Aggregation patterns in ectoparasites: revisiting the Tasty Chick Hypothesis

Abstract

Parasites, in a near-universal pattern, are often aggregated on their host populations. Whereby few host individuals harbour many parasites, many host individuals have few or no parasites. Although processes that explain aggregated parasite distribution are multiple, immunocompetence often plays a key role. This article revises the Tasty Chick hypothesis, which explains the patterns of asynchronous hatching as an adaptation to aggregation of hematophagous ectoparasites (e.g. mites, fleas, flies) within broods of altricial birds. This hypothesis states that ectoparasites would tend to aggregate on low-quality nestlings and, thus, lower immunocompetent nestlings, within broods. In altricial birds exhibiting hatching asynchrony, these nestlings typically correspond to late-hatched nestlings. By aggregating on these nestlings (the so-called ‘tasty chicks’ in the hypothesis framework), ectoparasites would increase their fitness, without negatively affecting the development and survival of the remaining siblings. A review of the studies testing the Tasty Chick hypothesis reveals that evidence in favour of the hypothesis is weak. Most studies show that ectoparasites tend to aggregate on larger and high-quality nestlings within broods, rather than late-hatched nestlings. Furthermore, studies also show that late-hatched nestlings are not always less immunocompetent than remaining nestlings in the brood. In conclusion, most studies do not provide evidence in favour of the Tasty Chick hypothesis, thus aggregation patterns of ectoparasites have not driven the evolution of asynchronous hatching in birds.