Both birds and mammals have four-chambered hearts. but birds are more closely related to crocodiles, which have three-chambered hearts, than they are to mammals. the correct interpretation of these observations is that:

The four-chambered hearts of birds and mammals are best described as homoplasies.

Explanation:

In evolution, homoplasies are the trait shared in different species despite that their ancestors didn't have it at all.  In other words, these are traits that can be gained or lost during evolution in different lineages independently. For example, wings of birds and wings of bats do the same function (help in flying) but they were not there in the ancestors so both animals developed these structures independently.

In the given example, birds and mammals have a four-chambered heart but their ancestors have 3 chamber hearts. They evolved independently and developed 4 chamber heart. This might be due to several reasons. However, their ancestors didn't have the feature so it is an example of homoplasy.

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