Long-Term Effects of Chronic Light Pollution on Seasonal Functions of European Blackbirds (Turdus merula)

Abstract

Light pollution is known to affect important biological functions of wild animals, including daily and annual cycles. However, knowledge about long-term effects of chronic exposure to artificial light at night is still very limited. Here we present data on reproductive physiology, molt and locomotor activity during two-year cycles of European blackbirds (Turdus merula) exposed to either dark nights or 0.3 lux at night. As expected, control birds kept under dark nights exhibited two regular testicular and testosterone cycles during the two-year experiment. Control urban birds developed testes faster than their control rural conspecifics. Conversely, while in the first year blackbirds exposed to light at night showed a normal but earlier gonadal cycle compared to control birds, during the second year the reproductive system did not develop at all: both testicular size and testosterone concentration were at baseline levels in all birds. In addition, molt sequence in light-treated birds was more irregular than in control birds in both years. Analysis of locomotor activity showed that birds were still synchronized to the underlying light-dark cycle. We suggest that the lack of reproductive activity and irregular molt progression were possibly the results of i) birds being stuck in a photorefractory state and/or ii) chronic stress. Our data show that chronic low intensities of light at night can dramatically affect the reproductive system. Future studies are needed in order to investigate if and how urban animals avoid such negative impact and to elucidate the physiological mechanisms behind these profound long-term effects of artificial light at night. Finally we call for collaboration between scientists and policy makers to limit the impact of light pollution on animals and ecosystems.

Melatonin: a possible link between the presence of artificial light at night and reductions in biological fitness

“…Human studies are typically correlational, but similar carcinogenic and immunosuppressant effects of artificial night lighting have been observed from laboratory experiments on mice and rats [23][24][25][26], although immune-enhancing effects of dim night lighting have also been reported [27]. Convincing support for the link between LAN and significant modifications to both behaviour (activity patterns) and physiology (moult dates and reproductive status) also comes from a series of long-term studies combining field and experimental trials on European blackbirds (Turdus merula) [12,18,28,29]. However, despite the recent surge in research on the impacts of night lighting within the fields of ecology and medicine, we still lack broad-scale, cross-species experimental evidence of the negative effects of LAN (but see [30]).…”

The effects of light pollution on biological rhythms of birds: an integrated, mechanistic perspective

“…A possible link could be a reduction in melatonin due to light at night, but unfortunately melatonin was only measured during the second reproductive year, when birds exposed to light at night did not show any reproductive function. Indeed, in the second year of this study blackbirds held under light at night failed to develop their gonads, testosterone levels were at their baseline, and molt was interrupted (Dominoni et al 2013d ). It is possible that constant, chronic levels of dim light at night "trapped" blackbirds into a photorefractory state after the first gonadal cycle, but this hypothesis remains untested.…”

Experimental illumination of natural habitat—an experimental set-up to assess the direct and indirect ecological consequences of artificial light of different spectral composition

“…Effects of artificial light on seasonal timing of birds have also been observed [18,19]. Changes in timing may be mediated by changes in hormone regulation [20], especially by melatonin [6,21].…”