"Differentiating", "distinguishing," or another word for "that helps to tell apart easily"?

I am trying to come up with the right word that means that helps to tell apart easily, for example, one that fits into the following sentences:

• Beak shape is a __ feature among birds.
• Language is usually the __ feature for ethnic groups.

I tried differentiating and distinguishing, but was not quite sure.

The standard word in this context is distinguishing...

In principle, as OP suspects, differentiating ought to be useable in this context, but it just isn't. Apart from anything else, differentiation has largely been appropriated by the mathematicians.

For many decades, many linguists have worked within the framework of Distinctive Feature Theory, but that's a bit specialised. And in general parlance we often refer to the distinctive features of all sorts of things, but by that we normally mean features which are particularly noticeable, rather than features which are crucial to identifying exactly what we're looking at.

Also identifying features, which for some reason seem to be more associated with contexts outside biology/taxonomy. All these terms (less commonly) occur with characteristic instead of feature.

Discriminative and discriminating are also valid.

In my opinion, 'Beak shape is a characteristic feature among birds' seems to fit well. You could as well use the word 'individualizing'.