The sound of a bird singing long after sunset can be puzzling, as most avian species are active only during daylight hours. Bird songs are complex forms of communication, typically used by males to defend territory and attract a mate. The reasons for these unexpected nighttime vocalizations are varied, stemming from natural biological necessity and adaptations to the human-altered environment.
Confusing Dawn: The Impact of Artificial Light
The most common modern cause for nocturnal singing in diurnal birds is the widespread presence of artificial light at night (ALAN). Sources like streetlights, security lamps, and commercial lighting disrupt a bird’s internal biological clock, known as the circadian rhythm. This light pollution tricks the birds into misperceiving the time of day, confusing the transition between dusk and dawn.
This results in an extension of their active period into the night, causing non-nocturnal species to start their dawn chorus earlier and continue singing later. In the brightest urban areas, some diurnal species vocalize for nearly an hour longer each day compared to birds in naturally dark environments. The light acts as a premature trigger for the hormonal and behavioral cues that signal the start of the day’s activities, such as territorial defense and mating calls.
Birds are not uniformly affected; species with larger eyes, which are more sensitive to light, show a stronger response to artificial illumination. The onset of singing in these susceptible species can begin up to 35 minutes earlier than normal. The effect is most pronounced during the breeding season when the pressure to establish territory and find a mate is highest. This disruption may impact the energy reserves of the birds, as the extra singing time reduces the period available for rest or foraging.
Naturally Nocturnal Vocalizers
While many night singers are confused by human infrastructure, another category of birds vocalizes after dark because it is natural for their species. These birds are equipped by biology and evolutionary history to be crepuscular or truly nocturnal. Their nighttime songs fulfill the same purposes as daytime songs: establishing territory and attracting a mate.
The Northern Mockingbird is a prime example in North America, famous for its varied singing throughout the night, particularly during the early summer breeding season. Unpaired males are the most active nocturnal singers, using the quiet hours to advertise their availability to potential mates. Similarly, the Common Nightingale in Europe is known for its powerful, flutelike song at night, which males use to attract or guard a female.
Other species are active during the night for reasons related to their hunting or survival strategies. Nightjars, such as the Eastern Whippoorwill, are insect-eaters that camouflage during the day. They rely on the cover of darkness to hunt flying insects and communicate with distinctive, repetitive calls. For these naturally active species, the quiet night environment represents their normal time to conduct activities necessary for survival and reproduction.
Acoustic Adaptation: Singing When It’s Quietest
A third reason for nighttime singing is a behavioral strategy adopted by birds to overcome high levels of noise pollution in urban environments during the day. This phenomenon is explained by the “Acoustic Masking Hypothesis.” Daytime noise, primarily from traffic and construction, contains low-frequency sounds that overlap with and drown out the birds’ vocalizations.
Bird songs are crucial for territorial defense and attracting mates, and if these signals cannot be heard, reproductive success is compromised. By shifting their singing to the quieter hours after human activity ceases, their songs travel farther and are detected more easily by rivals and mates. This is a deliberate adaptation to a noisy environment, separate from the confusion caused by light pollution.
Studies of species like the European robin show a direct correlation between high daytime noise levels and a shift to nocturnal singing. Robins in areas with high average daytime noise, sometimes exceeding 60 decibels, are significantly more likely to sing at night. This choice, despite the metabolic cost of being active instead of sleeping, demonstrates the importance of clear acoustic communication. The quiet night offers a window where the bird’s signal is not masked, ensuring the successful transmission of their messages.