The nests of altricial bird species are host to a variety of nest ectoparasites that may develop high infestation levels and have a negative impact on chick growth, hematocrit, metabolic capacity, and survival before or after fledging. If ectoparasites affect chick development and fitness, then one would expect chicks to express behavior that might serve to limit the impact of parasites. We compared the behavior of nestling Blue Tits (Parus caeruleus) in nests with and without ectoparasitic blowfly larvae (Protocalliphora spp.). In parasitized nests, only 14.5% of the 15-s sampling periods were devoted to rest, whereas all chicks were resting in 32.5% of sampling periods for unparasitized nests. When nests were classed as active, repositioning occurred nearly twice as often in parasitized nests (83.5%) as in unparasitized ones (48.5%). Comfort behavior (e.g., preening) occurred more frequently in unparasitized nests (35.1%) than in parasitized (18.1%). The proportion of time spent repositioning increased linearly with parasite load. The daily energy expenditure of parasitized nestlings (2.3 kJ/g/d) did not differ significantly from that of unparasitized nestlings (3.1 kJ/g/d), and the tendency was towards lower energy expenditure in parasitized chicks. We cannot rule out the fact that our anti-parasite treatment increased the thermoregulatory costs of unparasitized chicks and obscured the energetic costs of parasites. However, we argue that Protocalliphora also can cause a depression in nestling body temperature that reduces metabolic rate, thus offsetting the costs of increased activity, while also reducing tissue growth rates.