Upgrading lenses is so intimidating.

Please read full post before replying please. (I promise I'll keep it short and sweet)

I've been shooting with my D90 18-105mm Kit for about two years.

Recently I've been feeling myself being sucked into the wildlife area. To be more precise, birds. So, naturally I'm out growing my current lens and looking at telephotos.

This I feel is the best shot I've been able to get out of my lens thus far:
(In terms of fast subjects)

What would you reccomend that would give me more clarity and a longer reach, without setting my bank account aflame? I'm thinking in range would be up to $600.

I know this isn't much but the urge to try something new is almost overwhelming and even if it's just a lens to tide me over until I saved for a prime would be helpful.

Thank you in advance.