The lens is the eyes of our camera. To see something we must have good eyes, if not, then we use eyeglasses. The lens is expensive and there is not alibi or excuse for not having it if you want to go birding. You maybe a birder or photographer, you need the best lens you can afford just the same.
The lens coat can be categorized as an accessory but I consider it now as a need. The coat can protect your lens from scratches, bumps, water (as much it could take, but not recommended to get drenched), cold, being contaminated by body oil; it could also be a shock or vibration damper instead of using another lens support that can be an additional to camera weight.
It can be expensive when you have to import it but if you pay attention, their are local makers of this rare and expensive accessory and need.