Unless it was a predator kill or something obvious such as internal laying most of my bird deaths are mysteries. By that I mean that I almost never have two birds die in a flock within a month of each other and most of the time there is nothing that I notice outwardly about them. I just find them dead on the ground. Out of four layer tractors, a static hen yard, and a rooster pen I lose maybe a bird every two to three months for reasons that I cannot readily discern. "Natural causes" is what a coroner will usually put down for people who expire for no apparent reason and that's what I attribute to my mystery bird losses.
If I had several closer together in time, especially if they were in the same flock, I'd definitely go looking for a reason and I'd start with the feed as centrarchid suggests along with any outward symptoms they may be showing.