Twitter dropped the boring & ugly image for default avatars and replaced it with a much better twitter bird image as part of ongoing enhancements to the web site interface. In case you need a comparison of the old avatar with the new one, here it is.
The default avatars are in different colors for different people, but not sure on what basis the color are assigned to them. We noticed orange, blue, maroon and green color images.
Along with the changes to the avatar, twitter also tweaked the follow bar. It is extended along the width of the page and also hover menu options on the right side that allow – @mention, follow and block operations.