Ecological Effects of Light Pollution: How Can We Improve Our Understanding Using Light Loggers on Individual Animals?

Abstract

Light pollution has become an important theme of both scientific research and policy-making. Although in recent years we have seen a boost of research on this topic, there is still surprisingly little knowledge on the levels of artificial light at night that wild animals really experience. I made use of miniature light loggers attached to individual free-living European blackbirds (Turdus merula) to measure the light intensity to which these birds are exposed to in forest and urban areas. I have first shown that male blackbirds living in a city are indeed exposed to higher levels of light at night compared to forest conspecifics, but these levels are substantially lower to what can be measured underneath typical street lamps. Recently I have offered new perspectives by estimating the subjective day length to which urban and rural blackbirds are exposed to and by analysing the overall light intensity to which blackbirds are exposed daily. In a series of studies, I have interpreted these data in the context of daily patterns of activity as well as seasonal biology. European blackbirds which were exposed to a longer photoperiod than their rural counterparts extended their activity into the night and showed reduced levels of melatonin production in the early morning, suggesting that this could be the biophysical process underlying the early onset of daily activity, but also the advanced breeding season observed in many avian species that successfully colonize urban areas. Indeed, I found a remarkable similarity between the difference in the photoperiod experienced by rural and urban blackbirds and the difference in timing of reproduction and onset of daily activity between my two study populations. I will discuss these findings and underlie several outstanding questions that still remain unresolved.