Artificial light at night advances avian reproductive physiology

Abstract

Artificial light at night is a rapidly increasing phenomenon and it is presumed to have global implications. Light at night has been associated with health problems in humans as a consequence of altered biological rhythms. Effects on wild animals have been less investigated, but light at night has often been assumed to affect seasonal cycles of urban dwellers. Using light loggers attached to free-living European blackbirds (Turdus merula), we first measured light intensity at night which forest and city birds are subjected to in the wild. Then we used these measurements to test for the effect of light at night on timing of reproductive physiology. Captive city and forest blackbirds were exposed to either dark nights or very low light intensities at night (0.3 lux). Birds exposed to light at night developed their reproductive system up to one month earlier, and also moulted earlier, than birds kept under dark nights. Furthermore, city birds responded differently than forest individuals to the light at night treatment, suggesting that urbanization can alter the physiological phenotype of songbirds. Our results emphasize the impact of human-induced lighting on the ecology of millions of animals living in cities and call for an understanding of the fitness consequences of light pollution.

Figure 1.

Natural exposure to light at night of free-living city European blackbirds. Light grey bars represent data obtained from loggers deployed on individual birds (n = 8) in their natural urban environment. For each city bird we calculated the median and maximum of the third quartiles of all nights, and used the mean of those values from all birds for presentation in the figure. Data are represented as means ± s.e.m. Dark grey striped bar represents the light intensity measured under a representative street lamp of our study site (6 lux). Black bar represents the light intensity (0.3 lux) we used in the experiment to simulate urban exposure to light at night in the experimental group.

Figure 2.

(a) Effects of light at night on seasonal variation in testicular width, testosterone production (b), moult initiation (a) and (c) first morning song in adult male European blackbirds (Turdus merula). (a) City (triangles, dashed lines) and forest (circles, solid lines) adult male European blackbirds were tested to simulated natural photoperiods but with different light intensities at night. Control birds (blue) experienced nights without any light, whereas experimental birds (red) were exposed to constant light of 0.3 lux at night. Dashed horizontal line represents testicular size (width of 5 mm) above which males are assumed to be able to produce fertile sperms. Data represent mean ± s.e.m. In addition, we show time of moult initiation as horizontal box plots on the top right of the (a, control/experimental, blue/red; city/forest, filled/blank background). Sample sizes: control = 20 (10 forest and 10 city birds), experimental = 20 (10 forest and 10 city birds). (b) Seasonal variation in plasma testosterone production. For symbols legend see (a). (c) Time of onset of morning chorus in the two experimental groups (blue circles/solid lines, control, dark night; red circles/dashed lines, experimental/light at night) measured every three to four weeks during testicular cycle. Horizontal dashed line indicates beginning of morning twilight phase. Each circle represents first song of first individual in each group.