It's all about where the light is coming from. For bird photographers, there are three possibilities; the light can be from in front of us and behind the bird thus backlighting the bird, the light can come from one of the sides of the bird giving us side-lighting, and the light can come from behind us and thus lighting up the bird without shadows. If there are "rules" kinda like the "Rule of Thirds," then Pointing Your Shadow at the bird has become one of the rules of bird photography. And as a general "rule" or best practice, it's almost always going to give you a good photograph. But like the Rule of Thirds, there are other compositions and directions of light that will work and sometimes work better and create a stunning image. Backlighting or creating a bird silhouette at sunset can be very dramatic, side-lighting can show more character, etc.
In this image, the Sandhill Crane is fully lit up with some soft morning light. There are no harsh shadows because the sun is behind me over my left shoulder.
In this image at the same pond, different day, the crane has flown a little past me, and you can begin to see the shadows on its back. The sun is still coming from behind me and over my left shoulder, but where I'm standing in relation to the crane isn't as optimal as the prior image. In this photo, I need to move to the left to get a little more in front of the crane. And that's the big secret; we need to move around and put ourselves in the right position in relationship to the sun and the bird. And since birds fly into the wind, if the sun and the wind are at your back, well, that's a perfect combination. So each morning at Bosque del Apache, where these images were created, we always try to place ourselves in the right spot in relation to the sun, the wind and the birds.
Enjoy & Thanks