Are Clutch and Brood Size Patterns in Birds Shaped by Ectoparasites?

Abstract

Ectoparasites may influence the optimum values of important life history traits such as clutch size and brood size by having different fitness effects for large and for small trait values. We propose here that the life-cycle length of the common ectoparasite species of a host determines whether it is more profitable for the host to raise a large or a small brood. The hypothesis rests on the following argument: (1) the length of an ectoparasite's life-cycle relative to the timespan that the host nestlings are available as a resource determines the total parasite load per nest during the nestlings' growth phase, and therefore (2) also the parasite load per nestling, which in turn (3) determines the parasite impact on the nestlings. Populations of long-cycled ectoparasites (i.e. the life-cycle length of the ectoparasite is similar to the length of time that nestlings take from hatching to fledging) do not build up considerably during the nestling phase and, consequently, parasites become increasingly diluted with an increase in brood size. This predicts no correlation between parasite load and brood size, but a negative correlation between parasite load per nestling and brood size. Larger broods will be favoured and brood size should be reduced only when feeding conditions become increasingly inadequate. In contrast, populations of short-cycled ectoparasites (i.e. the life-cycle length of the ectoparasite is much shorter than the length of time that nestlings take from hatching to fledging) can build up quickly and may reach the carrying capacity given by the number of host nestlings. This predicts a positive correlation between parasite load and brood size, but no correlation between parasite load per nestling and brood size. Smaller broods may then be at an advantage because they can be more adequately provisioned with food. Whether females should adjust clutch size will largely depend on whether they can, when laying their clutch, predict the parasite load after hatching. When future infestation can be predicted, females of species that are commonly infested with short-cycled ectoparasites should lay a smaller clutch, but females commonly infested with long-cycled parasites should lay a larger clutch. When future infestation cannot be predicted at laying, with shortcycled ectoparasites, females should lay a normal clutch and reduce it when the nest becomes infested, but with longcycled ectoparasites, females should lay a larger clutch and maintain brood size as long as feeding conditions are adequate. If parasite pressure is constant over many breeding season, we may expect selection for smaller or larger clutches depending on the cycle length of the common ectoparasite. If parasite pressure fluctuates stochastically, a behavioural response will be more appropriate. Patterns from intraand interspecific studies are in agreement with most predictions outlined above. Clutch size in birds varies widely both among and within species and much of this variation has been attributed proximately to variance in phenotypic quality of the parents, variance in food abundance, nest predation, nestling competition, nest parasitism by other birds, phylogenetic inertia, physiological constraints, and ectoparasites (for recent reviews see e.g. Murphy and Haukioja 1986, Godfray et al. 1991, Poiani 1993a, b). Ultimately much of this variation can be understood in terms of variation in reproductive trade-offs, such as clutch size with offspring or adult survival and fecundity (for reviews see Linden and M0ller 1989, Dijkstra et al. 1990, Stearns 1992). Ectoparasites can strongly reduce reproductive success (Moss and Camin 1970, M0ller et al. 1990, M0ller 1993, Richner et al. 1993, Clayton and Tompkins 1994) of their hosts, and are therefore most likely to affect reproductive trade-offs. This life-history point of view predicts that hosts may reduce the impact of parasites by altering their own reproductive effort (Forbes 1993, Poulin et al. 1994). In this note we address the question of how ectoparasites are expected to influence brood size by considering (1) the relationship between life-cycle length of ectoparasites, host brood size and parasite load, (2) the expected relationship between life-cycle length of ectoparasites and the effect of ectoparasites on nestlings of small and large broods, (3) the expected host response in terms of a change in clutch size and brood size. Empirical evidence supporting the predictions are presented.