Just to be clear, I'm as confused as you :) However, http://www.studyspanish.com/lessons/gustar.htm from this page I figured out, that 'gustan' is probably the right version because it's not 'us' who is the subject, but the 'birds'. So if the subject is singular - gusta, if plural - gustan. Let's hope someone who actually speaks Spanish will correct me if I'm wrong :)
"A nosotros nos gustan los pájaros, pero también las tortugas."
This is what I was going for but I neglected the 'A' and typed 'los tortugas' by mistake. This makes me wonder what the purpose of 'A' beginning the sentence. When is it necessary and would it be OK to neglect it in this case? I'm baffled.
The "a nosotros" is there for emphasis/clarification. You can omit it and still have a correct sentence, but you cannot just use "nosotros" which is a subject pronoun. (Loose literal equivalent of what you are saying in Spanish: The birds are pleasing to us, but the turtles are also pleasing to us).