Which chick is tasty to parasites? The importance of host immunology vs. parasite life history

1. The Tasty Chick Hypothesis (TCH) proposes that hatching asynchrony evolved as an antiparasite strategy. Hosts would benefit if ectoparasites aggregate more on the offspring that are of lowest reproductive value within a brood, i.e. on the last-hatched chicks, because offspring reproductive value generally decreases with hatching rank. The poor body condition of the later-hatched chicks would impair parasite resistance and render them especially attractive to ectoparasites. Thus, the TCH predicts a decline in chick parasite load with hatching order in avian broods. 2. We investigated the main assumption of the TCH that junior chicks are less immunocompetent than their senior siblings. We also examine the prediction of the TCH that junior chicks bear more ectoparasites than their senior siblings. 3. Conform to the assumption of the TCH for hosts, junior chicks in broods of the barn owl (Tyto alba L.) showed a lower humoral immune response than their senior siblings. In contrast, the cell-mediated immune response of senior chicks in broods of the great tit (Parus major L.) was not significantly greater than that of their junior siblings. 4. In line with the prediction of the TCH for the distribution of parasites among hosts, the fly Carnus haemapterus Nitzsch infested junior chicks in larger numbers than senior chicks in both barn owl and kestrel (Falco tinnunculus L.) broods. 5. In conflict with the TCH, ticks (Ixodes ricinus L.) were distributed randomly with respect to hatching rank in broods of the barn owl and the great tit. Moreover, louse-flies Crataerina melbae Rondani infested mainly senior chicks instead of junior chicks in the Alpine swift (Apus melba L.). 6. Summarizing, the present descriptive study indicates that the distribution of ectoparasites within-broods is not generally governed by rank-related variation in host defence of chicks as initially suggested by the TCH. We argue that specific aspects of the morphology, life history and ecological requirements of various ectoparasite species need more consideration as to explain the dynamics and evolution of host-parasite interactions.