MAJOR FINDINGS

(a) Exposure to light at night in urban areas

The novelty of my study is that light intensity was measured directly on individual free-ranging birds that were captured, tagged with light loggers, and then released in their natural urban and rural environment. The data presented in Chapter 3 and 6 show that urban birds are exposed to higher light intensities at night than rural birds. However, this level of light intensity is still quite low, and 30-fold lower than the intensity measured

urban blackbirds actively avoid very bright roosting places, and they rather escape from light at night. This is a fascinating hypothesis, but it remains at present untested.

(b) Light at night and daily cycles

The data collected by the light loggers, together with additional radio-telemetry, allowed me to relate the individual exposure to light at night to the timing of daily activities in the field. The average light intensity at night to which urban blackbirds are exposed predicted the time of onset and end of activity: the higher the nightlight intensity, the earlier the onset of morning activity and the later the end of evening activity. This effect was particularly strong for the morning activity. However, light at night explained only a small part of the variation in activity timing, suggesting that other environmental factors might be involved. Noise and weather conditions seemed to explain little of this variation, but social cues were not considered in my study. Social stimuli are known to synchronize circadian rhythms of behaviour and activity (Gwinner 1967; Davidson &

Menaker 2003). I hypothesize that, given the apparent higher breeding density in the urban compared to the rural population, the difference in the timing of daily activity during the breeding season could be partially driven by a higher amount and frequency of social interactions.

But why do birds extend their activity into the night, and how do they accomplish that? A plausible answer could reside in the evidence that urban blackbirds have a faster and more labile circadian clock than rural conspecifics, as shown in Chapter 4. Circadian clocks are endogenous mechanisms that enable animals to predict and keep track of changes in the environment (Merrow, Spoelstra, & Roenneberg 2005), and are thought to

be evolved through natural selection (Foster & Kreitzmann 2004). Therefore shifts in the period of the circadian rhythmicity have been suggested to signal adaptation to environmental change (Tauber et al. 2007). One possible explanation for the variation in the circadian properties of urban European blackbirds might be a selective advantage of being active early in the morning, for example through increased extra-pair paternity gain (Kempenaers et al. 2010). However, further experiments are needed in order to test this theory. An alternative, non-mutually exclusive hypothesis, involves the hormone melatonin. In Chapter 5 I showed that light at night is able to reduce melatonin concentration in the plasma, especially in the early morning and in the late evening. The change in melatonin in the early morning, before dawn, predicted the amount of locomotor activity at this time of night: the lower the melatonin concentration, the higher the amount of activity. However to what extent light at night can reduce melatonin release of birds in the wild is currently unknown.

(c) Light at night and seasonal functions

Previous data from our study populations showed that urban blackbirds, both males and females, are ready to breed (i.e. have functional testes size) three weeks before the rural conspecifics (Partecke, Van’t Hof, & Gwinner 2005). I theorized that artificial light at night may be the strongest factor affecting the timing of reproductive physiology, because light is the most reliable cue that birds in temperate latitudes can use to predict the right time to breed (Dawson et al. 2001). Other environmental cues, such as temperature, food availability and social interactions, although crucial to mediate

dates (Dawson 2008a; Schaper et al. 2012). I tested my hypothesis exposing urban and rural blackbirds to two different light treatments simulating the nocturnal light environment in city and forest habitat. To this end, I used data obtained from the light loggers deployed on wild urban and rural blackbirds. During the first year of experiment, blackbirds exposed to light at night of 0.3 lux developed the reproductive system earlier than individuals exposed to dark nights. It is interesting to notice that the effect size of our light at night treatment is comparable to the difference observed in the wild (~ three weeks), suggesting that light pollution may be the major factor responsible of the earlier onset of the reproductive physiology in urban blackbirds. However, during the second year of study birds under light at night did not grow their reproductive system at all. This might have been an effect of birds being stuck in a photorefractory state because they interpreted light at night as a constant long day (Dawson & Sharp 2007). However, data of locomotor activity profiles seem not to confirm this hypothesis: birds under light at night did not free-run but rather experienced the seasonal change in daylength.

Alternatively, birds might have been chronically stressed, a physiological state that in some cases has been shown to suppress reproductive functions (Cyr & Romero 2007). In addition, we showed that moult sequence becomes irregular in response to long-term exposition to light at night. Since the natural alternation of long and short days is necessary for a regular moult (Dawson 2008b), this further suggests that the light at night treatment might have been interpreted as a constant long day. The data collected with the light loggers suggest that birds may try to avoid bright roosting places at night, thus it is unlikely that birds thriving in urban habitats are exposed to chronic light intensities at night similar to that I used in my laboratory experiment. Nevertheless, our results

highlight the potential deleterious effects that light pollution may have on the ability of urban avian species to maintain functional seasonal cycles of reproduction and moult.