No abstract

Artificial night lighting is expanding globally, but its ecological consequences remain little understood. Animals often use changes in day length as a cue to time seasonal behaviour. Artificial night lighting may influence the perception of day length, and may thus affect both circadian and circannual rhythms. Over a 3.5 month period, from winter to breeding, we recorded daily singing activity of six common songbird species in 12 woodland sites, half of which were affected by street lighting. We previously reported on analyses suggesting that artificial night lighting affects the daily timing of singing in five species. The main aim of this study was to investigate whether the presence of artificial night lighting is also associated with the seasonal occurrence of dawn and dusk singing. We found that in four species dawn and dusk singing developed earlier in the year at sites exposed to light pollution. We also examined the effects of weather conditions and found that rain and low temperatures negatively affected the occurrence of dawn and dusk singing. Our results support the hypothesis that artificial night lighting alters natural seasonal rhythms, independently of other effects of urbanization. The fitness consequences of the observed changes in seasonal timing of behaviour remain unknown.

The effects of artificial night lighting on animal behaviour and fitness are largely unknown. Most studies report short-term consequences in locations that are also exposed to other anthropogenic disturbance. We know little about how the effects of nocturnal illumination vary with different light colour compositions. This is increasingly relevant as the use of LED lights becomes more common, and LED light colour composition can be easily adjusted. We experimentally illuminated previously dark natural habitat with white, green and red light, and measured the effects on life-history decisions and fitness in two free-living songbird species, the great tit ( Parus major ) and pied flycatcher ( Ficedula hypoleuca ) in two consecutive years. In 2013, but not in 2014, we found an effect of light treatment on lay date, and of the interaction of treatment and distance to the nearest lamp post on chick mass in great tits but not in pied flycatchers. We did not find an effect in either species of light treatment on breeding densities, clutch size, probability of brood failure, number of fledglings and adult survival. The finding that light colour may have differential effects opens up the possibility to mitigate negative ecological effects of nocturnal illumination by using different light spectra.

The disruption of daily rhythms is one of the most studied ecological consequences of light pollution. Previous work showed that several songbird species initiated dawn song earlier in areas with light pollution. However, the mechanisms underlying this shift are still unknown. Individuals may immediately adjust their timing of singing to the presence of artificial light (behavioural plasticity), but the observed effect may also be due to phenotype-dependent habitat choice, effects of conditions during early life or micro-evolution. The main aim of this study was to experimentally investigate how males of four common passerine species respond to day-to-day variation in the presence of artificial night lighting in terms of the timing of singing. During two consecutive breeding seasons, we manipulated the presence of light throughout the night in a cyclic fashion in several naturally undisturbed forest patches. We show that individuals of all four species immediately and reversibly adjusted their onset of dawn singing in response to artificial light. The effect was strongest in the European robin, but relatively small in the blue tit, the great tit and the blackbird. The effect in the latter two species was smaller than expected from the correlational studies. This may be coincidence (small sample size of this study), but it could also indicate that there are longer-term effects of living in light-polluted urban areas on timing of dawn singing, or that birds use compensatory behaviours such as light avoidance. We found no evidence that our light treatment had carryover effects into the subsequent dark period, but robins progressively advanced their dawn singing during the light treatment.

Nest and territory defence are risky and potentially dangerous behaviours. If the resolution of life history trade-offs differs between individuals, the level of defence may also vary among individuals. Because melanin-based colour traits can be associated with life history strategies, differently coloured individuals may display different nest and territory defence strategies. We investigated this issue in the colour polymorphic tawny owl (Strix aluco) for which plumage varies from dark to light reddish melanic. Accordingly, we found that (1) our presence induced a greater response (flying around) from dark-coloured than light-coloured females and (2) dark reddish males suffered lower nest predation rates than light-coloured males. In experimentally enlarged broods, the probability that females reacted after we played back the hoot calls of a stranger male was higher if these females were lighter reddish; the opposite pattern was found in experimentally reduced broods with dark parents being more reactive than light parents. Finally, darker females alarmed more frequently when paired with a light than with a dark male, suggesting that partners adjust their behaviour to each other. We also tested whether colouration is used as a signal by conspecifics to adjust the level of their defensive behaviour. Accordingly, breeding females responded more vigorously to a dark than a light reddish stuffed tawny owl placed beside their nest. We conclude that melaninbased colouration is a signal of alternative nest and territory defence behaviour that depends on ecological factors.

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Animals breeding at northern latitudes experience drastic changes in daily light conditions during the breeding season with decreasing periods of darkness, whereas those living at lower latitudes are exposed to naturally dark nights throughout the year. Nowadays, many animals are also exposed to artificial night lighting (often referred to as light pollution). Animals strongly rely on variation in light levels to time their daily and seasonal behaviour. Previous work on passerine birds showed that artificial night lighting leads to earlier onset of dawn song. However, these studies were carried out at intermediate latitudes with more limited seasonal changes in daylength, and we still lack an understanding of the impact of artificial night lighting in relation to variation in natural light conditions. We investigated the influence of natural and artificial light conditions on the timing of dawn singing in five common songbird species in each of three regions in Europe that differed in natural variation in daylength (northern Finland, 65°N; southern Germany, 48°N; southern Spain, 37°N). In each region, we selected five peri-urban forest sites with and five without street lighting, and we recorded dawn singing at the beginning of the local breeding season. Our results show that the earliest natural singers, that is, European robins (Erithacus rubecula) and common blackbirds (Turdus merula), started dawn singing earlier along with the natural increase in night brightness in Finland, with no additional effects of artificial night lighting. In contrast, the later singers, such as, great tits (Parus major), blue tits (Cyanistes caeruleus) and chaffinches (Fringilla coelebs), showed similar onsets of dawn song relative to sunrise across the season and similar effects of artificial night lighting at all latitudes. Artificial night lighting affected great tits, blue tits and chaffinches even in northern Finland where nights became very bright. Proximate factors such as differential light sensitivities may explain why early singers showed more plastic behavioural responses to naturally and artificially bright nights. The maintenance of rhythmicity in the late singers during bright northern nights and under artificial night lighting may also be an adaptive response to predation risk or costs of sleep loss.

The ecological effects of light pollution are becoming better understood, especially in birds. Recent studies have shown that several bird species can use street lighting to extend activity into the night during the breeding season. However, most of these studies are correlational and little is known about the effects of artificial night lighting on the timing of activities outside the breeding season. During winter, low temperatures and short days may limit foraging opportunities and can negatively affect survival of resident birds. However, night lighting may allow them to expand the time niche available for foraging. Here, we report on a study where we repeatedly manipulated the amount of night lighting during early winter at automated feeding stations in a natural forest. We used video-recordings at the feeders to determine the time of the first (at dawn) and last (at dusk) foraging visits for six songbird species. We predicted that all species, and in particular the naturally early-foraging species, would advance their daily onset of foraging during the mornings with night lighting, but would show minimal or no delays in their daily cessation of foraging during the lighted evenings. We found that two early-foraging species, the blue tit and the great tit, started foraging earlier during the experimentally lighted mornings. However, in great tits, this effect was weak and restricted to nights with inclement weather. The light treatment did not have any effect on the start of foraging in the willow/marsh tit, the nuthatch, the European jay, and the blackbird. Artificial night lighting did not cause later foraging at dusk in any of the six species. Overall, our results suggest that artificial light during winter has only small effects on timing of foraging. We discuss these findings and the importance of temperature and winter weather in shaping the observed foraging patterns.