These two minute drawings were done with a 5B pencil from photos I have of my hummers.
While searching online today for gesture drawing information, I came across this blog / syllabus for a course taught by Marlene Angeja at San Jose State University. She is using Kimon Nicolaides' book, "The Natural Way to Draw" as the text book for her class which is shown in full here on this blog. This link takes you to the main page and a short video demonstration of gesture drawing. The introductory caption there reads; "The key to gesture drawing is speed. Work fast and trust your instincts. Practice holding the charcoal loosely between your thumb and index finger."