Is There Seasonal Variation in Stored Energy Reserves and Catabolic Enzyme Activities in Superb Fairy-Wrens?

Abstract

Birds face several energetic challenges during their annual cycle and have a diverse array of adaptations that help them to meet the additional energy expenditures involved. We determined whether there was adaptive seasonal variation in lipid and glycogen storage and the activities of some of the associated catabolic enzymes in a population of Superb fairy-wrens (Malurus cyaneus), 10 g insectivorous birds inhabiting heath vegetation in temperate SE Australia. Mean body lipid content and activity of β-hydroxyl-CoA dehydrogenase did not vary seasonally. In spring, mean liver glycogen concentration (μmol glucose [g tissue]⁻¹) was more than double that in winter and mean pectoralis muscle glycogen concentration was more than twice that in any other season. The spring activities of hexokinase, phosphorylase and citrate synthase were significantly greater than those in at least two of the other seasons. The lack of winter fattening may reflect the relatively mild winter climate at low altitude at 37°S in SE Australia. Spring increases in stored glycogen and glycolytic enzyme activity were correlated with the additional energy expenditures of breeding, but no comparable variation was associated with autumnal moulting. More research is required to determine whether climatic and life history differences between north and south temperate birds have resulted in disparate strategies for coping with annual energy bottlenecks.