I say, with Allah's guidance: If a bird or something similar dies in the well, it is considered: If the object in the water has swollen, all the water in the well must be removed. However, if it has not swollen, 20 buckets must be drawn from the well as an obligation, and up to 30 as a recommendation, after removing the object from it. See: Al-Ikhtiyar 1:26, Al-Wiqayah p.102, and Allah knows best.