"The only materials needed for sketching from live animals in the field are two pencils, one soft and blunt, and one fairly hard and well sharpened; a sketch-book, preferably of the loose leaf kind, of small size; and a soft eraser."
So begins a 1907 article on animal drawing in the children's magazine St. Nicholas Illustrated Magazine for Boys and Girls.
After drawing the main outline, the observer should make notes about key markings or coloration.
If you get nothing else, "first get the slant of the figure and the angles formed by the head and neck, legs, wings and tail, and then, if the bird is still before you, proceed with the smaller details."
Don't waste time on details that you could get from a mounted specimen.
The authors recommend that when the young artist returns home, he or she should fill scrapbooks of assorted outline sketches by cutting out the best examples of each species and putting them together with other similar poses.
Later, when you want to produce a finished picture, you can choose from the quick silhouettes and develop them into a more complete painting or drawing.
St. Nicholas: A Children's Magazine