Sewing Up, Stuffing, and Finishing
Ok, so you've knitted all your knitted bird pieces, so now what? First of all, you need to sew in any loose ends on your work, including where you changed colour mid-row, as in the photo below, of Barbara the Blue Tit.
The gold-standard way of dealing with loose ends is to sew absolutely all of them in, and then use new lengths of yarn to sew your pieces together. I have to say, that personally I don't sew in all the loose lengths on the edges, I keep some out to sew seams or to sew wings onto the birds. I've never had any problems with this technique but if you would like to sew in all the ends, please do!
I don't recommend blocking, or pressing the knitted bird pieces. This is a process where the knitting is dampened, shaped and often pressed with an iron to even out stitching and give the knitted piece a defined shape. While you definitely should do this with knitted garments, it's not necessary with these small birds. Any uneven stitching and shaping will be hidden once the bird is stuffed and shaped by hand. Additionally, pressing acrylic yarn can make the yarn melt if you use too high a temperature!
Once you've sewn in your loose ends it's time to stuff your bird. I'd recommend taking a little time with the stuffing as it can make all the difference to the appearance of your finished bird. Take the stuffing out of the bag and fluff it up, to get rid of any lumps. I like to partially sew a seam and then stuff as I go, as it gives me more control over how the stuffing looks. As you stuff, roll and shape the bird in your hand to give it a smooth, uniform shape. Before you finish sewing up the seam, just give it a final check that you are happy with the shape and feel of the bird before sewing up the seam.
When it comes to sewing up, I like to use mattress stitch as it gives a smooth, almost seamless finish. Here's a guide to mattress stitch by Alexis Winslow: