Given a steep slope, would the bees be able to forage at the top of the hill? Or would they get altitude sickness?
Given a multi-storey building and a lift. Do you feel any different when you get up to about the third or fourth floor? No, I didn't think so. Some could even use the stairs and feel OK at the 4th floor!
Why would flowering plants have insect pollinated flowers that high if insects were not able to fly that high?
Bees can forage a couple of miles in the horizontal, so I would think 15m vertical should be little trouble to them.
Do you know, you were right when you said "Why on earth wouldn't they?". It's obvious really!
Bumblebees have been discovered living at over 5,000m and tests in a chamber found they could still fly at a simulated altitude of 9,000m.
For honey bees I think, within limits, they can fly as high as they need to, i.e. to reach the top of a multi-story building if that is where their hive is or to the the top of a tree if there were flower or aphids to milk for honeydew.
My house overlooks the head of a valley and I have been watching bees spiral up from the hive for about 10m then fly horizontally across the lttle valley at which point they are at least 50 or 60m above the ground.
I don't think they have any trouble reaching the top of any tree in the UK.
I have a similar experience to Rooftops: my bees are in a small patch of woodland, surrounding by fairly tall trees. Instead of flying out through the gaps between trees, they seem to spiral up from the hive to the height of the tree tops (20m?) before disappearing to find their forage.
My physics textbook tells me they might be using a bit of extra energy doing this, but they seem to prefer it as a navigation route so it can't cost them very much.
Keithgrimes - don't be put off asking questions by the occasional short answer. I'm a beginner too and the folks here are usually very constructive. Well worth brushing off the odd grumpy answer for.