They because disoriented, chirping when they should be sleeping. This is especially true for birds like the American Robin and the Song Sparrow who mainly live in urban areas.
Light pollution is more than just a nuisance to birds. Light confusion can interrupt migration cycles and threaten breeding activities. Bird enthusiasts eagerly await the late spring arrival of birds from the southern regions, but the most observant birders will notice that fewer birds return from their trek.
For a bird, migration is a physically taxing logistical dance, one that is dependent on evolutionary instincts practiced over centuries.
Because most songbirds migrate at night, light pollution interrupts their circadian rhythms and disorients them.
Lights from buildings attract the birds, leading to fatal collisions during the trip. In the Northeast region of the country, there is a gauntlet of major, brightly-lit cities that the birds must navigate in order to make it to their warm-weather destinations safely. Birds live precarious lives, often spending their days protecting themselves and their offspring from predators of all types. A bird may chirp and sing at night to warn other birds of predators in the area.
Birds may team up and chirp loudly in order to create a menacing cacophony of voices meant to scare away a roaming cat or circling hawk. If there is a clear and immediate danger, birds may emit high-pitched sounds in order to warn other birds to leave the area. Birds not only send out the warning call to their own species but to others as well. A robin may send up the distress call, warning the goldfinches, sparrows, nuthatches and finches that a predator is nearby.
Some of their calls are barely audible to humans, while others sound like a pleasant song in the middle of the night. Birders often race outside to see the songstress, only to be met with a view of empty trees. Almost any bird can sing at night if the situation warrants it, but there are some birds that exclusively sing at night.
The Eastern Whipoorwill is not technically a songbird, but its distinctive chirp has been known to punctuate the summer night in many urban areas.
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A primary reason male birds sing is to attract mates, and it has been found that unmated mockingbirds and nightingales sing at night more frequently than mated males. Until relatively recently, not too many other species of diurnal songbirds were known to regularly sing at night. In the last decade or so, researchers have determined that more bird species are singing at night in urban areas so that they do not have to compete with ambient noise such as traffic sounds that are more common in the daytime.
In addition, some species have started singing earlier in the day or have increased the volume of their songs in noisy places. Not only is human-created environmental noise often simply loud, but it tends to be generated at lower acoustic frequencies. A number of species, including Song Sparrow and House Finch, have been found to sing with modified acoustic frequency in response to human-generated lower-frequency noise, reducing the masking of their lower-frequency notes by ambient clamor.
Being heard clearly is important to birds for mate attraction, territory defense, and communicating threats to other individuals. Noise pollution has had similar impacts on other types of creatures, including amphibians and insects.
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