Besides catching the proverbial worm, there’s another good reason for birds to get up and going early in the morning: Sleeping in can give their mates an opportunity to step out on them, leaving them to care for another bird’s chicks. 

The Great Tit is a Eurasian songbird with some complicated family dynamics. Males and females form strong partnerships for breeding and sharing the job of raising their young. These are open relationships, though. The birds frequently mate outside of the pairs they’ve formed and many nests contain offspring from more than one father. These “affairs” usually happen around dawn and when one partner in a pair is still asleep, suggesting that early birds have a better chance at doing a little mating on the side and can better guard their own partners from their neighbors, while birds that sleep late miss out on both these opportunities.

To see how wake-up times affected the birds’ mating success and if snoozers were losers, biologist Timothy Greives did an experiment where he manipulated their biological clocks. He captured a few dozen male tits in Germany and tagged all of them with radio transmitters to track their location. Half the birds were also implanted with a small tube that steadily released melatonin—a hormone normally produced by the body at night to promote sleep and sync an animal’s daily rhythm with the light-dark cycle—while the rest got an empty implant. After the birds were released, Greives kept an eye on their comings and goings and when they started their day. He also monitored their nests and ran paternity tests on any eggs that appeared to see how each group fared in fathering chicks. 

Grieves and his team found that the tits with the melatonin implants woke up each day about 10 minutes later than the other birds. That doesn’t seem like a much of a delay, but it was long enough to cost them in the mating game. Compared to the tits with unaltered biological clocks that woke up at their normal times, the late risers didn’t produce as many offspring with their partners or outside of their pairs. They also wound up getting cuckolded more frequently and had more eggs in their nests that were fathered by other birds. 

The results hint at a relationship between reproduction and animals’ circadian rhythms, and the pressure of finding mates and producing offspring may help set the biological clock. Grieves’ team now wants to see if they’ll find the same effects when males naturally wake up later without any hormone manipulation, and look at how the female birds’ daily rhythms and preferences for late versus early risers affect things.