European Robin (Erithacus rubecula) Science Article 3
Partially migratory behavior, the occurrence of resident and migratory individuals in a population, has been reported in free-living birds. I examined this behavior in the European Robin (Erithacus rubecula) under controlled experimental conditions. In an offspring group from a partially migratory field population, 36 (78%) individuals exhibited migratory restlessness (migrants), whereas 10 did not (residents).
HERBERT BIEBACH, The Auk 100: 601-606. July 1983