"According to Robert Claiborne in Loose Cannons and Red Herrings, it refers to city streets as they were before cars. "When I was a youngster on the streets of New York, one could both see and smell the emissions of horse-drawn wagons. Since there was no way of controlling these emissions, they, or the undigested oats in them, served to nourish a large population of English sparrows. If you say something's for the birds, you're politely saying it's horseshit."
Calling shitty generic deathcore bands 'progressive death/grind' is for the birds