Science news from the Year of the Great Tit edited by Marko Mägi, Bird ecology researcher at University of Tartu
Translation Liis
As autumn progresses the nights become longer and street lightning in cities is switched on ever earlier. It shines in through many windows disturbing in addition to TVs and smart IT devices sleep at night. We can make our sleeping rooms darker with window covers but what can the birds do who haven’t the option to close the nest opening in the tree hollow with a curtain and whose sleep is disturbed by the street lights in settlements?
The night and day rhythm of animals generally follows the sun – to have energy to begin the next day they go to sleep in the evening at sunset and get up early in the morning. The negative effects of night-time illumination on birds and other animals were known: city birds for instance start singing earlier in the mornings because for them morning arrives significantly earlier than for their co-specifics living in forests. However there was a lack of information on the relation between the sleep of birds and artificial light and an answer to the question whether the sleep of birds living in settlements is restless and waking up in mornings more difficult?
The relation between the sleep of birds and artificial light was studied by a group of Belgian researchers assisted by none others than great tits. In the breeding period an IR camera and a small LED lamp that lit up the interior of the nest box were installed. The study started when there were 10 days old chicks in the nest and while the female birds still came to spend the night in the nestboxes. The events in the nest box were monitored on three consecutive nights – during the first and third night the birds were allowed to sleep peacefully, on the second night however a lamp was burning in the nestbox.
Sleeping great tit / Image from the web camera: Felis silvestris 29.04.2016 LK forum
On monitoring the night-time behaviour and sleep rhythm of birds the conclusion was reached that additional light at night at 1,6 lux significantly affects the sleep of birds. Compared to the first night, on the second night when the lamp was burning the females arrived to the nestbox at the same time but fell asleep on average 95 minutes later and woke 74 minutes earlier in the morning. Usually great tits leave the nest box during the day most often to feed the chicks at sunrise but the artificial light disrupted the night-and-day rhythm of the birds and made them leave the nestbox at a time when it was still dark outside.
On summarising the experiments it turned out that in lighted conditions the birds slept 56% less than in normal circumstances. The total length of sleep cycles in a night is a little less than 4 hours; when the lamp was burning they could however sleep only barely one hour and a half. The length of the sleep cycles of the birds did not change (if sleep came they slept as long as usual), but the periods of being awake were prolonged. At nights with the lamp burning the chicks were more active and begged for food more than usual. Such extreme changes in the sleeping regime are however draining on the organism and so the females had a sleeping deficiency on the third night and slept 25% more than in the first night.
The results of the study point clearly indicate that artificial lighting has definite negative effects on the sleep of birds which in turn may affect the success of breeding. For instance food begging by the chicks at night can raise the interest of unwished predators. A deficit of sleep can also influence the ability of the adult to care for the chicks.
Great tits breeding in Estonia are evidently not as greatly influenced by artificial lighting in the breeding period as in Belgium since nights in that period are somewhat shorter and lighter here. However, it is evidently not a good idea to place a nest box for birds directly at a street street light post.
Raap T, Pinxten R, Eens M, 2016. Artificial light at night disrupts sleep in female great tits (Parus major) during the nestling period, and is followed by a sleep rebound. Environmental Pollution 215: 125–134. http://dx.doi.org/10.1016/j.envpol.2016.04.100
