ARE CLUTCH AND BROOD SIZE PATTERNS IN BIRDS SHAPED BY ECTOPARASITES

Ectoparasites may influence the optimum values of important life histo ry traits such as clutch size and brood size by having different fitne ss effects far large and for small trait values. We propose here that the life-cycle length of the common ectoparasite species of a host det ermines whether it is more profitable for the host to raise a large or a small brood. The hypothesis rests on the following argument: (1) th e length of an ectoparasite's life-cycle relative to the timespan that the host nestlings are available as a resource determines the total p arasite load per nest during the nestlings' growth phase, and therefor e (2) also the parasite load per nestling, which in turn (3) determine s the parasite impact on the nestlings. Populations of long-cycled ect oparasites (i.e. the life-cycle length of the ectoparasite is similar to the length of time that nestlings take from hatching to fledging) d o not build up considerably during the nestling phase and, consequentl y, parasites become increasingly diluted with an increase in brood siz e. This predicts no correlation between parasite load and brood size, but a negative correlation between parasite load per nestling and broo d size. Larger broods will be favoured and brood size should be reduce d only when feeding. conditions become increasingly inadequate. In con trast, populations of short-cycled ectoparasites (i.e. the life-cycle length of the ectoparasite is much shorter than the length of time tha t nestlings take from hatching to fledging) can build up quickly and m ay reach the carrying capacity given by the number of host nestlings. This predicts a positive correlation between parasite load and brood s ize, but no correlation between parasite load per nestling and brood s ize. Smaller broods may then be at an advantage because they can be mo re adequately provisioned with food. Whether females should adjust clu tch size will largely depend on whether they can, when laying their cl utch, predict the parasite load after hatching. When future infestatio n can be predicted, females of species that ace commonly infested with short-cycled ectoparasites should lay a smaller clutch, but females c ommonly infested with long-cycled parasites should lay a larger clutch . When future infestation cannot be predicted at laying, with short-cy cled ectoparasites, females should lay a normal clutch and reduce it w hen the nest becomes infested, but with long-cycled ectoparasites, fem ales should lay a larger clutch and maintain brood size as long as fee ding conditions are adequate. If parasite pressure is constant over ma ny breeding season, we may expect selection for smaller or larger clut ches depending on the cycle length of the common ectoparasite. If para site pressure fluctuates stochastically, a behavioural response will b e more appropriate. Patterns from intra- and interspecific studies are in agreement with most predictions outlined above.