Migratory birds like the garden warbler are hormone-controlled. (Photo: Wolfgang Goymann)

Migratory birds often stop along their long journeys to replenish their fat stores. The purpose of these stopovers – rest and refuelling – is clear. To date, however, it had been unclear which physiological signals triggered the birds’ decision to continue their flight. A team led by researchers from Vetmeduni Vienna has now identified, for the first time, the hormone ghrelin as a signal for the birds’ brains. Ghrelin, which is known to be an appetite-regulating hormone in humans, was measured at high levels in satiated garden warblers. Moreover, birds injected with additional ghrelin exhibited decreased appetite and increased the highly active state of migratory restlessness. The results, which were published in the journal PNAS, confirm the hormonal influence on avian migratory behaviour and could even lead to an improved understanding of eating disorders among humans.