My pre K, Kindergarten and first grade classes started by using round container lids (recycle those sour cream and yogurt tops, y'all!) to make four different sized circles. We then added beaks, eyes, wings and long skinny bird legs.
Next, we traced over the lines with black Sharpie markers, an art teacher's best friend, and colored in with crayon. To make a great crayon resist, you have to be sure the kids color in darkly, and make a good shiny surface to repell the watercolor. After we finished coloring the birds we used a "magic" white crayon to color scribble-scrabbly lines around the birds to be our snow clouds..
The second art class is when the magic happens. By painting on top of the negative space with blue watercolor, the white snow clouds "magically appear..." causing lots of oohs and aahs from my little ones.