CHIHUAHUA, Mexico (NBC) – Security footage shows how a flock of birds drop dead mid-flight in the northern Mexican city of chihuahua.

Sectional police of Alvaro Obregon reported that close to a hundred yellow-headed blackbirds that migrate from northern Canada to Mexico for the winter.

A local veterinarian reported to the police that the birds could have died after inhaling toxic fumes from a heater nearby or because of an overcharge from electricity cables.

However, two days later, local reports said the incident was caused by the birds becoming disoriented. A radio station, La Ranchera de Cuauhtemoc 89.7 FM, reported that the birds were flying at speed when they lost stability and collided into one another, causing blows strong enough to kill many of them.

Nevertheless, the authorities still don’t know what caused the deaths.

A similar incident was reported in Wales in March 2020 when dozens of starlings were found dead on a street. Experts speculated the birds had gone into a dive murmuration, or formation, together and hit the pavement.

In 2014, USA Today reported that birds had fallen from the sky in St. Louis, Michigan, because they ate insects and grubs from contaminated soil. A forensic study found lethal levels of DDT in the birds.