Energy expenditure during flight in relation to body mass: effects of natural increases in mass and artificial load in Rose Coloured Starlings

摘要

Rose Coloured Starlings (Sturnus roseus) flew repeatedly for several hours in a wind tunnel while undergoing spontaneous variation in body mass. The treatments were as follows: flying unrestrained (U), with a control harness of 1.2% of their body mass (C), or with a harness of 7.4% of their body mass, which was either applied immediately before the flight (L_S) or at least 9 days in advance (L_L). Energy expenditure during flight (e_f in W) was measured with the Doubly Labelled Water method. Flight costs in L_S and L_L were not significantly different and therefore were pooled (L). The harness itself did not affect e_f, i.e. U and C flights were not different. e_f was allometrically related with body mass m (in g). The slopes were not significantly different between the treatments, but e_f was increased by 5.4% in L compared to C flights (log₁₀(e_f) = 0.050 + 0.47 × log₁₀(m) for C, and log₁₀(e_f) = 0.073 + 0.47 × log₁₀(m) for L). The difference in e_f between C, L_S and L_L was best explained by taking the transported mass m _transp (in g) instead of m into account (log₁₀(e_f) = −0.08 + 0.54 × log₁₀(m _transp)). Flight costs increased to a lesser extent than expected from interspecific allometric comparison or aerodynamic theory, regardless of whether the increase in mass occurred naturally or artificially. We did not observe an effect of treatment on breast muscle size and wingbeat frequency. We propose that the relatively low costs at a high mass are rather a consequence of immediate adjustments in physiology and/or flight behaviour than of long-term adaptations.