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Light pollution has more dramatic effect on circadian rhythms of social birds than isolated birds, study finds

Light pollution, or artificial light at night (ALAN), is a widespread phenomenon in areas with dense human populations. Normally, animals use natural external cues, like sunlight and temperature, to synchronize their biological rhythms with the day-night cycle. However, ALAN is known to affect the biological rhythms of animals living within its range by altering physiological, molecular and behavioral mechanisms related to sleep-wake cycles (circadian rhythms).

In a , published in the Proceedings of the Royal Society B, researchers found that ALAN impacted the circadian rhythms of zebra finches (Taeniopygia guttata) more heavily when they were placed in groups, compared to isolated birds.

Although it was known that social interactions often affect the biological rhythms of many organisms, prior studies on the effects of ALAN have focused mostly on isolated animals. This study provides more insight into the effects of ALAN in a social context. This is particularly useful, since many animals, such as zebra finches, are more likely to live in social communities in the wild.

The experiment consisted of 104 birds split up into two groups. Several of the birds died throughout the study, but this did not have any lasting effects on the experiment. One half of the birds were kept alone in isolated cages and the other half were kept in small groups of six, with three males and three females per group.

For the first three weeks, all birds were subjected to 12 hours of light and 12 hours of darkness. Then, each group was again split into two approximately equal groups, with two groups (one isolated, one social) serving as a control and the other groups (one isolated, one social) being subjected to ALAN of around 5 lux at night (compared to 675 lux for daytime light) for 10 days.

The researchers monitored the birds throughout the night with video and recorded hop activity with a mechanized perch. They found that, while both ALAN groups showed increased activity compared to birds with dark nights, the social groups showed significantly more activity and became active earlier. Changes were also found in gene expression associated with circadian rhythms in the hypothalamus and liver, which correlated strongly with changes in activity in the ALAN groups.

Although the study showed marked disruption in circadian cycles, there was no significant change in levels of melatonin, a hormone important for the regulation of the sleep-wake cycle. The researchers also note that there was some degree of acclimation to ALAN over time, but that significant disruption was still occurring by the end of the ten days.

More research is needed to determine the long-term effects of ALAN in a social context and to study the effects at different levels of ALAN light intensity. The study authors write, "We used a light intensity of 5 lux, which is at the higher end of ecologically relevant light exposures, and using dimmer light intensities will likely result in differences in behavior, gene expression and physiology."

Future studies like this can aid conservation efforts to mitigate the effects of light pollution on the surrounding ecosystems by better understanding its effects.