The period of nesting development of Wilson's storm petrels (approx. 60 days) could be divided into three stages: first from hatching to 8th—10th day of development; second, from 10th to approx. 25th day and third from 25th day until nestlings leave the nests. During the first stage hemoglobin concentration in the blood decreases significantly while total surface of erythrocytes and the hematocrit increases. At that time nestlings do not grow very fast. In the second stage of development the values of all studied parameters do not change, while the growth of body weight is very intensive. The last stage is characterized by significantly reduced growth rate accompanied by important changes of all hematological parameters responsible for the respiratory function of blood volume unit.