The big tree in the background, the bees have already collected pollen from its catkins. Now I can hear them in that tree and around the crab apple in the foreground. The bees I see just look like they are walking on the leaves. I can’t see any holes in the tree where there would be a nest so, are they collecting propolis?
Hard to know for sure, but you could look at their pollen sacs (corbiculae) as they return to the landing board. If they are stuffed with dark golden to brownish material, it may well be propolis. Nothing that you can do about it, because bees will be bees, as Winnie-the-Pooh knows! 
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