Trade-offs are central to life history theory, yet few studies have ex
amined how geographic variation in trade-offs can lead to geographic v
ariation in life history characters. I examined whether or not trade-o
ffs for future fecundity or offspring survivorship could explain why t
ropical birds lay smaller clutches than their temperate relatives. I s
tudied a tropical population (in Monteverde, Costa Pica) 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 br
ood manipulation experiments showed weak effects of brood size on both
future fecundity and offspring survivorship. Females that raised broo
ds enlarged by two nestlings laid subsequent clutches, in the same bre
eding season, that were one-third of an egg smaller than those of fema
les that did not raise enlarged first broods. Clutches in the year fol
lowing brood manipulation were about a half an egg smaller for females
raising enlarged broods than for females raising control or reduced b
roods. However, brood manipulation had no effect on male or female sur
vivorship in any year of the study, despite the observation that both
sexes increased their foraging rate to compensate for rearing larger b
roods. 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 on
e year, nestlings in enlarged broods hedged lighter and had lower fled
gling survival than those in control or reduced broods. Predation of b
roods was unrelated to brood size, so food limitation appeared to be t
he mechanism causing the trade-off between brood number and offspring
production. The pattern in tropical House Wrens is similar to that fou
nd 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 dutch 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 stu
dy population showed that the trade-off for offspring survivorship had
a greater influence on fitness than the trade-off for future fecundit
y. Also, the clutch size strategy accruing the highest fitness depende
d on temporally varying conditions for reproduction. A strategy of lay
ing 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 duri
ng 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 ter
m. Alternatively, tropical birds may lay fewer eggs so that they can i
nvest more care in each offspring, enhancing the chance that their off
spring will survive and compete successfully in social contests for br
eeding territories. This offspring-quality hypothesis is supported by
the observation that tropical House Wrens devote more time to the diff
erent stages of the reproductive cycle than do temperate House Wrens.