Mechanisms of host preference in ectoparasites are important to the understanding of host-parasite interactions. Since ectoparasites negatively affect the condition of their hosts, while the hosts’ condition itself may affect the parasites’ choice, separating the factors that drive host preference from parasite impact asks for experiments. We combined the data of two choice experiments to investigate the preference of the nidicolous tick Ixodes arboricola when exposed to the nestlings of a passerine bird (Parus major). In the first experiment, in which complete broods at hatching were exposed to an ecologically relevant number of ticks, the relationship between tick loads and nestlings’ developmental status was characterized by a distribution with the highest tick loads on the more developed nestlings. Host preference became more apparent at a smaller brood size, suggesting a role for host density. In a second experiment we evaluated host choice in a pairwise choice experiment, exposing pairs of siblings with contrasting developmental status to eight ticks. In the first and the second pair, a median developed nestling was linked with the most developed and the least developed nestling, respectively. Seventy-two h after tick exposure we measured the innate constitutive humoral immunity and haematocrit. No differences were found in innate immunity, but the least developed nestlings had on average a lower haematocrit than the median and most developed nestlings. Significantly fewer ticks attached on the least developed nestling compared to the median nestling, and this difference was more pronounced when the innate immunity of the median developed nestling was higher. No difference in tick load was found among the median and best developed nestlings. The linkage between host preference and host physiological condition provide further insight in the mechanisms driving ectoparasite aggregation, which is important for the population dynamics of host, ticks and tick-transmitted pathogens.