Depends what birds and where they are.
Canon 17-85mm kit lens.. and about a kilo of peanuts.. and a set-up only really practical for a back garden (but you don't say where you want to take the shots of birds).
There was a neat solution to getting more naturalistic setting for garden bird photography a week or two ago in AP. Making up a fake pool in a raised tray with careful arrangement of the window dressing to persuade the birds to go to a fixed point to drink. This autumn/winter I plan on applying some of this idea to improving the backdrops.. but I don't plan going beyond 105mm focal length.
At last a true common sense answer to problems, AND reminding everyone that size doesn't matter!, it's what you do ......., and so true, about the lens, however why spend all that time as AP say making a fake pool, when in four hours you can have the real thing, photo at pool, and kit by pool.
Though I prefer your train shots.
So easy sitting with a coffee at 50' wireless
Also the pool has the benefit of attracting all wildlife through the year, frogs, newts, toads, birds and now we have a fox and hedgehog, cost of pool, logs free from local cemetery, liner £10, time 4 hours.
Go for a fixed 400+ if all you want is birds, but will you, 100% of the time only want a 400+mm, what about 100/200/300 only a zoom or a LOT of money can offer that.
400mm can more often than not be more than enough..........
Aviemore, "Scottish sunrise"
I used to live in Inverness, photographers paradise, Scotland.