Plumage moult is a costly maintenance process which conflicts with the requirements for breeding, explaining the avoidance of moult-breeding overlaps in many species. However, moult-breeding overlaps are especially frequent in the tropics, a fact which has been related to protracted breeding seasons and small clutches linked to high nest predation. Here, a new hypothesis explaining the high incidence of moult-breeding overlaps and low fecundity in tropical birds is proposed. Parasites and pathogens of birds may be more prevalent in the tropics than in temperate regions, requiring a higher level of immune responsiveness. There is observational and experimental evidence that moult interferes with immunity, and that the induction of an immune response delays the initiation of postnuptial moult. To avoid this conflict between a necessarily high preparedness of the immune system and plumage renewal, moult would have to be slowed down. Protracted moult processes would imply a high incidence of moult-breeding overlap. This, in turn, would reduce resources necessary for reproduction, leading to small clutches. Immunity and moult would limit fecundity in tropical birds.
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