Variation during breeding in parameters that influence blood oxygen carrying capacity in shearwaters

Parameters that influence blood oxygen carrying capacity (whole-blood haemo globin content, haematocrit and red blood cell count) were measured in samp les of 30 breeding, adult short-tailed shearwaters (Puffinus tenuirostris) on Phillip Island, Victoria at seven key stages of their reproductive cycle . The aim of the investigation was to determine whether variation in blood oxygen carrying capacity during the birds' 7-month breeding cycle was corre lated with variation in the energy demands they experienced or was an incid ental by-product of other physiological changes. All the blood parameters v aried significantly during breeding, but the pattern of variation was only partly correlated with the likely pattern of changing energy demand imposed on parents by their schedule of breeding activities. The main trend concei vably related to energy demand was that significantly higher values were re corded for these blood parameters during the nestling stage than earlier in the breeding cycle. This could have reflected the high costs of the very l ong foraging trips undertaken by parents feeding nestlings, but it could al so have occurred in preparation for the long migration undertaken soon afte r breeding finished. It involved an similar to 10% increase in blood oxygen carrying capacity above the lowest mean value recorded during the breeding cycle and so other mechanisms must also be employed to achieve the increas e in aerobic metabolism likely to be required at this stage. The lack of ad justment of blood oxygen carrying capacity to energy demand early in the br eeding cycle suggests that either oxygen delivery was not a rate-limiting p rocess for aerobic metabolism at that time or that delivery was enhanced th rough other mechanisms. At egg laying, females had a lower haematocrit and erythrocyte count than males, which could be attributable to either estroge nic suppression of erythropoiesis or an increase in osmotic pressure of the blood associated with yolk synthesis. Immature, non-breeding birds attendi ng the colony were of similar mass to adults, but did not show the increase in the parameters determining blood oxygen carrying capacity that occurred in adults later in the breeding cycle. Factors other than changing energy requirements (dehydration, burrow hypoxia and differential responsiveness t o capture stress) that might have influenced the pattern of variation in bl ood oxygen carrying capacity of adults during breeding are discussed.