I always remember reading that bumblebees like to specialise on one plant at a time, say only feeding from lavender for a few weeks at a time.
I just noticed a queen B.hypnorum going from feeding on jostaberry to blackcurrant (which are similar) but then saw one go from edible honeysuckle to blackcurrant which are quite different shaped flowers. Is this just a queen behaviour when flowers may be in short supply?
Although bees are supposed to show what is known as “Flower Constancy” this is much stronger in honey bees than bumblebees.
If you watch them foraging, I think you’ll find that bumblebees of both sexes and both workers and queens are likely to nip over to try out a nearby flower of different type as they forage. Much of this will be for nectar, but with pollen collecting a way of telling is supposed to be looking for pollen loads that have different colours in them - like stripes.
I’ve not seen this yet, but have seen photos of it.
By bringing back pollen from different sources bees widen their diet and so their protein base.
Honey bees are extremely flower constant, but even in them a few bees do try out different flower types on a foraging trip. If they find a good source they will come back and communicate it to their colony and a foraging switch may occur.
There is much detail about bumblebee foraging in Dave Goulso’s book Bumblebees; and someone called Heinrich wrote a famous book called Bumblebee Economics, where he investigates the energy balance of foraging.
So, keep watching, but maybe also have a read too !
I hope this helps. Yours Clive