Nestling development is among the most energy-demanding periods of a bird's lifetime and altricial species, in particular,  require extensive parental energy investment in the form of feeding and heating. In species under high predation pressure it seems advantageous to shorten a risky nesting period and to reduce parental activity at the nest. It can be achieved by a strategy that shifts allocation of resources between growth and endothetmy. Shortening of the nesting period may be achieved by rapid body growth in poikilothermic nestlings with the minimum possible feedings, and the onset of endothermy at a body size enabling them to leave the nest. As a result, the frequent feedings necessary for maintenance of warm-blooded young would be constrained to a short period. Such an optimal strategy would reduce both parental cost of reproduction and the risk of predation. In the present study we tested the energy allocation hypothesis in the Blackcap Sylvia atricapilla, a small passerine suffering from high nest predation. We analyzed the relation and trade-offs between nestling growth, development of thermoregulation and feeding rate. Our findings revealed that Blackcap nestlings were characterized by rapid growth but they achieved only 80% of their mature weight prior to fledging. Body mass showed highest relative growth rate before nestlings achieved homeothermy. The onset of endothermy, indicated on day 7, coincided with 90% of nestling fledgling weight, indicating that the two processes are separated in time. A strong negative correlation between feeding rate and growth rate demonstrated that Blackcap nestlings develop their bodies under relatively low feeding rates and more feeding is needed for maintenance of body temperature than for body growth. The study indicates the high cost of endothermy for parents - endothermic nestlings received over 100% more feedings than ectothermic ones. Separating in time growth and development of endothermy may reduce the risk of predation by shortening nesting period and decreasing parental activity at the nest.