I see from the EXIF that -
Shutter speed is 1/1000, that is usually a good startup speed for birds.
ISO - 5000, am I reading this correctly?
If true, at that high an ISO images will come out softer then they would be at lower isos.
For next time -
- always focus on the eye or as close to it if that is not possible.
- angle the shots in such as way that you get some catchlight in the eyes and if possible some light falloff on the feathers.
- use a smaller aperture to get a deeper DOF that will ensure that more of the bird is within focus.
- use a lower ISO. I personally do not go above 1600.
- for stationary birds one can get pretty good results @1/500 to 1/800. That will help reduce the need to up the ISO.
- for more natural looking shots avoid getting the feeder in the frame. Best option is place the feeder within a clump of bare(ish) branches. Alternatively you could try "installing" branches around the feeder area.
Since you are using a tripod there is no need at this time to get a new lens. Just improve your skills. You will know when it is time to upgrade.
Hope this helps.
Looking forward to your future pics.