I'm currently reading the book "Alone" which is part of Winston Churchill's series on World War II. At one point Winston mentions that spotters looking for - and giving warning of - German air raids were eventually known as "Jim Crows. How did this come about? Was this a common nickname?
It was certainly in common use. The [October 10th issue of Flight] (https://www.flightglobal.com/pdfarchive/view/1940/1940%20-%202884.html), for example, has an article on "Jim Crowing: Roof Watching Without Tears". In RAF jargon, the lookout meaning was extended to reconnaissance flights, e.g. in [The Royal Air Force 1939-1945 Volume 1] (https://www.ibiblio.org/hyperwar/UN/UK/UK-RAF-I/UK-RAF-I-12.html), "...we had arranged no other patrols in the Channel except the routine Fighter Command reconnaissance known as 'Jim Crow'".
The precise etymology is uncertain; [wordorigins.org] (http://www.wordorigins.org/index.php/site/comments/1940_words/) suggests "From either thieves’ cant for a look-out or from crow’s nest". A footnote in [Newsweek] (https://books.google.co.uk/books?id=kG06AQAAIAAJ&focus=searchwithinvolume&q=%22jim+crow%22) concurs with the latter, saying "A term evidently meaning the lookout in a crow's nest. The American expression "Jim Crow", applying to segregation of Negroes, is not commonly used in England."