Why are our dogs throwing up?

Ok, so our 2 year old female Boxer and our 10 year old female German Shepard/Chow mix are throwing up ALOT! I've seen our Boxer and Chow mix throw up in the last 2 days. Its not the food because we've been feeding them the same things. When both of them throw up its kinda like water, mucus, and spit. Is it because they killed a pigeon and licked up its blood ( We stopped them before they could do anymore than that)? Why are they throwing up all of the sudden?! Please Help :)