A new study showed that northern cardinals that live year-round in urban San Antonio, Texas, had eyes about 5% smaller than members of the same species from the less bright outskirts.
When we misuse or overuse artificial light at night, we create nighttime light pollution, which makes it harder to see the night sky and causes negative effects on wildlife and human health.
For example, sea turtle hatchlings usually wait until night to emerge from their nests to reduce exposure to daytime predators. Artificial lights can draw them away from the ocean. Light pollution can also alter bird reproduction, disturb wildlife habitats and disrupt melatonin production, which can affect human health. Now, we’re learning that outdoor lights at night could be a significant risk factor in Alzheimer’s disease.
Astoundingly, the bright lights of big cities could additionally be causing an evolutionary adaptation for smaller eyes in some birds. In general, smaller eyes in birds could harm their vision since larger eyes allow for better light capture, leading to improved visual acuity and sensitivity, especially in low-light conditions. Larger eyes that gather more light are crucial for seeing in dim environments or detecting subtle details. A bigger eye can accommodate more photoreceptor cells on the retina, further enhancing visual resolution. And birds with different ecological niches often have eye sizes adapted to their needs, with nocturnal birds typically having larger eyes to see better in darkness.
Nighttime light pollution, such as this in Anchorage, Alaska, may influence the risk for Alzheimer’s disease. Luckily, light pollution is a modifiable environmental factor.
We’re learning that good visual acuity is important for birds for a reason we didn’t expect: a brand-new discovery rocks our belief that only humans and great apes use visual, gestural communication. The behavior has now been documented in birds.
Nighttime light pollution contributes to Alzheimer’s disease
While legislation in some states aims to reduce light pollution, levels of nighttime light remain high in many parts of the country. Now, researchers at Chicago’s Rush University System for Health have shown that there is an association in the U.S. between Alzheimer’s disease prevalence and exposure to light at night, particularly in those under the age of 65. While it’s known that light pollution is associated with increased risk for some disorders and diseases, this is the first time it has been associated with Alzheimer’s disease.
In studying light pollution maps, the researchers, who published their findings in the journal Frontiers in Neuroscience in September 2024, looked at the Lower 48 states and incorporated medical data associated with Alzheimer’s disease risk factors and divided the groups by light intensity. In the five groups, they found that higher nighttime light intensity was correlated with Alzheimer’s disease prevalence more than any other risk factor examined in the study for those under the age of 65, even when some well-established disease factors were not. That suggests that younger people may be more sensitive to the effects of light exposure at night.
The association between Alzheimer’s disease prevalence and the exposure to light at night is especially strong in those under the age of 65.
While the cause is unknown, explain the researchers, certain genes can influence early-onset Alzheimer’s, and these same genes may cause increased vulnerability to the effects of nighttime light exposure. Additionally, younger people are more likely to live in urban areas and have lifestyles that may increase exposure to outdoor light at night.
Exposure to light influences the body’s natural sleep-wake pattern, which is called “circadian rhythm.” Your circadian rhythm can be disrupted by exposure to light at night, which can promote inflammation and make you less resilient and more prone to disease.
The good news, say the scientists, is that simple changes can be made with minimal effort to reduce exposure to light at night, such as adding blackout curtains or sleeping with an eye mask. They conclude that further testing is needed to better understand the connection between evening outdoor light and Alzheimer’s disease.
Canada and the U.S. have lost 29% of their bird populations since 1970. The birds that have dwindled the most include blackbirds, finches, meadowlarks, sparrows and warblers. The common house sparrow (shown here) is at the top of the list for losses.
Urban light pollution links to smaller eyes in birds
We humans aren’t the only ones being changed by nighttime lights.
Canada and the U.S. have lost 29% of their bird populations (or almost 3 billion birds) since 1970. Scientists believe that habitat fragmentation is the primary driver of the decline in birds, but a new study suggests that sensory pollutants, such as human-caused light, may also play a role in the birds’ ability to cope with city life.
To find out, researchers from Texas A&M University–San Antonio studied more than 500 birds from central and edge areas of San Antonio. They compared body and eye sizes of the birds and analyzed light and noise measurements during the day and night of each area.
Along with northern cardinals, Carolina wrens that live in urban San Antonio, Texas, were found to have smaller eyes than their cousins from less bright areas. Birds with bigger eyes can be somewhat blinded by the glare of city lights or be unable to sleep well, putting them at a disadvantage in urban centers.
Their findings, published in the journal Global Change Biology in September 2023, showed that there was no difference among the body sizes of birds in different areas, except for one species: the painted bunting. Upon further analysis, the researchers found that this size difference was mostly due to age. Younger, smaller male buntings, who cannot compete as well for mates as their more colorful elders, were more often found in the brighter, noisier central locations, which are likely less desirable.
However, two common songbirds, the northern cardinal and the Carolina wren, that live year-round in the urban core of San Antonio, Texas, had eyes about 5% smaller than members of the same species from the less bright outskirts. Researchers found no eye-size difference for two species of migratory birds, the painted bunting and the white-eyed vireo, no matter which part of the city they lived in for most of the year.
It’s postulated that the smaller eye size may enable birds to deal with the brighter and more constant light in city environments. Birds with bigger eyes can be somewhat blinded by the glare of city lights or be unable to sleep well, putting them at a disadvantage in urban areas.
In the study conducted in San Antonio, Texas, there was no eye-size difference for migratory birds, such as painted buntings.
The researchers say that their results have implications for conservation efforts amid the rapid decline of bird populations across the U.S. Residential birds may adapt over time to urban areas, but migratory birds are not adapting; probably because in the locations where they spend the winter, they are less likely to have the same human-caused light and noise pressures. That may make it more difficult for them to adjust to city life during the breeding season.
Understanding what the benefits and costs are for animals living in a sensory polluted world will be important for any protection efforts.
Birds communicate visually with gestures
A thumbs-up, a wave goodbye or pointing out a book on a shelf. These gestures and many more are an integral part of how we communicate. Such gestures were once thought to be used exclusively by humans, until closer observations of great apes—such as bonobos and chimpanzees—revealed that they, too, move their bodies to communicate nonverbally.
Ravens use “deictic gesturing,” meaning that they use their beaks to point out objects and direct the attention of other ravens to specific things, similar to how humans point fingers. This behavior is considered a sophisticated form of communication in birds.
In more recent years, studies on other animals, such as ravens and fish, have shown that they also use simple gestures to, for example, point out objects or show something of interest, called deictic gesturing. However, symbolic gestures, such as showing an open hand to signal “after you,” require complex cognitive skills; and there was no conclusive evidence supporting the existence of such abilities in animals other than humans.
That’s why researchers at Japan’s University of Tokyo were surprised to find clear evidence of symbolic gesturing by a small, wild bird, the Japanese tit. These birds not only use specific calls to convey particular meanings, they also combine different calls into phrases using syntactic rules. These diverse vocalizations led the scientists to initiate research into their potential use of physical gestures.
In spring, Japanese tits form mating pairs and build their nests inside tree cavities with small entrances. The University of Tokyo scientists were able to observe the behavior of 16 parent birds (eight pairs) breeding in nest boxes. The researchers found that when feeding their young, the birds enter the nest one at a time. When carrying food back to the nest, the birds would often find a nearby perch first, and then one would flutter its wings toward the other. By analyzing over 320 nest visitations in detail, the researchers saw that the wing-fluttering display prompted the mate who was being fluttered at to enter the nest box first, while the one who fluttered entered second, determining the order of nest entry and mirroring the “after you” gesture observed in human communication.
Japanese tits use gestures to communicate with their mates. One bird will flutter its wings at the other, meaning “you enter the nest box first,” thus mirroring the “after you” gesture in human communication.
The scientists were surprised to find that the results, published in the journal Current Biology in March 2024, were much clearer than what they had expected. Japanese tits were seen fluttering their wings exclusively in the presence of their mates; and upon witnessing this behavior, the mates almost always entered the nest box first. The gesture was performed more often by female birds, after which the male usually entered the nest box, regardless of which had arrived first. If the female didn’t flutter her wings, then she usually entered the nest box before the male.
The researchers believe this behavior fulfills the criteria to be classified as a symbolic gesture because it only occurred in the presence of a mate, it stopped after the mate entered the nest box, and it encouraged the mate to enter the nest box without any physical contact. They also noted that the wing-fluttering “after-you” gesture was aimed at the mate and not the nest box, meaning that it wasn’t being used as a deictic gesture to indicate the position of something of interest.
There is a hypothesis, say the researchers, that walking on two legs allowed humans to maintain an upright posture, freeing up their hands for greater mobility, which in turn contributed to the evolution of gestures. Similarly, when birds perch on branches, their wings become free, which may have facilitated the development of gestural communication.
It’s hypothesized that walking on two legs allowed humans to maintain an upright posture, freeing up their hands and contributing to the evolution of gestures, such as thumbs-up or thumbs-down. Our fellow more-than-humans could be more like us than we realize.
Illumination on light pollution
We’ve known for some time that nighttime light pollution can harm us. We’re just finding out, though, that our nighttime lighting could have some unintended consequences for birds, too, such as changing their morphology. While it’s not clear what outcomes these adaptations will have for birds down the road, considering that urban environments aren’t going away anytime soon, it’s important to understand how to manage such habitats for our neighborhood birds, as well as for those that might not be urban adapted.
And in another, new revelation on sight, being able to see gestures is more important for birds than we thought.
I hope that we have the foresight to use these illuminations on light pollution to make the world just a bit better for own health and for that of our fellow more-than-human travelers in time.
Here’s to finding your true places and natural habitats,
Candy
