In "The Birds" by Daphne du Maurier, what are some details that suggest that an evil force might be directing the birds to turn on people?

In "The Birds," there are a number of details which suggest that the birds might be under the influence of an evil force. When Nat goes to pick up Jill from the bus stop, for example, he notes that the birds appear as though they are waiting for their next command:

It was as though they waited upon some signal. As though some decision had yet to be given.

This quote implies a hierarchy or some chain of command, of which the birds occupy the lowest position. This idea is further reinforced by some of Nat's observations of the birds' movements:

They headed, in bands of thousands, to the four compass points.

This suggests that their movements are prescribed and rehearsed; that somebody or something else is in control.

For Nat, the tide and the east wind are the evil forces which direct the birds. They hold power over the birds and signal the beginning and the end of each attack:

There was some law the birds obeyed, and it was all to do with the east wind and the tide.