![]() ![]() ![]() This result was consistent over the 2 years, with opportunities for error being assessed and eliminated in the second year. Dressing percentage decreased by 0.08/kg as liveweight increased. An interaction of sex with full liveweight in predicting carcass weight and subcutaneous fat indicated that as carcass size increased, the sex of the lamb became an increasingly important consideration when assessing characteristics important in describing lamb carcasses. Full liveweight, condition score, sex and year explained 80% of the variation in subcutaneous fat, 47% in GR and 57% in fat score. Condition score, sex and year explained 45% of variation in GR, 52% in fat score and 57% in subcutaneous fat. Full liveweight, sex and year explained 41% of variation in GR, 48% in fat score and 79% in subcutaneous fat. Carcass weight and fatness, measured either as indicators of fat (GR measurement or fat score) or as total subcutaneous fat, could be predicted from assessments made on live lambs. When good quality feed was again available, rams gained more weight than wethers, as did wethers compared with ewes, while rams and wethers gained more condition than ewes. The rams grew 0.16 kg/week faster when pasture availability was limiting for a period in winter but lost significant (P<0.01) body condition, whilst ewes maintained both weight and condition. In general, when pasture availability and quality was high, rams grew faster than wethers by 0.3 kg/week and ewes by 0.5 kg/week. In all stages rams grew significantly faster than wethers and ewes, whereas wether growth rate exceeded that of ewes in only 3 stages. Multivariate repeated measures analyses were used to compare growth rates for rams, wethers and ewes within 2 periods of growth in 1983, and 4 in 1984. In birds breeding in the northern hemisphere, the importance of a timely arrival to the breeding sites could explain why the control of the migratory programme is more rigid in spring.Lambs from the Poll Dorset x (Border Leicester x Merino) cross were raised from a late lambing in 1983 (a year when pastures grew unchecked and had high clover content) and 1984 (a year with intermittent dry periods and a low clover content in pastures). The comparison with a similar study carried out with the same modalities during spring migration suggests that there are seasonal differences in the sensitivity of the migratory programme to hormonal factors. To our knowledge, this is one of the first demonstrations that the autumn migratory programme can be altered by hormonal manipulation in migrating birds. The change in body mass during the time spent in the recording cage depended on food availability, but not on any of the other factors.ĬONCLUSIONS: This study shows that the migratory programme of two Sylvia warblers can be manipulated by administration of exogenous melatonin and confirms that this hormone is involved in the control of migratory behaviour. Fuel deposits also had a strong effect on the amount of nocturnal activity: lean birds with a fat score of 1 showed significantly less Zugunruhe than fatter birds. ![]() RESULTS: The melatonin treatment significantly decreased the amount of Zugunruhe, while the availability of food only tended to increase the amount of Zugunruhe. Food availability in the cage (yes/no) was added as a second factor because previous work showed that it enhanced Zugunruhe. atricapilla subject to temporary captivity at an autumnal stopover site. Here we tested whether a non-invasive treatment with melatonin can alter Zugunruhe in wild garden warblers S. However, previous studies indicated that the response to melatonin manipulation could differ between spring and autumn migration, which are in fact separate life history stages. In a recent study we found no effects of melatonin administration on Zugunruhe during spring migration. Previous work with Sylvia warblers suggested an involvement of melatonin, a hormone associated with day-night cycles in most vertebrates. The mechanisms that control this major change in activity are mostly unknown. The switch in activity from purely diurnal to also nocturnal is evident even in caged birds that during migratory periods develop an intense nocturnal restlessness, termed Zugunruhe. INTRODUCTION: A remarkable aspect of bird migration is its nocturnality, particularly common in Passeriformes. ![]()
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