Migrant blackbirds (Turdus merula) on stopover have higher plasma PYY compared to residents: A validation and...

Migratory birds expend substantial energy during flight and often rely on stopovers to refuel. Foraging behavior during stopover might vary with fuel loss and food availability though likely involves higher food intake rates than in non-migratory (resident) individuals. In mammals, hormones such as leptin signal information about energy balance and satiety. In contrast the gut hormone peptide YY (PYY), which rises rapidly after feeding and suppresses appetite in mammals, might serve a similar functional role in songbirds, which lack functional leptin. We validated and measured plasma PYY in migrant and resident Common Blackbirds (Turdus merula) on Helgoland, a key North Sea stopover site, during peak autumn migration. Migrants exhibited significantly higher circulating PYY levels than residents, possibly reflecting increased or differential foraging. PYY was unrelated to body condition or fat stores. This proof of concept study justifies future work on plasma PYY and other metabolic signals and their potential influence on avian migratory decision making.