Abstract

The responses of small birds to many seasonal energy challenges include enhancement of aspects of aerobic metabolism, sometimes involving an increase in the rate of oxygen delivery to the metabolising tissues. One such mechanism that enhances oxygen delivery seasonally is an increase in blood oxygen carrying capacity. This response is enhanced in birds because of their rapid erythrocyte turnover rate. Some small birds have also evolved winter fattening, which helps them to meet the energy challenge presented by winter conditions. Such adaptations, while well documented for North Temperate birds, have received little attention in birds inhabiting temperate Australia.

Over a two-year period, we examined seasonal changes in mass, an approximate indicator of fattening, and the parameters determining blood oxygen carrying capacity in a population of superb fairy-wrens (Malurus cyaneus) in outer Melbourne, Australia. Body mass did not vary significantly seasonally, but haematocrit and whole blood haemoglobin were significantly higher in the breeding season than at other times of year and the erythrocyte count was significantly higher in spring than in autumn. We conclude that the failure of the fairy-wrens to increase mass in winter (i.e. show marked winter fattening) was probably due to the comparative mildness of the climate and to the known fitness costs of fat storage. The significant 18% increase in blood oxygen carrying capacity in spring probably helped the birds to meet the additional energy requirements of breeding, particularly the likely increase in flight activity. However, given the magnitude of the increase, other mechanisms must have been involved in meeting breeding costs. The seasonal peak in blood oxygen carrying capacity did not coincide with the time when moulting was most pronounced.