Solution: Proverbial rarities - NYT Crossword

This isn’t a new entry, but it hasn’t been in a puzzle since the 1980s, and it’s a great phrase that I still hear all of the time for something that hardly comes around — its rarer than HENS TEETH. The contrarian will say that birds do have an egg tooth or a gizzard or some other technicality, but it’s more interesting to me that it’s taken such a long time to posit why birds don’t have teeth, since they’re modern dinosaurs. A paper from 2018 theorizes that birds evolved to incubate ever more quickly for their improved survival, and removing teeth from the equation allowed an embryo in an egg to mature after days instead of months.