Having Friends Makes Crows More Resilient to Light Pollution

Featured image caption: The Indian house crow (Corvus splendens) is a social species of bird that is negatively impacted by light at night (Image source: “House Crows at the Gateway of India” by Mustang Joe, CC0 public domain, via Flickr).

Source article: Buniyaadi, A., Bhardwaj, S. K., & Kumar, V. (2025). Conspecifics confer survival advantage in the face of a light-polluted environment: Evidence from melatonin secretion, sleep, mood, and cognitive performance in Indian house crows. Hormones and Behavior, 174, 105793. https://doi.org/10.1016/j.yhbeh.2025.105793

Bright nights can disrupt wildlife

When you think of pollution, you likely picture smog in city skies or chemical runoff in rivers and oceans. Light pollution—excessive artificial light at night—may receive less attention than other forms of pollution, but it can also have harmful impacts on wildlife. Nocturnal light from streetlights, buildings, and other sources can disrupt the sleep, physiology, and behavior of city-dwelling animals.

Why is light at night so bad? Animals (humans included) have an internal clock called the circadian rhythm. This clock is tuned to environmental signals, including light, food, and social interactions. Light pollution can disrupt this rhythm. For example, when it gets dark at night, the body releases the hormone melatonin, which promotes sleep. But with light pollution, nights aren’t truly dark, meaning that melatonin and sleep can be disrupted.

In a recent study published in Hormones and Behavior, researchers investigated whether an animal’s social environment influences its response to light pollution. Scientists from the University of Delhi and Meerut University sought to answer two key questions: (1) how does light pollution affect physiology, behavior, and gene expression, and (2) can living in groups mitigate some of the adverse effects of light pollution? The researchers predicted that social cues from group members could help animals adjust their internal clocks and adapt to light pollution. 

In a challenging light environment, crows in groups do better than solitary crows

To investigate whether group living may mitigate the effects of light pollution, researchers exposed Indian house crows (Corvus splendens) to different light and social conditions. House crows make a great model system for this study because they are highly social, typically feeding and roosting in large groups, and they often live in urban environments where light pollution is common.

Researchers exposed the crows to a normal light condition (12 hours light, 12 hours dark) or a light pollution condition that mimicked the nighttime light environment of a university campus (12 hours light, 12 hours dim). Within each light condition, crows were split into one of two social conditions: housed alone or housed with two other crows. This allowed researchers to investigate the interaction between the light environment and the social environment. 

After 10 days, researchers quantified the effects of these conditions on several metrics: the amount and quality of sleep, mood, cognitive performance on a problem-solving task, and blood melatonin levels. Researchers also collected the crows’ brains to quantify the expression of several candidate genes that regulate sleep, mood, and cognition.

As the researchers expected, light pollution had several adverse effects on the crows. Crows in dim light had poorer sleep quality and lower melatonin levels at night than those in the dark condition, suggesting that light pollution suppresses normal melatonin secretion and disrupts sleep.

Interestingly, light pollution also disrupted the crows’ behavior, but only when they were housed alone. Solitary crows in dim lighting exhibited a depressed mood and reduced problem-solving ability. Mood and cognition were not affected by light pollution when crows were kept in groups, even though their melatonin levels remained irregular, and sleep was impaired. This suggested to researchers that something about group living may support the crows’ emotional state and cognition even when the crows didn’t sleep well.

Social enrichment may boost mood and cognition via effects on the brain

How can the social environment improve mood and cognition? The gene expression data revealed two potential molecular pathways that may help to explain why group-housed crows did better when faced with light pollution.

Researchers found that solitary crows in dim light showed reduced expression of genes involved in dopamine synthesis and signaling in the brain, such as th and darpp-32. Dopamine is a neurotransmitter that influences motivation and mood; therefore, a reduction in dopamine could contribute to the crows’ depressed mood. These crows also had reduced expression of genes such as dcx, which is involved in synaptic plasticity and is crucial for learning and memory. A reduction in the expression of dcx could contribute to impaired cognition. 

Crows in groups didn’t show reduced expression of these genes even when they were housed in the dim light condition. Why not? The researchers proposed that social interactions could have a positive effect on the brain, promoting the expression of these genes and helping maintain normal mood and cognition despite the challenging light conditions.

This study presents an interesting example of how, by living in groups, animals can protect their cognitive and emotional well-being in the face of light pollution. It will be interesting to see in future research whether social interactions can buffer animals against other anthropogenic challenges as well. 

What can you do to reduce light pollution?

Streetlights and other nighttime lights are essential for safety, whether you are driving home from work late at night or walking up the steps to your home. Because of this, we can’t eliminate all lights, but there are ways we can modify lighting to reduce the potential harm to animals. Lights that are particularly bright and illuminate a broad area are the most problematic. More targeted lights, for example, motion-activated lights with narrow beams, are less harmful. By making small, thoughtful changes to lighting, we can mitigate the adverse effects of light pollution on wildlife. 

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