Originally Posted by Andrew Whitehouse
There's also fairly liberal use of more impressionistic, 'jizzy', in-habitat illustrations of how birds might look in real life - something I like to see and that the Collins pioneered.
"jizzy" in-habitat illustrations are actually frequent in pre-Collins guides as well. I've misplaced most of my oldest guides, but still have my 1970 version of The Hamlyn Guide to Birds of Britain and Europe. It has numerous such illustrations, e.g. on the bottom of p. 131 is an illustration titled "flock of Sanderlings and Ringed Plovers on a beach." (The book was illustrated by the American artist Arthur Singer, who also illustrated the 60s era Birds of North America by Robbins & Brunn, afaik the first guide to put maps, text, and illustrations all on the same page).
More to the point of the present thread, I have the eighth edition of Simpson and Day's Birds of Australia. It utilizes in-habitat jizzy illustrations to a greater extent than any other guide I've seen (except the photo-based Crossley guides). Not sure if the pre-Collins editions of that guide did the same, but it wouldn't surprise me.