giving birth to live young) are intercorrelated, meaning they are connected and often found together in certain groups of animals. This is because they are both related to reproductive strategies and adaptations.

If Something Flies and Builds a Nest, It Usually Lays