What is the expiation for torturing a chick to death at a young age, and are multiple good deeds sufficient for the forgiveness of this sin and the acceptance of repentance?

Allah accepts repentance from any sin, including animal torture and killing, a sin not greater than disbelief (kufr). Repentance is achieved by regretting the sin and resolving not to return to it in the future. This applies if the sin was committed while you were held accountable (mukallaf). However, if it occurred before accountability, there is no sin upon one who is not accountable. Treating animals well, feeding them, giving charity to the poor, and frequently seeking forgiveness are good deeds that increase your success and are a sign of sincere repentance, for the mention of good deeds has been coupled with repentance in the Holy Quran.