Well, Jesus was resurrected and made new after he died on the cross so that we could repent of our sins and be made fresh and new. Spring and Easter is kinda symbolic of this. New life is born in the spring (mostly). Baby chicks, bunnies, and lots of new born animals. Like above said it doesn't have a whole lot to do with the resurrection but spring and Easter is just a nice time where everything is fresh and new.
Nothing ... but they are part of the Spring celebration of Ester, which the Christians "converted" to Easter. Much like they "converted" other pagan celebrations into Christmas - and so we have Christmas trees, decorative lights, and presents.