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Unless someone has better ideas, my guess for the specimens of 13 May is Bibio hortulanus, the specimen of 26 July resembles the B. pomonae (conspicuous red femora) and the 28 April specimen is not B. marci but I suspect B. ferruginatus.

The latter cannot be B. marci because the basal part of vein R4+5 is about equal in length to the crossvein r-m. In the right wing of the pictured fly you look slightly more to the base of the wing and you will see two veins meeting in the direction from the base of the wing to the tip of the wing. These two dark parts are of similar length. In B. marci the posterior one should be about twice as long.