<p>Urbanization is a rapidly increasing phenomenon. Expansion of urban habitat has led to two major changes: i) Use of the forest area for agricultural practices and ii) Intensive increase of use of artificial light at night (ALAN). The sources of ALAN produce much higher-intensity light in many urbanized areas than on full-moon nights, blurring the contrast between day and night. Most animals are light-sensitive and use sunrise and sunset to track the time of day and year. Birds are an indicator of environmental health. Most birds use the daily light–dark cycle to regulate their physiology, both daily and seasonally. The increase in urbanization and the use of ALAN pose a serious threat to many of these birds’ ability to cope with a changing environment. In this review, we address the effects of ALAN on various physiological functions in birds, including behavior and biological clocks, as well as migration and reproduction.</p>