Crataegus monogyna (48°11' N 16°29' E)
2011-04-25 Vienna XXII. district (Lobau national park, 155 msm Quadrant 7864/2).
This is very early for Crataegus to flower: typically, here in Vienna flowering period begins early in may (between 1st and 10th or so); and late april definitely is unusual.
German name: Einkern-Weissdorn