Poop ?

Sounds like insect poop.  Theirs tends to be mustardy coloured.  If you look on your windows or the cills there may be mustardy dots and this is insect poop.    They have to go to the loo somewhere and why not in mid flight eh!   maralyn

Speaking as a beekeeper, your description sounds similar to the one product of apis mellifera which has nil commercial value. So I think you're right to suspect it's an insect of some sort. No idea what kind, however. Al

I'm afraid there's really nothing (short of shooting down every wild and domesticated bee overflying your terrace) that you or the beekeeper can do about this. Bees have an instinct not to void their bowels in or near the hive, but I've not heard of anyone ever researching exactly what prompts them to do their business where they do it. All I can say is that, while it does look distasteful, it is only "processed" pollen and flower nectar! While the feces of bees that are suffering from some bee diseases can contain microorganisms which will infect other bees with the disease, even this poses absolutely no risk to human health. Perhaps the best you can make of the situation would be to have a word with the beekeeper in a lighthearted way and suggest that the bees owe you a bit of compensation in the form of a jar of their honey. Al