Immune defence is fundamentally important for the survival prospects of young animals. While innate immunity offers initial protection from a variety of pathogens, acquired immunity responds more specifically to pathogens, but is considered to be more costly and to respond slower. Moreover, the acquired immunity is not yet fully developed in neonatal chicks. Little is known about the ontogeny of the immune system of wild birds. Long-lived seabirds, with their slow chick development, are good models to investigate how young birds invest in both arms of their immune system. We determined leucocyte profiles and heterophil to lymphocyte (H/L) ratios of Red-tailed Tropicbirds (Phaeton rubricauda westralis) on Christmas Island, Indian Ocean. Young chicks (N = 10) had significantly higher H/L ratios than older chicks (N = 19), while adults (N = 47) showed intermediate values and did not differ from either chick age class. High H/L ratios in young chicks were caused by high initial numbers of heterophils per 10,000 erythrocytes that declined with age. In contrast, the number of lymphocytes per 10,000 erythrocytes was similar for young and older chicks. These data suggest that young chicks invested heavily in innate immunity to protect themselves from pathogens, while investment into acquired immunity became more important in older chicks with a functional acquired immune response. Body condition did not have a significant influence on any leucocyte parameter.