Comparative study of winter body composition of resident and migrant grey-breasted silvereyes

Abstract

A comparative study of the winter body composition between resident and migrant birds was made on the partially migratory Tasmanian race of the Grey-breasted Silvereye (Zosterops I. lateralis). Each year some members migrate from Tasmania to winter on mainland sites, while other members remain in Tasmania year-round. Resident birds were those captured at Mt. Nelson, Tasmania, during midwinter; migrant birds were those captured during midwinter at Armidale, New South Wales, 1,500 km to the north. Nighttime winter temperatures at the two locations are very similar. Both residents and migrants showed diurnal increases in lipid content, but not in other major body components. Despite differences in photoperiod and feeding conditions at the winter grounds, lipid content accumulated in the late afternoon was similar (8% of live body mass) for residents and migrants. Fractional differences in major body components between residents and migrants were negligible, being less than 0.01 in all cases. Therefore, body composition is not related to whether an individual bird is a resident or a migrant. Low night temperatures alone do not explain why the species migrate from Tasmania to places like Armidale in New South Wales. The birds could migrate for reasons such as better food conditions, higher daytime temperatures, and longer daylength for foraging on the mainland.