Annual Cycle of Physiological Stress and Condition of the Silvereye, Zosterops-Lateralis (Aves)

Abstract

The physiological condition of a natural population of silvereyes was monitored near Margaret River in Western Australia in each month for a complete year. Plasma samples were analysed for total corticosteroids, glucose, osmolality, urea, sodium, potassium and chloride; blood haematocrit was recorded. Carcasses were processed to yield body weight, total body water, fat content, lean dry-body weight and lean dry weight of the breast muscles. These results showed that in March total corticosteroids were high, haematocrit was low and fat contents were low. In June, fat contents were low and urea levels were high. Although breast muscle weights did not differ significantly between months, lower mean values during June may indicate that the labile protein content is 58% lower than in May. These results indicate that silvereyes are stressed in March and in poor condition in March and June. March and June may be the periods when natural mortality limits the population size, even in years when the March food supply is adequate.