Before we even discuss which are the factors that you need to take care of while selecting the best birding binoculars I think I must tell you that it is definitely not an exact science and a lot depends what your own individual tastes and preferences are. In fact the factors for buying these birding binoculars may just well vary from individual to individual so much.
That is why you will see people buying Vortex and some people buying Leupold to some going in for Nikon’s and Bushnell’s.
That said there are same features that you should know which will help you select the right binoculars for bird watching.
Magnification – You should go in for a 7, 8or at max a 10 magnification and the reason being that with anything above 10 the overall available area which is called as Field of View will become less and less. That may not be good with bird watching as you need to have a bigger area to view all the birds which are there.
For detailed discussion on magnification so you read this page here.
Objective Lens diameter – this has been discussed here a number of times whoever for the sake of completeness I must say that the best objective lens diameter for birding binoculars is to be tween 30mm – 40 mm. The bigger the objective lens diameter the expensive and bulkier it will be. No doubt it will give more light however there has to be a balance between all three factors.
Exit pupil – The basics about exit pupil can be read here however the best binoculars which are required by bird watchers should have an exit pupil of 4mm.
Filed of View – This is something which is a must to know so that you can view the birds properly. The requirement of birding binoculars is very different than hunting binoculars when it comes to Field of View and the reason is that you are looking at small creatures as opposed to some large animals like deer etc. Here is a primer on Field of View.
The field view is in inverse relation to magnification. So that means that higher the magnification the smaller will be the field of view. It is here the individual preference will come in and you may need to strike a balance between these two.
Prism design – Roof vs Porro prism design.
You can read about Porro prism and roof prism however the decision on which one to use for bird watching is something that you will need to take. Roof prism binoculars are more costly but they are more lightweight. Take your pick !
Lens coating and light transmission. – I have done a detailed article on binoculars lens coating and that should be read before you make a decision.
Apart from these I do not think you should be worried about anything while selecting the best binoculars for bird watching. In fact there are a lot of small things like binocular waterproofing and binoculars ruggedness however these are better left as individual decision as opposed to something which is a must have for selection of a good pair of binoculars.