The weather isn't obvious in the photo, but it was dismal. The temperature at 8 am was 34 F, and rose to only 37 F. Frosty temps are no problem, but the nonstop rain that ranged from light to moderate showers was an issue. To me, there are no worse weather conditions than drenching rain at temperatures just above freezing. Makes it much harder to find birds.
Sorry for the dreaded white sky background in these photos, but there's nothing I could do about that. White skies are the absolute worse for photographic backdrops, and we get a lot of those skies in Ohio winters.
One effect of having a big flock of active robins in a large woodland is that their hustle and bustle attracts lots of other birds. The sumac entices the robins. Their conspicuous activity draws many other birds who then forage in the vicinity even though the hangers-on aren't necessarily after the sumac's fruit. I probably had at least a dozen species in the mixed flock with the robin nucleus. In the songbird world, plants ultimately orchestrate the show.
In the above photo, we see Smooth Sumac in full flower in mid-summer. The oceans of tiny greenish-yellow flowers attract legions of interesting pollinating insects. When I am out and about, camera in hand, and spot flowering sumac I always veer over for a look. I've obtained many a great insect image at these flowers.
Keep in mind next summer's Midwest Native Plant Conference in Dayton, Ohio: August 1st thru 3rd. That's an awesome venue to learn more about native flora, buy quality plants, and generally have a great time.