There is no shortage of folks in the world who claim that the Beatles’ music was often prophetic, but I wonder if even they had any idea of the growing importance of those lyrics from one of my favorite songs of theirs - “Blackbird singing in the dead of night….”
It is growing more common these days for one to hear the songs of some unexpected songbird species well into the dark of night. In Europe, common nightingales were traditionally known for this behavior, but today, with their populations seriously declining, their night songs are being replaced by those of European robins, common blackbirds and song thrushes. The nightingale’s nocturnal-singing counterpart in North America is the northern mockingbird, but today it is often joined by the American robin and, where it is found on the continent, the Nuttall’s white-crowned sparrow. Thus, the dark of night is no longer owned by the owls, nightjars (e.g. whip-poor-wills, etc.), and night-herons.
This is not happenstance. Nor is it merely a function of the light emanating from a full moon as often seen in the mockingbird’s case. Sadly in today’s world, the ambient light is much brighter than normal due to various sources of anthropogenic light, e.g. streetlights, sports stadiums. It is not always just singing behavior being affected either. The bright lights of the ballpark used by the San Francisco Giants apparently attracts ashy storm-petrels from their nests on the Farallon Islands, some 30 miles away. Speaking of such well-lit sports stadiums, I will never forget being in the stands of a late evening game of the Montreal Expos (Rest In Peace!) and being more entertained by an American kestrel flying about the bright lights of the Olympic Stadium hawking for moths and other large flying insects. While hunting at dusk is rather common for many falcon species throughout the world, peregrine falcons breeding in our brightly lit cities are regularly hunting in the twilight hours, and in recent years, ospreys have been recorded migrating in the dark of night. The latter behavior though has long been documented in songbirds and scientists regularly use the silhouette of the moon as a background for their spotting scopes to count migrating warblers, sparrows, thrushes and other birds.
Thanks to startling photographs taken by NASA, we now know that two-thirds of the world is seriously affected by ‘light pollution’. In the contiguous United States as well as the European Union, city lights can be almost five times brighter than the natural night sky. Today, one has to travel much farther north into the boreal forests and tundra of northern Canada or Alaska to catch glimpses of the Milky Way, a phenomenon that city-living folks never get to see. Some scientists now refer to it as the “Loss of Night”.
So, with all of this light emanating from our myriad streetlights, shopping centers, stadiums, skyscrapers, and houses turning night into day, how significantly is this pollution affecting the singing, mating and foraging behavior of those birds adapted to a day-time existence? For example, if a songbird is expending energy singing in the night instead of resting, will it still have enough energy left over to carry on with its territory defense, mate attraction, food gathering, and raising of its young during the daylight hours?
You see, just like we humans, birds have a circadian rhythm or internal body clock that affects their behavior. The seasonal change in day length is a key environmental signal in controlling daytime rhythms (e.g. sleep-wake cycles) and seasonal ones (e.g. breeding season). Based on ambient light levels and using light receptors in their retinas and in their pineal glands, as well as in other parts of their brains to detect them, birds know when it is time to mate, breed, forage or migrate. The pineal gland in particular secretes melatonin, the so-called “jet-lag hormone”, at night, which guides the internal biological clocks controlling body function, growth and behavior.
Breeders of poultry and even wild bird species such as northern-nesting gyrfalcons have long known how ambient light affects breeding cycles and they regularly use artificial lighting to stimulate both the onset of breeding and egg production. It stands to reason that if the natural day and night rhythms of wild birds are also affected by artificial light, then their natural behavioral patterns may also change.
To better understand the impact of light pollution on wild birds, scientists in Europe have turned to the blackbird. Originally a forest dweller, this songbird species has adjusted well to conditions in the city since the early 19th century and is now widespread in cities and easily identifiable through its distinctive song.
As part of a research program called "BILL" (Birds in ILluminated Landscapes), a working group led by Dr Reinhard Klenke at the Helmholtz Centre for Environmental Research and the Leipzig University captured 200 blackbirds in an area of roughly 50 acres in the area of the city of Leipzig, Germany. The study area spanned a 3-kilometer strip stretching from the well-lit city center to the more naturally lit areas in a flood-plain forest. The birds were measured, weighed, leg-banded for individual identification, released, and observed.
On the short days in March, the blackbirds in the forest stopped foraging almost one hour before their counterparts in the well-lit city. The longer the days grew, the smaller the difference became. In the illuminated city centre, the males were considerably more likely to be last to leave the foraging grounds, while in the forest, there were no differences between males and females. Male blackbirds tend to be slightly larger than females, including their eye size. With superior visual capabilities in poor light conditions, males could more easily forage for food in the twilight than females and thus, remain active in the twilight much longer.
So what levels of light intensities are the birds actually experiencing at night? A group of scientists working with Jesko Partecke from the Max Planck Institute for Ornithology in Radolfzell, Germany tagged several urban common blackbirds with light loggers to measure the average light intensities the birds were exposed to during the night. According to the study, the intensities were very low, i.e. about 0.2 lux, which is just about one-thirtieth of the light emanating from a typical street lamp.
Even so, these low values were still enough to cause the testes of male blackbirds to mature earlier. In a subsequent study, Partecke’s team exposed wild-caught city and forest blackbirds to lighting intensities of 0.3 lux at night for a period of ten months. As a result, the birds’ gonads grew on average almost a full month earlier than those allowed to sleep in the dark! They also measured the level of testosterone in the birds' blood as another indicator of their readiness for breeding and found that the level rose earlier if the birds had been exposed to light at night. Even singing behaviour got out of rhythm as a result of the low night-time light intensity, i.e. the birds began singing around one hour earlier. In short, the blackbirds exposed to night-time light are ready to breed earlier. Not only that, they also moulted their feathers a lot earlier towards the end of the breeding season than birds that spent their nights in the dark.
What causes the advanced onset of breeding? Possibly, the artificial night-time light has the effect of extending the day length for the animals. Alternatively, perhaps the light makes the birds continue foraging for food at night and put the additional energy into reproduction. The light could also influence the animals' metabolism to the point of causing earlier gonad growth.
Nor is it clear whether the city blackbirds' advanced breeding offers an advantage or whether it is merely an unintended side effect of the lighting. While the birds might be able to breed earlier in the year due to the artificial light and produce more young in a year as a result, this will depend on whether the nestlings have access to enough food. Otherwise, the advanced onset of breeding could turn out to be an evolutionary step backward for the blackbirds.
From a physiological point of view, how exactly is the low level of light at night affecting these behavior changes? Some scientists have postulated that this may be due to an inability of the birds to detect how long days are, but the mechanism behind that is not yet known. Davide Dominoni, another scientists at the Max Planck Institute for Ornithology, and his colleagues tested how melatonin levels were affected by artificial light levels in the blackbirds. They exposed birds to low intensity light at night and looked at whether this altered the nocturnal production of melatonin and activity compared to birds that were exposed to near darkness at night. Daily patterns of melatonin concentrations were decreased in the birds exposed to low intensity light at night in both summer and winter. Combined with the observations of altered activity during the night, Dominomi’s research team suggested that the light-induced decreases in melatonin production could lead to an altered perception of day length, causing the birds to behave as if they were exposed to longer days when compared to birds kept under dark nights.
It is one thing to measure the impact of light-at-night on bird behavior in naturally dark and “bright” habitats, but how about using powerful LED lights to purposefully illuminate large areas of forest as a means of actively manipulating ambient light conditions?!
Katharina Mahr and Herbert Hoi from the Konrad Lorenz Institute of Ethology at the Vetmeduni Vienna are doing exactly that in the Viennese forest, using not blackbirds but blue tits as the model species because they have been so well studied over the decades and also frequently breed in cities where they are exposed to artificial light. Over a 3-week period at the beginning of 2014, LED lights illuminated various areas of the Viennese Forests for two additional hours in the morning, before sunrise, and in the evening after sunset. Meanwhile, Mahr’s team are examining activity patterns such as singing and mating behavior, growth and development of the nestlings, as well as levels of stress hormones. They wonder whether light at night will affect the birds' strategies of choosing partners. Since males like to be in the "limelight" whereas females might prefer to remain in the shadows, the increased ambient light could affect the love lives of the two sexes. Male blue tits are also known to be "morning singers" and particularly fit males begin warbling before dawn even unveils itself. And since female blue tits tend to be sneakily unfaithful to their partners, how will the light-at-night affect that behavior? Basically, Mahr and Hoi want to determine whether artificial light generates a certain conflict between the sexes to the point of impairing reproduction. And what about the fledglings? Will the longer daylight lead to shorter rest periods and impose additional stresses on them? Perhaps even more important, what impact will the artificial light have on insect populations in the Viennese woods? They are a critical source of food for not just the tits but myriad other species. Stay tuned.
European scientists are not the only ones studying the effects of light pollution on birds. As recent as June 2014, a note published in the Wilson Journal of Ornithology by Christine Stracey of the University of Florida in Gainesville and two associates described a study of northern mockingbirds rearing their nestlings in various local sites which varied in the level of artificial light. The pairs in well-lit parking lots fed their young 15 minutes later than the other pairs, suggesting that this species at least can actually exploit light pollution.
While that might at first blush seem like a good thing in the case of those mockingbirds in Gainesville, most scientists will agree that this growing form of pollution poses a serious threat to the reproductive performance of many birds species affected and to ecosystems in general. To my knowledge, no one has yet studied the impact of the lengthened day time and thus, longer foraging periods on the various prey organisms eaten by birds. And what about predation pressure on the songbirds? Will those songbirds being more active at night be more vulnerable to nocturnal predators, both winged and four-legged?
Most important, if our birds are being affected through hormonal changes caused by light pollution, the chances are good that we humans are also not immune to its effects.
So what can one do to help out?
First, reduce the light escaping from your own home, cottages, and camp sites by using dimmer switches and black-out blinds and just turning off lights that are not really needed.
Second, encourage your employers and fellow employees to turn off unnecessary lights at work.
Third, for business and travel, boycott companies that use excessive lighting and frequent eco-friendly establishments sensitive to the welfare of birds and other wildlife.
Fourth, get involved in municipal politics to reduce lighting consumption in government-run facilities (how many times have you seen sports fields brightly lit through the night with nary a human using it) and lobby for lights-out projects to enhance public awareness.
Fade to black.