Lesson No 1 .... Don't mix with people with big lenses. It's contagious.
I'm not into bird photos particularly, but I do photograph animals. I find being close to humanised animals tends to cause them to interact and want to approach me. That is a problem, so I wanted a bigger lens to be able to stand further away.
As everything here is strictly low budget, there were not a lot of options. I ended up with a second hand 100-400mm canon lens, which I notice quite a few bird photographers use. My choices were that lens, or the one MarkL uses, a Sigma 120-400mm. Price wise I had to pay about $100 extra for the canon, but, in return it was lighter. Having older arthritic hands, I thought the extra price for a lighter lens was worth it.