Buzz
Beyond the age-old saying about early birds and worms, there's another compelling reason for birds to rise early: lingering in the nest can lead to their partners straying, resulting in the unexpected responsibility of raising another bird's offspring.
The Great Tit, a Eurasian songbird, exhibits intricate family dynamics. Males and females establish strong bonds for breeding and jointly raising their young. However, these relationships are far from exclusive. The birds often mate outside their pairs, leading to nests where offspring include chicks from multiple fathers. These extramarital encounters typically occur at dawn, when one partner is still asleep. This suggests that early risers have a dual advantage: they can engage in side dalliances and protect their own mates from neighboring birds, while late sleepers miss out on both opportunities.
To investigate how wake-up times influenced mating success and whether late risers were at a disadvantage, biologist Timothy Greives conducted an experiment to alter the birds' internal clocks. He captured several dozen male Great Tits in Germany, tagging each with radio transmitters to monitor their movements. Half of the birds received implants that released melatonin—a hormone naturally produced at night to regulate sleep and align daily rhythms with the light-dark cycle—while the other half received placebo implants. After releasing the birds, Greives tracked their activities, noting when they began their day. He also observed their nests and conducted paternity tests on the eggs to determine the success of each group in fathering chicks.
Grieves and his team discovered that the Great Tits with melatonin implants began their day approximately 10 minutes later than their counterparts. While this delay might appear minor, it significantly impacted their reproductive success. Compared to the birds with unaltered internal clocks, the late risers sired fewer offspring, both within and outside their pairs. Additionally, they were more frequently cuckolded, with a higher number of eggs in their nests fathered by other males.
These findings suggest a link between reproductive success and circadian rhythms, with the urgency of mating and producing offspring potentially influencing biological clocks. Grieves’ team now aims to explore whether similar outcomes occur when males naturally wake later without hormonal intervention. They also plan to investigate how female birds’ daily rhythms and preferences for early or late risers play a role in these dynamics.
