Abstract
ABSTRACTFat storage is essential to the survival of many bird species, providing energy reserves, but can have an effect on locomotor performance with an associated potential increase in predation risk. In particular, the ability to initiate flight through jumping is critical to predator avoidance and may be influenced by changes in body mass (Mb). Here we investigate seasonal differences in the jump take-off performance of high Arctic Svalbard rock ptarmigan (Lagopus muta hyperborea) resulting from around a 50% increase in Mb during winter as a result of fat deposition. Using force-plate data and videography, we reveal that, in the absence of alterations to take-off angle, winter Svalbard rock ptarmigan are unable to increase hind-limb power output during jumping to compensate for their increased Mb. As a result, peak take-off velocity is reduced by 42% and jump duration is also extended during winter. The consequences of reduced jumping performance upon Svalbard ptarmigan during winter may be relatively small given their low risk of predation during this season. It may be, however, that the observed reduction in jumping performance when fat may contribute to the sub-maximal pattern of fat acquisition observed in other bird species.
Highlights
The seasonal and diurnal acquisition and maintenance of fat reserves is important to many avian species
The results of this study support the hypothesis that an increased Mb compromises jump performance in Svalbard rock ptarmigan and potentially other species as well
In summer, jumping performance was consistent with that found in other bird species
Summary
The seasonal and diurnal acquisition and maintenance of fat reserves is important to many avian species. Birds commonly rely on fat for insulation and as an energy source to be utilized during periods of food scarcity, low temperature or during migration (King, 1972). There are, associated costs that increase with the level of stored fat. Birds seldom maintain maximal fat reserves and stores are instead regulated through a cost versus benefit trade-off (Gosler et al, 1995; Witter and Cuthill, 1993). The primary benefit is a lowered risk of starvation, whereas costs are related to the increases in energy expenditure and predator susceptibility as a result of elevated body mass (Mb) (Brodin, 2001). The relative importance of these costs will vary
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