The first siskin shot is actually quite good considering it's through glass. Checking the EXIF I can see that it's shot at 1/200 second. Because you're presumably using image stabilisation, that shutter speed is good enough to stop camera shake at your 300mm focal length, but it won't stop subject blur. So the best time to snap is when the birds pause. They will do that as they stop and listen for danger. With some birds like bluetits, that pause is very brief indeed so you have take lots of shots to get some good ones.
Many birds will fly to your feeder even if you're outside in full view of them. Stand about 15 feet away and keep still and quiet. Once they start to come down to the feeder, edge closer until you reach a distance that both you and the birds are comfortable with. The first time you hit the shutter they may fly off but just stay put and they'll come back - they get used to it surprisingly quickly. Bluetits and robins are the least timid and will lead the others.
The tip about having a perch set up that they can land on before going to the feeder is a good one.