I came upon a variety of birds gobbling up something from the paved surface of a bike trail in Decorah, beneath the canopy of an elm tree (American elm, I believe). It took me a while to figure out the trail under the elm was littered with these tiny grubs, which seemed to be falling out of the tree. Further investigation revealed that many of the developing samaras on the tree hosted small galls -- more or less cone-shaped with an opening on the tapered end. As I recall I actually noticed one of these grubs emerging from a gall on a samara. There were plenty of birds feasting on the ground (see species list below) but a number of warblers were in the tree itself as well, foraging among the clusters of samaras, seemingly grabbing and tugging at individual samaras as if extricating the grubs from them (or grabbing them as they emerged from their galls). I dug around on the Internet and eventually found a paper by Dr. Raymond Gagne: Primavera (new genus) porrecta (Felt), the elm samara midge: description and biological notes (Diptera: Cecidomyiidae)
. No BG page for Primavera porrecta
so I can't compare these images with others comfirmed to be of that species, but seems like that's gotta be it, or a close relative, based on Dr. Gagne's paper.
Many birds were enjoying the bounty of grubs on a sunny morning! Here's a list of the bird species I saw picking the grubs off the trail surface:
Allow me to nerd out a bit more... :-) As I watched through binocs I timed how quickly some of the birds pecked the surface of the trail, to get a very rough sense for how many grubs they were finding...the numbers were pretty amazing. 30 pecks on the trail surface in 27 seconds (indigo bunting), 30 in 23 seconds (Tennessee warbler), 30 in 26 seconds (gray catbird), 16 in 18 seconds (yellow warbler), and so on...that's a lot of fly larvae!