Q: Does the amount of light, or its direction, have any bearing on the effectiveness of decoys during the course of a day?

A: There is no question at all that decoy patterns show up much better in bright sunlight, particularly when pigeons flighting toward the field have the sun behind them.

This means, in most circumstances, the sun is now in YOUR eyes, but I think that is a small price to pay if it means your layout is more visible to the birds. In situations where I have the choice, I will ALWAYS arrange things so that the pigeons have the sun behind.