Every year on Sept. 11, two soaring beams of light turn on over Lower Manhattan in a glowing tribute to those killed 18 years ago. Observers looking closely can spot dots and shapes gathering inside the towers of light, moving, multiplying, wheeling lower and lower.

These dots are actually birds, who are drawn to the light along with insects and bats. The birds circle these lights, seemingly unable to veer away, and are pulled off course, putting them at risk of deadly exhaustion or injury. Sometimes, their calls to one another can be heard, which increase, scientists say, with their confusion.

By twist of fate, the anniversary of the Sept. 11 attacks coincides with a much older yearly ritual: the migration of birds across New York City as fall approaches. The convergence creates a spectacle that is eerily beautiful, yet according to one study endangers some 160,000 birds a year, starkly illustrating the perils of humans and animals sharing an urban ecosystem.

The tiring detour through the beams of light can put birds at risk of starvation or injury to populations already threatened by light pollution, collisions with buildings, habitat destruction and climate change.