The birds can be done the same way. Technically, anything can be done this way. All you need is photo at the pose/angle you want (or something close). Just use an imaging program to re-scale the photo to the size/scale you want, then trace it out. You can also blend two or more photos. If you have some drawing ability, you can also change some aspects of the image as you draw it. I use these techniques in own scratch-built flats. I imagine others do - to one degree or another - as well. If you have any questions just ask. I check in at least once a day.
Cheers,
Glen