Abstract
The impact of artificial light at night (ALAN) on ecological and behavioural processes is increasingly recognized but we have limited knowledge on the impact of ALAN on wild species, and on the potential to mitigate any negative effects by using different light sources and colors. In birds, effects of ALAN on activity levels are reported for several species and, hence, their daily energy expenditure (DEE) may be affected. DEE is a potent mediator of life-history trade-offs and fitness and thus an important aspect to consider when examining the potential long-term ecological effects of ALAN. Previous work has suggested that birds exposed to ALAN show higher levels of provisioning and nocturnal activity, suggesting that white ALAN increases DEE. Other factors regulating DEE, such as provisioning behavior and food availability, might also respond to ALAN and thus indirectly affect DEE. We tested the hypothesis that ALAN increases DEE using an experimental setup where four previously unlit transects were illuminated with either white, green, or red LED light, or left dark as a control treatment. This setup was replicated in eight locations across the Netherlands. We measured DEE of our focal species, the great tit (Parus major), using a novel doubly labelled water technique that uses breath rather than blood samples. Contrary to our expectations, birds feeding their offspring under white and green ALAN showed lower DEE compared to birds in the control dark treatment. Differences in chick provisioning activity did not explain this result, as neither visit rates nor daily activity timing was affected by light treatment. However, food availability under white and green light was much higher compared to red light and the dark control. This difference strongly suggests that the lower DEE under white and green ALAN sites is a consequence of higher food availability in these treatments. This result shows that there can be positive, indirect effects of ALAN for breeding song birds which may balance against the negative direct effects shown in previous studies.
Highlights
The alteration of natural light levels in the outdoor environment due to artificial light sources, defined as light pollution (Cinzano et al, 2000), is a globally expanding phenomenon with an estimated increase of 6% per annum (Hölker et al, 2010)
Our study revealed an effect of artificial light at night (ALAN) on the energy expenditure of breeding wild great tits
Previous studies suggested that adult songbirds exposed to white ALAN showed higher provisioning rates during the day (Titulaer et al, 2012), as well as higher restlessness at night (Dominoni et al, 2013; de Jong et al, 2016a; Ouyang et al, in press), energy consumption was never directly measured
Summary
The alteration of natural light levels in the outdoor environment due to artificial light sources, defined as light pollution (Cinzano et al, 2000), is a globally expanding phenomenon with an estimated increase of 6% per annum (Hölker et al, 2010). It has been recently estimated that at mid-high latitudes more than 23% of land surfaces are exposed to artificial light at night (ALAN). Energetic Costs of Light Pollution (Falchi et al, 2016). ALAN is caused by a wide variety of light sources, such as public lighting of streets and buildings, vehicle lights, and light from advertising. ALAN is one of the first signs of expanding urbanization, and rural land is widely exposed to street lighting, especially in developed countries. A recent example is the widespread shift to LED lights (Gaston et al, 2012; Bennie et al, 2014). Since LEDs color is modifiable, it is important to fill this research gap in order to inform policy-makers and improve future lighting strategies
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