Wetenschap - 30 juni 2016

Light

tekst:
Roelof Kleis

Birds need a good night’s sleep just as we do. Excessive artificial light disturbs that rest. But how problematic is that? Not very, concludes Maaike de Jong from her doctoral research. And green light is not better than white light.

Illustration: Geert-Jan Bruins

Both humans and animals sleep better in the dark. But it is not easy for wild animals in the Netherlands to find deep darkness. By night, the Netherlands is one of the most lit-up countries in the world, as photos taken from space reveal. Biologists refer to this as light pollution and a lot of research has been done on its effects in recent years.

One good example is the Light on Nature project implemented by Wageningen UR and the NIOO KNAW. Since 2011 rows of lampposts have stood at locations in formerly dark nature areas around the country. The lamps shed white, red or green light. The effect of light and its colour on the nearby nature is monitored. Biologist Maaike de Jong got her PhD Friday the 24th of June for a study of the effects of nocturnal light on birds.

First egg

Over two years De Jong studied the effects of artificial light on the brooding behaviour of great tits and pied flycatchers in nesting boxes near the lampposts. Light appeared to affect their brooding behaviour but the results are not clear-cut. White and green artificial light brought forward the date when the first egg was laid by five days. However, this effect was seen in the first year but not the next.

De Jong has an explanation for this. ‘The two triggers for brooding are daylight hours and temperature. In 2013, when we did observe an effect, the spring was cold. Normally birds brood a bit later then, while the days get longer. The additional white and green light may have caused the tits to start brooding earlier. The spring of 2014 was warm, so daylight hours had less influence and the light had no effect on the date of the first egg.’

The pied flycatchers were unaffected however. But De Jong has a plausible explanation for that as well. Pied flycatchers are migrating birds. They only arrive in the brooding area just beforehand, not long enough for the artificial light to have an impact.

Meanwhile there is data from more recent years too. In 2015 (a warm spring) there was no effect, this year (a cold spring) there is an effect again. ‘So a pattern can be seen: in a cold year artificial light brings forward the date of the first egg. But the data needs better analysis first. For these kinds of differences we need measurements over several years.’

Laying their eggs earlier does not seem to harm the animals, however. In biologists’ terms: the animal’s fitness remains unchanged. The number of young that leave the nest, their average weight and the number of brooding birds returning the next year is not changed by artificial light. Laying eggs five days earlier might not seem like a big deal either but De Jong says it can make a difference. ‘Great tits try to time their brood by the annual caterpillar peak, which lasts approximately ten days. So a change by a couple of days could be decisive.’

Later to bed

The results of these kinds of field trials are often difficult to interpret because there is no control over the amount of artificial light the animals are subjected to. So De Jong also did extensive research on the behaviour of great tits and blue tits under controlled laboratory conditions. She looked at how the diurnal rhythm, or activity pattern, of tits responds to light of various colours and intensities. Activity was measured in this case by the number of times a bird makes contact with a perch in a cage. A tried and trusted method.

There is no reason to believe that birds do better in green light

The results are fascinating. The birds are up and about up to two hours earlier and go to bed at least half an hour later. The extent of the effect also correlates closely with the strength of the light. De Jong: ‘The effect is stronger in the morning than in the evening. I think they like to make use of the extra light so they can eat earlier in the morning.’

The earlier activity is in evidence with white, red and green light, although it is less with green light. That could be an argument to use green light, which is used increasingly in places where managers want light but do not want to disturb nature. De Jong says cautiously: ‘At higher intensities the difference between green and white light disappears.’ In her view green light is no better and no worse than any other light. ‘There is no evidence at all in the terrestrial ecosystems I have studied. Sure, you can demonstrate effects of green light on the behaviour of birds. But there is no reason to believe birds here do better in green light.’

Dimmable

In fact, De Jong’s research provides no basis for the claim that light pollution has a negative impact on the fitness of birds. ‘The effects do not seem so big. We know that many species can adapt very well to a new environment. On the other hand, it comes out clearly in my research that the less light there is, the less disturbance. So it might be better to use the money spent on green light on making lighting dimmable.’ But De Jong reserves judgement. ‘This is only three years of research. It might very well be the case that effects on the population or on individual animals only become visible in the long term. What is more, for other animal species such as nocturnal animals, insects or humans, negative effects of artificial light have been proven.’


More infidelity in the dark

Great tits are known for their tendency to infidelity. A nest often contains young from different fathers. Artificial light has an influence on this, De Jong’s study shows. The further the nesting boxes were from the lampposts, the more fathers were represented in the nest. That is, in the presence of red and white light at least. It seems as though red and white light functions like a kind of traffic light: stop, no adultery here! De Jong guesses that female great tits are more faithful under lighter conditions because their mates can keep a better eye on them.


Re:ageer