Hundreds of yellow-headed black birds swarmed together over the Mexican state of Chihuahua before they suddenly fell out of the sky, leaving the streets covered with dozens of dead birds.

Security footage shows the birds circling a house en masse and then hurtling toward a home on in Alvaro Obregon area of Cuauhtemoc city. While most birds fly back upward following the plunge, others continued downward, striking the sidewalk.

While clip started going viral on social media this week, officials said the gruesome incident actually occurred around 5 a.m. on February 7, according to local reports. It’s likely the bird species were in the process of migrating to Mexico from northern Canada when they collapsed.

Nearly a week later and local authorities are still not exactly what caused the birds to hit the ground.

Experts have posed a range of reasons for the freak incident, including that the birds lost stability mid-flight and then slammed into each other. Other theories include that they inhaled toxic fumes or struck electrical wires.