Mees

Mees is a variant of Meus, which is a short form of Bartholomeus. In other words: it's not a diminutive. If it were a diminutive, the name would have contained one of the Dutch diminutive suffixes, namely -je and -ke (resulting in respectively Meesje and Meeske).

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Also see:
• https://nvb.meertens.knaw.nl/verklaring/naam/Mees (in Dutch; "Verkorting Bartholomeus")
• -je: https://en.wiktionary.org/wiki/-je#Dutch (in English)
• -ke: https://en.wiktionary.org/wiki/-ke#Dutch (in English)
• https://en.wikipedia.org/wiki/List_of_diminutives_by_language#Dutch (in English) [noted -ed]

The name's modern use can partially be prescribed to the fact that "Mees" is also the Dutch name for the birds tits, chickadees, and titmice.