As we start to welcome migratory birds back to our shores, it’s a moment to consider the miraculous phenomenon of migration, and how and why it happens. One question that we’re often asked is: how do birds know when’s the right time to migrate?
Here Ranger Eloise introduces the world of biological clocks, and how they play such an important role in a bird’s 24-hour cycle.
When birds migrate, they need to start replenishing their energy as soon as they arrive at their destinations. Too early or too late, and they won’t be able to find enough food to sustain themselves. Timing is a matter of life and death.
Every winter, the Solent sees 125,000 migratory birds arriving to feed along the coast.
Black-tailed godwits are just one of the species that you will find there. As a colony, they split up over the winter and, on average, spend the colder months about 1000km apart. Yet, at the end of the season they manage to reunite with their life-long mate back in Iceland arriving within a few days of each other.
This incredible precision really does spark the question… how do the birds know when it’s time to migrate?
This process is down to a fascinating phenomenon, seen in all living organisms, known as biological clocks.
These are the internal timekeepers of daily and seasonal events.
How we stick to a 24-hour cycle, how crabs navigate the correct tide times, and how birds and butterflies know when to migrate, are all controlled by biological clocks.
Birds have three independent clocks that form a centralised clock system: the input, the pacemaker, and the output:
- The input to a bird’s clock starts with the retina of their eyes, constantly keeping track of environmental changes like daylight. The gradual change in day to night length throughout the year is a major factor in deciding when to migrate.
- As we head into our winter months and daylength gets shorter, this information is passed onto the bird’s pacemaker. In birds, it’s a special gland that sits in the middle of the brain. It takes all this information on day and night cycles from the eyes and uses it to initiate a release of hormones, such as melatonin, to make them tired.
- This then causes an output to the rest of the body, a circadian rhythm, that brings about physical or mental changes to the bird’s behaviour over a 24-hour cycle.
When environmental cues pass a certain threshold, birds go through a period of migratory restlessness.
Also known as ‘zugunruhe’, from the German words ‘zug’ meaning move or migration and ‘unruhe’ meaning anxiety or restlessness, it is the change to when a bird is most active. In a lot of songbirds, for example, they are typically more active during the day.
But, under the circumstances of zugunruhe, their behaviour shifts to being more active at nighttime, where they become restless and more energised to start their migrations.
This change is ultimately the leading cause of when birds start their long migration pathways to the UK, initiated by the clever ways of biological clocks that feed off environmental changes, such as light, temperature and the Earth’s magnetic field.
Birds rely heavily on this mechanism to ensure survival and, with the change in climate, it is still uncertain as to whether this will have an impact on migratory animals in the future.
One thing that is for certain is that these clocks are fundamental in, not just understanding birds, but all living organisms, and how we navigate our day to day lives.
