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.

The methods used to collect this dataset and data processing information can be found in the manuscript.