The big lesson behind this demonstration is flowing from one sketch to another as live birds move before you. Do not get hung up trying to finish one sketch. If your bird moves, start a second, and then a third, right there on the same page. You do not have to finish any of the sketches. If your bird returns to a pose that you have already started, jump back to that sketch and continue drawing.
In this case I made sketches in three poses. The first two birds are roughly the same size and their feet come to the same level in the drawing putting them on the same plane. The third bird is much smaller. I placed the head of the bird at the same level as the head of the foreground bird. This also puts the birds on the same plane. When I added distant birds, their heads too fall on the same line (the horizon). This simple trick of aligning the heads allowed me to join all the birds into one scene. It is also fine to just jot new birds anywhere you want on the page. You do not need to incorporate all of them into a unified sketch. You can turn any drawing into a little landscape. Just draw a window behind your bird (or other focus object) and draw your landscape within it. Let the focal object break the frame of the box, extending beyond it her or there.
Click on the first image to start a step-by-step slide show.