Every night, a murder of crows lays claim to entire blocks of downtown Denver – and the locals have taken notice.

Thousands of birds fill the Mile High City’s skies like clockwork each evening, flying in such large numbers that they look like black clouds descending upon the city. And as one might expect, they leave droppings that coat the streets.

The National Audubon Society describes a rookery as a “giant avian slumber party.” According to the organization, the rookery offers birds warmth, protection from predators, knowledge about food sources, and a chance to find a mate.

Every day at dusk, crows flock to the rookery on 17th Street between Larimer and Curtis. According to reporting from Denverite and Gath Spellman, an avian expert with the Denver Museum of Nature and Science, birds from Colorado’s mountains and further north congregate in the city to avoid harsh winter conditions.

According to Spellman, the rookery is not only for social purposes. It’s for safety as well, and the body heat helps keep the birds warm through frigid Colorado nights.

As the birds continue their nightly ritual they might not seem like they’re going anywhere. That said, next time you’re walking around the Mile High City and spot them in the sky, know that numbers should drop by the time warmer summer days hit.

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