Congrats on the DSLR, and welcome to the forums. Certainly birding can become quite addictive, and starting off is always fun but with a lot of things to learn. Most of us have been there, so no worries...you are receptive to input and critique, which is the key to learning and getting better.
I see you are shooting with a Sony A390...you mentioned 300mm, but what lens in particular are you using? In general, a zoom lens which extends to 300mm, especially if it's a fairly basic inexpensive zoom lens, will struggle a bit with softness, too small an aperture in lower light, and sometimes chromic abberations when used at full zoom extension. It can get you by in a pinch, but certainly if you think birding will become more of a hobby, you might start looking for slightly better and faster lenses for the job.
Without knowing your EXIF data, it's hard to tell - I'd say the glass window you were shooting through likely had some impact on sharpness and contrast...it usually does. Whenever possible, shoot with as little glass as possible between you and the subject - which includes windows, filters, etc. Also, it seems like you have slight motion blur, either by you or the birds, in almost all the shots - which tells me the shutter speed was likely just a bit too slow for the subject matter. In general, smaller birds are going to require shutter speeds of 1/250 or better most of the time, especially with a longer lens, to remain sharper and free of blur. Your camera has stabilization in the body (make sure it is turned on!!)...you should have a shake indicator in the viewfinder that lets you know when you are moving around - try to glance at that as you are ready to shoot - attempt to only shoot when the shake meter is on the first two bars or so - anytime it's maxed out at the 3-4 bar area, you may be moving too much for the IS system to counter you.
Light is a big factor - looks like you had some overcast while shooting. You can get by in this weather, but sometimes have to raise the ISO a bit to compensate. If you were shooting at ISO100/200, then try moving up to ISO400 or 800...you'll get some more noise/graininess in the shot, which can be removed or reduced in post processing, but you also will get better shutter speeds and less blur. You could try Auto ISO - though sometimes your camera may try to go higher than you really want.
If you aren't already, always half-press your shutter to lock and hold focus before shooting - I find using Autofocus Single mode to be best, as it will focus one time and fix, until I release half-press and ask it to focus again. AF Continuous can help with tracking a moving subject, but with a still subject, any minor movements by you can cause the focus to grab something else and refocus there before you take the shot. Stick with spot focus if you need to pinpoint just what to focus on, and use the half-press shutter with single focus mode to hold the focus only where you want it. Then, when you fully press the shutter to take the shot, there is no delay or lag - it snaps immediately.
When using spot meter, always try to put the focus crosshair on the birds' eyes - that keeps the proper focus area where it most needs to be. When shooting birds, don't be afraid to take a few shots...either using burst mode or by pressign the shutter a few additional times. Birds are so skittish and move about so often, that taking 3 or 4 shots lets you pick the best of the 4 instead of hoping you got it in 1...even if you do everything right, your 1 shot may be the one in which the bird blinked or turned it's head...taking 3 or 4 means one of them will likely get everything lined up.
Hope that helps a little!
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 150-600mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony A6000 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses