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. Received 14 January 1994, accepted 27 May 1994. INCLEMENT WEATHER and low food availability during winter are often implicated as the main forces in the evolution of bird migration (Co- hen 1967, Berthold 1975, Gauthreaux 1982). Small passetines breeding in temperate lati- tudes are of special interest because they have high metabolic rates and substantial increases in rates of energy expenditure when confronted with winter cold and diminishing food supplies (Dawson et al. 1983). In order to reduce ther- moregulatory costs in winter, many species have evolved adaptations that enable them to mi- grate to warmer regions. These include enlarge- ment of the pectoralis muscle (Lundgren and Kiessling 1988), flying at night (Moore and Ker- linger 1991), and substantial changes in body composition leading to massive fat accumula- tion (Ramenofsky 1990). The latter is known as migratory fattening, which normally develops in the premigratory period in which fat stores can more than double compared with the non- migratory period (e.g. Odum 1960, Berthold 1975, Dawson et al. 1983, Bairlein 1991). Birds that remain in their temperate breeding area in winter may reduce energy expenditure by