Brendan Body has a great post on his website which displays lots of great reference and observations on birds and flight.
Brendan Body (Birds)
Animators Resource
Animators Resource is a similar website to this one but focuses more on specific posts and videos rather than linking to other websites.
Sketchfab
Sketchfab allows you to upload full 3D scenes for others to view directly in their browser. They are the world’s largest platform to publish, share and discover 3D content online and in VR.
Quickposes
Quickposes is a tool for art students, illustrators or anyone who wants to focus on improving their drawing skills. By practicing gesture drawing you will not only get better at recognizing certain aspects of poses, but you will also build a visual library of characters and models.
Living Lines Library
Living Lines Library is an amazing resource full of pencil tests, drawings, documentaries and more from the animation legends!
Mixamo Auto-Rigger (FREE)
Mixamo Auto-Rigger is a free online tool that allows you to upload any 3D human model with two arms and two legs and get it rigged with a full skeleton and skinning weights.
OpenToonz (FREE)
OpenToonz is a free open-source software for producing 2D animation and is known for being used by Studio Ghibli
Source Filmmaker (FREE)
Source Filmmaker is a free movie-making tool built and used by Valve to make movies inside the Source game engine. It is a full package and can be used to make cinematics using Valve models and environments.
Animator Island
Animator Island is a treasure trove of animation secrets, techniques, tips, and information on your quest to become a Master Animator
Animator's Resource Kit 