Larger bumblebees are more likely to go out foraging in the low light of dawn, new research shows. University of Exeter scientists used RFID - similar technology to contactless card payments - to monitor when bumblebees of different sizes left and returned to their nest.
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.
"Our finding that more experienced bees are more likely to fly in lower light suggests that knowledge of food locations helps them navigate safely."
The study tracked the bees' behaviour over five days during warm periods of the flowering season.
Only a small proportion of foragers left the colony at dawn when light levels were below 10 lux.
Katie Hall's work was funded by the Biotechnology and Biological Sciences Research Council - South West Biosciences Doctoral Training Partnership.
The study was carried out as part of a wider collaboration with the South Devon Area of Outstanding Natural Beauty.
The paper, published in the journal Ecology and Evolution, is entitled: "Onset of morning activity in bumblebee foragers under natural low light conditions."
Ecology and Evolution