These are Ash Flower galls, found on male Ash trees. They are caused by the trees' reaction to tiny eriophyid mites feeding on male flowers. Good news is the galls are mostly cosmetic and don't cause significant damage to the tree. They'll show up year after year, but last year we saw a particular abundance of these ash galls (was a good year for the eriophyid mites!).

Here is a bit more info (From Iowa State Extension) in case you're curious:
https://www.extension.iastate.edu/news/news/2006/jul/072801.htm

From (Wisconsin-Madison Extension): https://hort.extension.wisc.edu/files/2014/11/Ash-Flower-Gall.pdf