An Experimental Analysis of Small Clutch Size in Tropical House Wrens

Trade-offs are central to life history theory, yet few studies have examined how geographic variation in trade-offs can lead to geographic variation in life history characters. I examined whether or not trade-offs for future fecundity or offspring survivorship could explain why tropical birds lay smaller clutches than their temperate relatives. I studied a tropical population (in Monteverde, Costa Rica) of the House Wren (Troglodytes aedon), a species that ranges in average clutch size from 6 in the temperate zone to 3.5 in the tropics. Three years of brood manipulation experiments showed weak effects of brood size on both future fecundity and offspring survivorship. Females that raised broods enlarged by two nestlings laid subsequent clutches, in the same breeding season, that were one-third of an egg smaller than those of females that did not raise enlarged first broods. Clutches in the year following brood manip- ulation were about a half an egg smaller for females raising enlarged broods than for females raising control or reduced broods. However, brood manipulation had no effect on male or female survivorship in any year of the study, despite the observation that both sexes increased their foraging rate to compensate for rearing larger broods. In two of three years, House Wrens were able to raise enlarged broods just as successfully as control and reduced broods, as measured by fledgling mass and survivorship of nestlings and fledglings. In one year, nestlings in enlarged broods fledged lighter and had lower fledgling survival than those in control or reduced broods. Predation of broods was unrelated to brood size, so food limitation appeared to be the mechanism causing the trade-off between brood number and offspring production. The pattern in tropical House Wrens is similar to that found in many studies of temperate passerines: in most years, brood sizes larger than the modal brood size appear to produce the most offspring. Thus, the same mechanism that controls clutch size in temperate birds may be at work in the tropics, but the level at which clutch size is controlled is lower in tropical birds, resulting in smaller clutches. A population model based on demographic parameters measured in the study population showed that the trade-off for offspring survivorship had a greater influence on fitness than the trade-off for future fecundity. Also, the clutch size strategy accruing the highest fitness depended on temporally varying conditions for reproduction. A strategy of laying 5-6 eggs had higher fitness than laying smaller clutches in years when conditions were favorable for reproduction, but clutch sizes of 3-4 (the observed clutch sizes in Monteverde) were most productive during less favorable years. Depending on the frequency of favorable years, House Wrens may be responding to a "bad-years effect" by lowering variance in reproductive success to maximize fitness over the long term. Alternatively, tropical birds may lay fewer eggs so that they can invest more care in each offspring, enhancing the chance that their offspring will survive and compete successfully in social contests for breeding territories. This offspring-quality hypothesis is supported by the observation that tropical House Wrens devote more time to the different stages of the reproductive cycle than do temperate House Wrens.