Does light pollution alter daylength? A test using light loggers on free-ranging European blackbirds (Turdus merula)

Abstract

Artificial light at night is one of the most apparent environmental changes accompanying anthropogenic habitat change. The global increase in light pollution poses new challenges to wild species, but we still have limited understanding of the temporal and spatial pattern of exposure to light at night. In particular, it has been suggested by several studies that animals exposed to light pollution, such as songbirds, perceive a longer daylength compared with conspecifics living in natural darker areas, but direct tests of such a hypothesis are still lacking. Here, we use a combination of light loggers deployed on individual European blackbirds, as well as automated radio-telemetry, to examine whether urban birds are exposed to a longer daylength than forest counterparts. We first used activity data from forest birds to determine the level of light intensity which defines the onset and offset of daily activity in rural areas. We then used this value as threshold to calculate the subjective perceived daylength of both forest and urban blackbirds. In March, when reproductive growth occurs, urban birds were exposed on average to a 49-min longer subjective perceived daylength than forest ones, which corresponds to a 19-day difference in photoperiod at this time of the year. In the field, urban blackbirds reached reproductive maturity 19 day earlier than rural birds, suggesting that light pollution could be responsible of most of the variation in reproductive timing found between urban and rural dwellers. We conclude that light at night is the most relevant change in ambient light affecting biological rhythms in avian urban-dwellers, most likely via a modification of the perceived photoperiod.

Behavioural plasticity in the onset of dawn song under intermittent experimental night lighting

“…3). Blackbirds and several tit species naturally show the earliest dawn singing during peak female fertility (Cuthill & Macdonald, 1990;Dominoni & Partecke, 2015;Mace, 1987;Welling, Koivula, & Lahti, 1995) and the response to light may be strongest at this time, as suggested previously (Da Silva et al, 2014;Dominoni & Partecke, 2015;Dominoni et al, 2014;Kempenaers et al, 2010). However, we did not find a significant effect of the interaction between date and light in our experiment, which does not support this claim.…”

Effects of experimental night lighting on the daily timing of winter foraging in common European songbirds

“…, Table ). However, we cannot exclude that blackbirds, which were frequently recorded foraging on the ground, extended ground feeding during the lighted evenings, as found in previous studies (Dominoni and Partecke , Russ et al ). The blue tit was the only species regularly foraging at the feeder after sunset, yet, it did not take advantage of the artificial light to extend its foraging time.…”

The biological impacts of artificial light at night: the research challenge

“…However, particularly given the spatial resolutions at which it is mapped, animals can often behaviourally avoid the typically more heterogeneous patterns of ALAN through the spatial and temporal habitat and movement choices that they make. In one of the few examples we are aware of to date, Dominoni & Partecke [40] show that urban blackbirds do actually experience longer subjective day lengths as a consequence of ALAN.…”