The tale of the city bird and the country bird

Once upon a time, time on our planet was divided by light. During the daytime, animals could forage, predators could hunt, and trees could grow. As day turned into night, some animals slept, while others, having slept while their predators prowled, went out fully rested in search of food. Whether diurnal or nocturnal, life on planet Earth was, at least in part, determined by the celestial dance between the sun and moon, with ourselves caught in between.

That predictable pattern, the constant shift between light and dark and light again, is incorporated into the very fabric of our behavior and biology. Our circadian clocks become entrained to the passage of each day according to zeitgebers, environmental stimuli that signify the passage of time. One of the most important zeitgebers is simply the presence or absence of daylight.

But light is no longer a useful way to tell day from night, thanks to the invention of the electric light bulb. Artificial light at night (LAN) diffuses in the atmosphere and in some cases makes the sky brighter than it would be even on a full moon night. It’s as if the night, in those places, is locked into a constant, persistent twilight. “This loss of the night has serious consequences for almost all groups of animals, including humans,” write researchers Anja Russ, Annika Ruger, and Reinhard Klenke, in a recent issue of the Journal of Ornithology. And that leads to changes in physiology and behavior. Our biological clocks have a hard time identifying the shift from day to night in a world of persistent illumination. Some animals become disoriented while they migrate; for others, LAN means habitat loss. A few animals can take advantage of lights, such as by feeding on swarms of insects gathering around artificial lights, or by extending their foraging opportunities into the nighttime hours. But even then, those shifts can upset the delicate balance of their ecosystems.

Light at night is a ubiquitous feature of our planet now, but it’s perhaps worst in cities. As a result, populations of a single species can have different physiologies and behaviors, depending on whether they’re living in the city or in more rural areas. In just two hundred years, blackbirds have become quite well adapted to city life. But how? That’s why Russ and her colleagues turned to the blackbirds of Leipzig, Germany. It’s a city that has quite a built-up area, but also a fairly natural riparian forest running through the center. They looked at birds in the forest and in the city, as well as in urban parks, which represented a sort of middle ground between the two.

They discovered that one of the primary reasons that European blackbirds can thrive in cities is because they can extend their foraging activities into the night, which they say is “probably” due to the increased amount of artificial light available in urban environments throughout the night. The difference in terms of the end of foraging activities between urban and forest blackbirds was most evident in late winter and early spring, ostensibly because the day is so much shorter during that period. During the summer months, the difference was less evident, because the forest birds had so many more daylight hours to forage.

Consistent with the additional foraging opportunity, urban blackbirds’ body condition was better during the breeding season as well. Interestingly, that was not the case during the rest of the year. That led the researchers to wonder why the urban birds spend so much more time foraging, compared to their country counterparts, if that’s not reflected in their body condition. They suspect that it’s not their extra foraging time itself that allows city birds to become healthier, but other factors, such as the availability of food or reduced predation risk. – Jason G. Goldman | 07 November 2014

Source: Russ, A., Ruger, A., & Klenke, R. (2014). Seize the night: European Blackbirds (Turdus merula) extend their foraging activity under artificial illumination. Journal of Ornithology. DOI: 10.1007/s10336-014-1105-1.

Image: ©camdoc3/