Incubation temperature as a constraint on clutch size evolution

1. Elucidating factors that limit the number of offspring produced is
fundamental to understanding life-history evolution. Here, we examine the
hypothesis that parental ability to maintain an optimal physical
developmental environment for all offspring constrains clutch size via
effects on offspring quality. 2. Experimental laboratory studies of birds
have shown that a <1°C difference in average incubation temperature
has diverse effects on fitness-related post-hatching offspring phenotypes.
Thus, the inability of parents to maintain optimal incubation temperatures
could constrain clutch sizes. 3. A fundamental question that has not been
sufficiently addressed is whether larger clutch sizes lead to within nest
variation in egg temperature that is large enough to produce offspring
with different phenotypes within a brood. This could lead to differential
survival among offspring, and could create a tradeoff between offspring
number and quality. 4. We manipulated clutch size in nests of free-living
wood ducks and measured incubation temperature among and within clutches
using multiple temperature loggers. 5. As clutch size increased, average
incubation temperatures were lower and more variable, and eggs took longer
to hatch. Notably, the range in average incubation temperature among eggs
within nests increased with clutch size and exceeded 1°C in large
clutches. 6. Clutch size did not affect hatch success. In conjunction with
our companion laboratory studies that used artificial incubation to
document the effects of temperature variation on fitness-related traits in
this species, our work suggests that suboptimal incubation temperatures
could be a factor that limits clutch size through diminishing returns on
post-hatch offspring quality.