I'm assuming they mean light colored birds. Some breeds are predominatly a certain color, so perhaps they were referring to those who are usually solid white. Hawks do seem to give special attention to white birds, but I don't blame them. They stick out like a sore thumb
However, it wouldn't be any different if you had 40 white pigeons and one black one. He'd stick out too even though he's not a light colored bird.
Are you talking about a chicken or a pigeon? "Light breed roo" in chicken terminology would refer to a light, in terms of weight, breed of chicken....usually white egg layers, such as Leghorns and the Mediterranean class of birds.....
Conversely, heavy breed birds would be the heavier weights, ones that are dual purpose for meat and eggs, usually are brown layers.