Fat and weight variations of blackcaps wintering in southern spain

Abstract

Weight and fat variations of Blackcaps Sylvia atricapilla have been studied during several winters, and a “winter rest period” with a slight increase in weight and fat has been detected. These increases do not become substantial until March and April, coinciding with premigratory restlessness. Fat scores varied from 1.6 ± 0.2 in October to 3.6 ± 0.8 and 4.5 ± 0.9 in March and April respectively. Weights were correlated with fat scores, ranging between 17.7 ± 0.7 in October to 20.6 ± 1.6 and 21.1 ± 2.3 in March and April respectively. Interannual differences in weight and fat were recorded between the 1981–82 winter and the subsequent winters 1982–86, mainly due to variations in the resources available in the Pilas area. Changes in the weight and fat scores of recaptured birds and their diurnal variation were analysed, and an average increase during the day of 0.4 g in weight and 0.91 in fat score were detected. The relationship of weight and fat score to the range of diurnal variation in temperature was studied: An increase in weight, but not in fat, being detected. There is a latitudinal pattern of body weight within the wintering range of Blackcaps. Higher weights are recorded in Central European areas than in the Mediterranean. Differences in weight in different habitats of southern Spain have been found during the same winter, this being related to food resources. A redistribution of the wintering birds may occur, depending on the available resources in each local habitat at the beginning of winter. © 1989 Taylor & Francis Group, LLC.