The blazing lights of Times Square in New York City may be impressive for tourists, yet this blindingly bright attraction can cause problems for urban wildlife. Artificial light at night, typical of cities and suburban areas around the globe, can cause problems for the animals that we share space with, although the impact on local wildlife is often disregarded. Given the current global COVID-19 pandemic, understanding how stressors, such as artificial nocturnal light, alter infectious disease transmission is now even more pressing. So, Daniel Becker and colleagues from Indiana University in the USA delved into this question, looking at how persistent artificial light at night alters immunity and infection risk in wild animal communities.

Becker and colleagues suspected that migratory species may be especially vulnerable to artificial nocturnal light, as they frequently migrate at night and use urban and suburban habitats as resting spots along their route. To test this theory, the team chose to study the dark-eyed junco (Junco hyemalis), a songbird which has populations that migrate and others that stay put to breed. Both of these population types are susceptible to blood-borne parasites during their summer breeding season that are transmitted by blood-sucking insects (such as mosquitoes), that initially cause a short infection, but can then remain dormant for months before rearing their ugly head when the host is either stressed or busy with other tasks, such as reproduction. The researchers suspected that the stress associated with artificial nocturnal light could make the birds more vulnerable to infection relapses, and that these effects would be stronger in the migratory birds than in their stay-at-home counterparts, as migration can suppress the immune system.

To test these ideas, the researchers rounded up resident and migratory juncos from their summer-time roosts in the Appalachian Mountains, USA, before transporting them back to an indoor aviary at Indiana University. For over 6 months, half of the migrants and stay-at-home birds were exposed to artificial light at night, while the remaining birds experienced a natural light cycle. Throughout this time, the researchers looked for signs of changes in the birds’ immunity (by looking at the number of white blood cells in their blood) as well as the risk of infection relapse, by keeping track of the number of parasites in the birds’ blood over time.

Becker and colleagues found that nocturnal artificial light increased the total number of white blood cells in both resident and migratory birds, indicating that their immune systems were surging, and when the researchers looked at the birds’ parasite numbers, these spikes weren't surprising. Following exposure to man-made nocturnal light, the number of parasites in the birds’ blood also boomed. This effect could be due to changes in the hormone melatonin, which is secreted when it is dark and boosts the immune system. Artificial light at night may suppress melatonin production, thus supressing the immune system and increasing susceptibility to infections.

The negative impacts of artificial light at night on wildlife in urban and suburban areas could be minimized by urban planners devising alterations to city lights in collaboration with physiologists. Possibilities include shifting street lighting to a yellower or redder tone, reorienting lighting structures to project light down, rather than up, and restricting the use of bright lighting at night during key times of year – such as those coinciding with annual migrations – to reduce the impacts of man-made light on nocturnal wildlife in and around our cities.