Development of some aspects of blood oxygen transport in nestling short-tailed shearwaters

Body mass of nestling short-tailed shearwaters (Puffinus tenuirostris) on Phillip Island, south-eastern Australia, increased about twelve-fold over the first 65 of the typical 94 days of development, peaked at 33% above adult mass and then declined to adult levels at fledging. Haematocrit and blood haemoglobin concentration of nestlings were constant throughout the initial phase of rapid mass gain, but increased respectively from 30% to 55% and from 9 to 15 g per 100 mL once asymptotic mass had been attained. The erythrocyte count increased throughout development, but most markedly after attainment of peak mass. Consequently, the estimated 70% increase in blood oxygen carrying capacity during nestling development mostly occurred when mass gain had ceased. At fledging, this capacity was 81% of that of adults shortly before they deserted the colony. Over the course of nestling development, erythrocytes decreased in volume by 20% and became more ellipsoidal, which could potentially increase the rate of oxygen delivery to the metabolising tissues. The pattern of development of blood oxygen carrying capacity differed from those reported for most other altricial birds in that the increase in capacity was not continuous throughout development. We suggest that the relatively delayed development of blood oxygen carrying capacity in shearwater nestlings is permitted by their slow growth rate. It may be adaptive in deferring the costs of producing, and particularly maintaining, large numbers of small erythrocytes. The magnitude of the respiratory problem potentially posed for the nestling by burrow hypoxia may not be great and should in any case be reduced by its slow growth rate and consequently relatively low daily energy requirements.