Illuminating a bird’s world

Natural night-time darkness has disappeared across large parts of the world as a result of light pollution, the alteration of light levels in the outdoor environment due to artificial light sources. This increase in sky brightness not only obscures our perception of the starry sky; it can also have severe effects on human health as well as detrimental impacts on nature. Many animals are attracted to light at night and thereby suffer direct fitness losses, but more subtle effects can also occur. One such effect is the timing of daily and seasonal activities. As organisms have evolved under a natural light-dark cycle, which is the main driver for circannual and circadian rhythms, these activities can be disturbed by anthropogenic light at night. One species group that is potentially greatly affected by nocturnal illumination is birds, as species in this group have excellent vision and also possess light sensitive tissue in their brain. Artificial light at night can thus affect many aspects of a bird’s life. Timing of dawn and dusk singing, foraging behaviour, and sleep pattern are for example known to be altered by the presence of street lights. Yet, so far, experimental and especially long term field studies investigating the effects of nocturnal illumination on birds are lacking. Amongst the options to reduce the effects of night-time light pollution on ecosystems is the adaption of the light spectrum. The LED lamps that are used more and more in outdoor lighting have large economic advantages, and their colour composition can be custom-designed. This could potentially mitigate the impact of light on flora and fauna by using a specific light colour that has minimal effects on biological processes. However, little is known about the effects of different colours of light on birds. The aim of this thesis was to investigate the effects of artificial light at night on the ecology of birds. More specifically, the effects of different light colours and light intensities are studied, to provide insight into the possibilities of mitigation. The focus was on investigating the effects of artificial light at night on several aspects of avian ecology, including effects on physiology, behaviour, life-history traits, and fitness, by studying common songbirds in the Netherlands. To this end, three different approaches were used. Firstly, avian timing of breeding was related to levels of light pollution, in a correlational study using long-term data from across the Netherlands. Nocturnal light levels were used as a proxy for the level of urbanisation of an area. Data from ten common, nest box breeding bird species, collected by a citizen science network, were used. For great tits (Parus major), blue tits (Cyanistes caeruleus) and pied flycatchers (Ficedula hypoleuca), correlations were found between light levels and first egg laying dates. However, these correlations were not apparent in all years, and were negative in some years and positive in others. A possible reason for this inconsistency is that this study included few data from the highly urbanized areas of the Netherlands. In order to truly quantify the impact of urbanisation on wild birds, data collection needs to be expanded to include such areas. Secondly, in order to study the breeding ecology of cavity-breeding passerines, a large-scale field experiment was set up in which formerly dark, natural habitat was experimentally illuminated with white, green or red LED light, in addition to a dark control treatment. The experimental nature of the set-up allowed the effects of nocturnal illumination to be tested independently of other anthropogenic disturbances that are normally associated with light at night. The effects of light at night on life-history traits and fitness components in two free-living songbird species, the great tit and the pied flycatcher, were measured in two consecutive years. In 2013, but not in 2014, white and green light advanced the first egg laying date of great tits. Pied flycatchers were unaffected by the light at night. In the same two years, the extra-pair behaviour of the great tit was studied. In 2014, the proportion of extra-pair young in broods increased with distance to the red and white lamps. In 2013, light had no effect. The discrepancies between years in both studies were probably linked to the very different climatic conditions in both years. The effects that were shown in these studies are with regard to the behaviour of birds; so far in our experimental study, no fitness consequences of breeding in lighted areas have been observed, in terms of the reproductive success of pairs, the reproductive success of males when extra-pair offspring is included, or of adult survival to the next breeding season. To study whether the behavioural effects found so far at this experimental set-up were due to the direct or the indirect effects of artificial light at night, male great tits were deployed with light loggers to measure their light exposure over a 24 hour period. Males from pairs breeding close to the lamp posts were not exposed to more light than males from pairs nesting further away. This suggests that male great tits avoid exposure to light at night and thus that the effects of artificial light on behaviour found so far might be indirect rather than direct. Finally, the effects of night-time light colour and intensity on the physiology and behaviour of captive birds were studied, in a controlled laboratory environment. In contrast to the field studies, in this set-up birds were exposed to artificial light levels with no possibility of escaping to darker places. Dose-dependent effects of artificial light at night on birds’ daily activity patterns and melatonin levels were demonstrated in great tits. Higher light intensities advanced activity onset and delayed activity offset. Night-time activity increased and melatonin levels (measured at midnight) decreased at higher light intensities. In two other experiments, the effects of light colour and light intensity on the daily activity patterns of blue tits were studied. In all colours, and most of all in red and white light, birds advanced their onset of activity in the morning. The effect of light intensity on activity onset was smaller in green than in white light in the lower range of intensities, but became equal at the highest intensities. These studies show that different light colours have different effects on activity patterns, and that disturbance to daily activity patterns can be partially mitigated by changes in the characteristics of outdoor lighting. Some of the findings presented in this thesis can be directly translated into advice for policy and conservation; others first need further investigation. Furthermore, all are based on studies of a few bird species. Since the effects of light at night of different spectral compositions do vary widely between species groups, the challenge is going to be in coming up with advice on outdoor lighting for areas with many different species, rather than for just one species or species group. This thesis has begun to uncover the effects of artificial light at night on avian ecology; some novel findings on birds in illuminated nights have been presented and recommendations for future research have been made. In recent years, much has been revealed regarding the biological impacts of anthropogenic nocturnal illumination. Yet, there is much still unknown. The experimental field set-up described here forms an excellent and valuable tool to continue to study the effects of ever-increasing night-time light levels on ecosystems.