a65 & lenses
Whilst what the others are saying is true, that to do birding "really" well you need a mega lens, please dont let that put you off. It CAN and DOES get done with much smaller lenses. However you will need a bit more patience and to pick your spots well.
There are plenty of folks on here (and also in the Pentax forum where the 55-300mm is a really popular lens) that shoot with a lens in the 250 - 300mm range and get great shots. If you can master birding with the smaller lenses, such time as you get plenty of money and can pick up a larger, possibly prime lens, you'll be in a fantastic postion to know how to get the most out of the outlay.
You can always add a 1.4 teleconverter as well, which while it would cost you a little light and speed would certainly increase your reach substancially.
From the reviews the Tamron 18-270 is a great lens but is a little soft at the far end, but I think you'll be fine!
After 10 years of picture taking - you'd have thought I'd have learnt to remove the lens cap...