This composite looks great. There are a few minor lighting issues but overall, it's very well done.
Backlighting is always a nightmare and it's hard to make it look believable. The two birds at the the top of the image (yellow circle) are great! Their buddy to our right (yellow arrow) is rotated just far enough that the lighting under his left wing is off. He needs to be rotated counterclockwise so that his wing is in a better position to be catching the light from the sunset.
The two birds with the red arrows, are lit from the front, and to me, do not look like they belong in this composite. I'm not overlooking the possibility of a second light source, but nothing about the lighting of these two birds suggest this. The 'out of focus' bird on the left is a bit distracting. While it's focus is correct for the perspective, it might look better without it.
The rest of the image works. You could use the Blur Tool with a low setting on the edges of the tree to make it blend a bit better. It's edges are a bit sharp.