Studies on migratory fattening in passerine birds

Abstract

The body composition of juvenile Willow Warblers, Phylloscopus trochilus, was examined during the autumn moult and premigratory period. Premigratory birds exhibited hyperphagia and lipid deposition. Weight decreases of the pectoralis muscles and their glycogen reserves during moult are interpreted as responses to increased thermoregulatory demands, and not to utilization of muscle amino-acids for feather growth. Apparent premigratory muscle 'hypertrophy' represents merely a recovery to pre-moult values. Mean liver weights do not vary between the post-juvenile moult and the premigratory period, but the times of day at which weight maxima of water and protein occur are earlier in premigratory birds. These birds also show an earlier increase in liver lipid levels during the day than moulting birds. Premigratory birds also have greater evening lipid concentrations than moulting birds. As the dawn lipid levels of moulting and premigratory birds do not differ, all lipid synthesized (or processed) by the latter group must be stored solely in adipose tissue. The marked diurnal cycle of liver glycogen of moulting birds is absent in the premigratory period. Samples of Willow and Grasshopper Warblers (locustella naevia ) killed during migratory flight had different potential ranges but similar potential flight times, due to a lipid-correlated increase in body water in the latter species Injections of prolactin in photosensitive Bramblings, Fringilla montifringilla, showed that the stimulation of appetite by thishormone was independent of the time of its injection, whereas lipid, deposition was strongly dependent. In Bramblings given restricted food body and liver weights still increased when birds were injected with prolactin confirming that prolactin acts directly on liver lipid synthesis, independently of its effects on appetite. Changes in liver size produced by prolactin injections were similar to size changes found in photostimulated birds. It is concluded that prolactin is the probable cause of changes in liver composition of autumnal premigratory Willow Warblers.