sounds like it's not actually that they asked you to bring a chook that bothers you... it's the inequity of it.... that they don't usually contribute.....

I guess you are going to have to ask them to bring something next time - perhaps the manners they are lacking is in the 'offering' to bring something to yours... rather than in asking you to bring a chicken....

(personally I reckon a chicken is WAY easier to arrange than a salad or something.... there's no preparation!)

But I agree with most of the others..... if you are going to ask, you are silly to be annoyed when they ask you to bring something you're not happy about..... I guess if you want to be polite, you offer to bring something that suits you...., or you say... "hmm sorry, I won't be have time to get a chicken, but I have a frozen pizza I can bring if that's okay?" (but prepared for them to be annoyed at you though if you did say that.... cause... if you're going to ask... well .....

(still you're entitled to feel irritated if you feel the friendship is a bit unbalanced..... just have to work out what you can do to feel better about that )