When we think of the night, we think of the sun setting and creating darkness. But today, our cities and towns create a lot of light that makes the night not so dark. Even when we turn off the lights in our houses, there are street lights, building lights, lights in parks, and so many more sources of light. These sources of human made light are called artificial lights. In our own houses, we can turn off the lights and close the blinds to sleep, but how is the sleep of other animals affected by the artificial lights? Scientists in Scotland tried to answer this question by looking at the Great Tit—a bird that can live in both cities (that have artificial light) and in forests (that do not have artificial light).
The pattern of sunrise and sunset provide a natural clock that helps tell animals, including humans, when to be awake and when to sleep. So when artificial light makes it still light outside when it is supposed to be night, scientists think that birds might behave differently. Sitting on their eggs, or incubating, is really important behavior for female Great Tits that helps baby birds be healthy. At night, the birds sit on their eggs while they sleep. But if artificial light makes the night shorter, do birds incubate their eggs for less time? In this experiment, scientists had three different groups of birds: 1. birds in the forest; 2. birds in the forest with artificial light shined on their nests; 3. birds in cities. Then, the scientists watched and recorded the behavior of birds when the sun rose and when the sun set to see if any of the groups of birds behaved differently.
The scientists discovered that urban birds and the forest birds exposed to artificial light moved around earlier in the morning than the normal forest birds—it is like the birds exposed to light started their morning before the normal forest birds who wake up due to the sunrise. In the night time, normal forest birds went to bed earlier as their eggs got older and closer to hatching. But for the urban birds and forest birds exposed to artificial light, their nighttime routine did not change at all. So while the forest females prepared for their new babies by sleeping longer, the other two types of birds did not change their behavior. The scientists also measured the temperature of the eggs the birds were sitting on. Over the night, the egg temperature for all the birds changed a little bit. But for the forest birds exposed to artificial light, their egg temperature varied a lot more, probably because they moved around more than the other birds. The scientists have two predictions for why this might happen. 1: The artificial light was in the nests of the forest birds but the artificial light was outside of the nest for the urban birds; think of the difference between having a light on right next to your bed and having a light on outside of your room but your door is closed. Or 2: the urban birds are more used to having artificial lights than the forest birds.
Overall, it is really important to understand what impact we as humans have on other animals around us. The results of this experiment show that the light we create can have big effects on birds. Keeping a normal pattern of temperature on eggs before they hatch is really important for making sure the baby birds will be healthy, so this experiment shows one way that light might be changing how birds behave.
McGlade, C. L., Capilla-Lasheras, P., Womack, R. J., Helm, B., & Dominoni, D. M. (2023). Experimental light at night explains differences in activity onset between urban and forest great tits. Biology Letters, 19(9), 20230194.