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To better understand the ecology of daily torpor, we completed the largest study to date on the intraspecific variation of daily torpor use in hummingbirds by exposing 149 individuals of two hummingbird species to ambient or experimentally cooled temperatures in a field setting.

The smaller species, a latitudinal migrant, used daily torpor frequently under ambient conditions. The larger species, an elevational migrant, also used daily torpor regularly, but further increased the frequency of daily torpor use when experiencing colder temperatures and prior to migration—indicating a facultative adaptation.

To place our results within a broader phylogenetic context, we combined our experimental results with a meta‐analysis, including 31 species and all major hummingbird clades, and found a broad taxonomic pattern in which smaller hummingbirds are more likely to use daily torpor than their larger counterparts. Smaller hummingbirds may be physiologically constrained, requiring nearly obligate daily torpor use, while larger hummingbirds are physiologically more flexible and can facultatively respond to changing environmental conditions.

Our results reveal how physiological traits, such as the frequency and depth of daily torpor, can provide a mechanism to understand how hummingbird species have established and persisted across broad environmental gradients.

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.

A fundamental question that has not been sufficiently addressed is whether larger clutch sizes lead towithinnest 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 trade‐off between offspring number and quality.

We manipulated clutch size in nests of free‐living wood ducks and measured incubation temperature among and within clutches using multiple temperature loggers.

As clutch size increased, average incubation temperatures were lower and more variable, and eggs took longer to hatch. Notably, the range inaverageincubation temperature among eggswithinnests increased with clutch size and exceeded 1°C in large clutches. 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.

Facultative hyperthermia has been proposed to minimize rates of water loss via three distinct mechanisms: M1) by maintaining body temperature (T_b) above environmental temperatures (T_e), heat can be lost non‐evaporatively, saving water; M2) by minimizing the thermal gradient whenT_e>T_b, environmental heat gain and evaporative water loss rates are reduced; and M3) by storing heat via increases inT_bwhich reduces evaporative heat loss demands and conserves water.

Although individuals may benefit from all three mechanisms during heat exposure, the relative importance of each mechanism has not been quantified among species that differ in their body size, heat tolerance and mechanisms of evaporative heat dissipation.

We measured resting metabolism, evaporative water loss and real‐timeT_bfrom 33 species of birds representing nine orders ranging in mass from 8 to 300 g and estimated the water savings associated with each proposed mechanism. We show that facultative hyperthermia varies in its benefits among species.

Small songbirds with comparatively low evaporative cooling capacities benefit most from (M1), and hyperthermia maintains a thermal gradient that allows non‐evaporative heat losses. Other species benefited most from (M2) minimizing evaporative losses via a reduced thermal gradient for heat gain at highT_e. We found that (M3), heat storage, only improved the water economy of the sandgrouse, providing little benefit to other species.

We propose that differences in the frequency and magnitude of hyperthermia will drive taxon‐specific differences in temperature sensitivity of tissues and enzymes and that the evolution of thermoregulatory mechanisms of evaporative heat dissipation may contribute to differences in basal metabolic rate among avian orders.

Understanding the mechanistic basis of heat tolerance is essential to advance our understanding of the ecology of birds living in hot environments that are warming rapidly, where extreme heat events are already re‐structuring avian communities.