Compensatory frugivory in migratory Sylvia warblers: geographical responses to season length

Seasonal shifts in the diets of Garden Warblers Sylvia borin, Blackcaps S. atricapilla and Lesser Whitethroats S. curruca at a North German site were studied from samples obtained by flushing the digestive tract of the birds. The significance of seven species of berries and pulpy fruits in the diet of these species while feeding in riparian vegetation during the premigrato ry and migratory seasons was assessed and compared with the pattern of frug ivory shown by the same species in southern Germany. Because of the short t ime span between hatching and onset of migration, first-year long- and medi um-distance migratory Sylvia warblers from northern Europe are forced to ac celerate their juvenile development. presumably to meet the higher nutritio nal demands of an accelerated development, juvenile Garden Warblers and Bla ckcaps in northern Germany ate relatively more fruit during premigration th an Blackcaps in southern Germany. Fruit occurred in about 90% of their diet samples during both premigration and migration. However, this might not ho ld true for experienced adult birds. Garden Warblers accelerated the rate o f fattening and reduced the time needed to prepare for migration when feedi ng on a mixed fruit diet. There were indications that Blackcaps directly in fluenced the fruit choice of Garden Warblers when fruits preferred by both species were scarce. Blackcaps and Garden Warblers showed subtle difference s in their seasonal preferences for black elder Sambucus nigra and woody ni ghtshade Solanum dulcamara.