I was slower to recognize Northern Rough-winged Swallows, probably because I had them confused with Bank Swallows when I first started looking at birds. At my local patch, Northern Rough-winged Swallows seem to form the majority on most days. It may be that the park has better nesting habitat for them than for the other swallow species. Tree Swallows are usually the second most numerous.
When I first started birding, it was a challenge to track swallows in flight with my binoculars well enough to see enough details of their plumage to identify. It is still not easy, but I have gotten much better at that over the years. These two Northern Rough-winged Swallows, though, did not present any such challenge. One seemed to be picking up nesting material from the parking lot, while the other just rested in place.
In addition to the Tree and Northern Rough-winged Swallows, I have also seen Bank and Barn Swallows at my patch regularly, though mainly during migration. Cliff Swallows started nesting nearby (at the Route 27 bridge) a few years ago. I have not seen them yet this year, but they will probably show up soon. Yesterday, I saw a Purple Martin at the park for the first time ever. I just happened to notice one swallow that looked darker than the rest, which was enough to make me take a closer look.