When animals live in cities, they have to adjust their behavior and life histories to novel environments. Noise pollution puts a severe constraint on vocal communication by interfering with the detection of acoustic signals. Recent studies show that city birds sing higher-frequency songs than their counterparts in non-urban habitats. This has been interpreted as an adaptation to counteract masking by traffic noise.
Similarly, anthropogenic light and noise have modified differences between day and night, and may thereby interfere with their circadian clocks. Urbanized birds as well as many other species are known to advance their activity into early morning and night hours. Studies indicate that city birds start their activity earlier and had faster but less robust circadian oscillation of locomotor activity than their forest relatives. Circadian period length predicted start of activity in the field, and this relationship was mainly explained by fast-paced and early-rising city birds.
In the article “Ecological Light Pollution” from the publication Frontiers in Ecology and the Environment by the Ecological Society of America it is reported that “through the various effects that light pollution has on individual species, the ecology of regions is affected. In the case where two species occupy an identical niche, the population frequency of each species may be changed by the introduction of artificial light if they are not equally affected by light at night. Changes in these species frequencies can then have knock-on effects, as the interactions between these species and others in the ecosystem are affected and food webs are altered. These ripple effects can eventually affect even diurnal plants and animals. As an example, changes in the activity of night active insects can change the survival rates of night blooming plants, which may provide food or shelter for diurnal animals.
The introduction of artificial light at night is said to be one of the most drastic anthropogenic changes to the Earth, comparable to toxic pollution, land use change, and climate change due to greenhouse gases.