The biggest bees, and some of the most experienced foragers (measured by number of trips out), were the most likely to leave in low light.
Bumblebee vision is poor in low light, so flying at dawn or dusk raises the risk of getting lost or being eaten by a predator.
However, the bees benefit from extra foraging time and fewer competitors for pollen in the early morning.
"Larger bumblebees have bigger eyes than their smaller-sized nest mates and many other bees, and can therefore see better in dim light," said lead author Katie Hall, of the University of Exeter.
"We might expect all bumblebee foragers to leave the colony to forage as soon as there is enough light to allow them to fly.
"In fact, colonies seem to regulate the start of foraging.
"There is a balance of risks and rewards in low light – and most bees wait for higher light levels when they can see better and fly faster, with less risk from predators or getting lost and running out of energy.
