Effects of artificial light at night on daily and seasonal organization of European blackbirds (Turdus merula)

Abstract

Urban areas are growing faster than any other land cover types. Associated with increasing urbanization, artificial light at night is now recognized as a public health issue and in recent years new interest has risen around the ecological effects of light pollution. Given that light through its diel and seasonal (daylength) changes is one of most important factors regulating daily and annual cycles of virtually all organisms, I hypothesized that modifications of the environment by means of artificial light at night could strongly affect important biological cycles of wild animals. In my doctoral dissertation I experimentally demonstrate the effects of light at night on the daily and seasonal cycles of an urbanized songbird, the European blackbird (Turdus merula), and describe the physiological mechanisms that may underlie such effects.I used a combination of radio-telemetry and light loggers to first record the light intensity to which free-ranging urban and rural blackbirds are exposed to during the night, and then to relate this information to the timing of onset and end of daily activity. Urban birds were exposed to higher intensities of light at night than rural birds. In addition, birds exposed to higher light at night started their activity earlier in the morning and ceased it later in the evening. However, a considerable amount of within-individual variation was present. Temperature, cloud cover and precipitation did not explain this source of variation, but other environmental factors such as noise, food availability and social stimuli could also play a role. I was able to explore the relative role played by noise and light at night through a natural experiment. Indeed, while both nocturnal and diurnal noise varied considerably between weekend and weekdays, light at night intensity and time of onset of morning activity did not, further proposing light at night as a major driver of altered timing of daily activity in urbanized avian species.The variation in the time of morning activity between urban and rural blackbirds was mirrored by the properties of their endogenous circadian rhythm. Urban birds showed faster and weaker circadian rhythms than rural individuals when kept under constant laboratory conditions. The relationship at the individual level between circadian traits and activity in the field was found to be significant, suggesting that urbanization may have shaped both the circadian clock and the behavioral phenotype of European blackbirds. Whether this is a consequence of micro-evolution, developmental (maternal/epigenetic) effects or phenotypic plasticity in response to different environmental characteristics is still unknown. However, I suggest that a plausible scenario could be selection for early birds in urban environments, and that the presence of artificial light at night may be the underlying environmental factor promoting such micro-evolutionary process.I then used a long-term laboratory experiment to test specific hypotheses about the effects of light at night on daily and seasonal cycles of blackbirds. Wild caught urban and rural birds were exposed for two consecutive years to either dark nights or very low light intensity at night (0.3 lux), calibrated on the data previously recorded on individual blackbirds in their natural urban environment.I first predicted that light at night was able to reduce plasma melatonin concentration. Indeed, birds exposed to light at night, irrespective of their origin, had lower melatonin concentration in the late evening and in the early morning, during both winter and summer. In addition, the lower the melatonin concentration was in the early morning, the higher the amount of activity was at this time of day. I therefore suggest that light at night is able to decrease melatonin release at night, and that this could be one potential physiological mechanism underlying the link between artificial lights in urban areas and alternative temporal activity tactics of urban birds. Using the same experimental set-up, I also monitored testicular size and plasma concentration of testosterone over two consecutive reproductive cycles. In the first year of experiment birds exposed to light at night grew their testes almost a month earlier, and also showed earlier increase in testosterone, than birds exposed to dark nights. At the end of the reproductive cycle birds exposed to light at night started to moult earlier. However, moult lasted longer in the light at night group, among which some individuals did not even finish to moult. In the second year, birds exposed to light at night did not grow the reproductive system at all and moult sequence was highly irregular, possibly because birds were stuck in a photorefractory state. Conversely, birds exposed to dark nights showed normal cycles of testicular development, testosterone and moult. The results of this experiment suggest that short-term exposure to light at night can significantly advanced important annual cycles such as reproduction and moult, but that long-term effects can dramatically impair the maintenance of seasonal functions. Although the chronic, constant exposure to light at night is unlikely in nature, my results call for an urgent understanding of the fitness consequences of light pollution.