Kyoufu no Kuchibashi

Saori grew up in a family with a crazy custom of feeding dead family members to crows. One day, Saori mistakenly believes that her only best friend, Kiyo, has betrayed her and dies in an accident. Saori's resentment turns into a swarm of creepy crows that begin to attack Kiyo and her classmates..!

(Source: MangaDex)