Mass and visible fat scores of skylarks alauda awensis in winter in south‐east Scotland

Abstract

Over six winters, from 1990/91 to 1995/96, Ski/larks were caught and ringed at Musselburgh, Lothian. The birds were sexedfrom biometrics, and mass and visible fat score were recorded in the field. This revealed that there ivas some sexual dimorphism in mass as well as size. Although males were significantly heavier than females (mean mass being 42.3g, range 35.1 ‐ 51.4g, female 36.3g, range 29.6 ‐ 45.4g), there was some overlap in mass between the sexes. Individual Skylarks showed variation in mass of up to 25% (maximum for male 21.8%, for female 24.4%). Mean mass for males increased through the autumn to peak at the end of January, while mean mass for females apparently peaked twice, in early January and in mid‐February, but sample sizes were small. In line with studies of other species, such variations in mass over time suggest that winter fattening occurs in the Skylark. For both sexes, there were more fat birds in January and. fewer in March than expected. Between the sexes it was found that, in February and March, relatively more females had visible fat deposits than males. On a broad scale, comparing “cold” winters (1990/ 91 and 1995/96) with “normal” ones, revealed that there were more fat females than expected in January (and possibly in February) in cold winters, but there was no relationship for males (except possibly in “cold” Februaries). On a narrower scale, looking at the effects of meteorological variables (mean minimum daily temperature and consecutive days of snow‐cover), significant relationships between these and fat scores were found for both sexes in January and February. Snow cover appears to have a greater effect than temperature on fat deposition. In line with other studies, it appears that the smaller (and hence probably subordinate) females were relatively fatter than the dominant males during the most stressful ivinter months for survival. This fat may be an insurance for sitting out severe conditions in situ, or may be utilised in emigration from severe conditions, since previous work by the present author, based on biometrics, suggested that “cold weather” flocks of Skylarks may be comprised largely of females and possibly subordinate males.