Blurry Photos

shutterbug5

First of all, I am not a newcomer to photography. I am using a Canon EF 70-300mm f/4-5.6 IS USM lens that I got about 5 years ago - the VR is on. I got a Canon T3i for Christmas 2013. I always use a shutter speed over 1/300, my aperture varies from f/5.6 to 8, and I know aperture affects depth of field and how much will be in focus. I support my lens with my left hand from underneath and keep my elbows in. I mostly use the center focus point and partial metering. I always try to focus on the animals eyes. I noticed a lot of the cardinal pictures I took this winter through the window were not sharp, however some were. I chalked it up to the window, but recently I went to a wildlife park where it was sunny and warm, car windows rolled down. Over half the pictures I took had to be deleted and even 6 I took were black. Some of them look ok as 4x6's, but if I zoom in on them on the computer they are blurry - nothing is in focus. Any ideas? Is it the camera, lens or something I am doing that I didn't do before this year? Thank you!