Birds like this Horned Grebe must balance incubation and foraging needs. Isidor Jeklin/CLO

Sitting on a nest may look easy, but it involves more trade-offs than meet the eye.When birds sit on eggs, they are not simply relaxing. They are regulating the temperature of the clutch. The optimal range is 96.8 °F to 104.9 °F (36 °C to 40.5 °C). If egg temperatures drop below the optimal range, embryonic development slows. Higher temperatures are lethal for the embryo.

Although most people think of incubation as a warming process, birds may need to cool their eggs by shading or moistening them in hot environments. For example, one pair of Black-necked Stilts at southern California's Salton Sea made 155 trips in one day to soak their belly feathers in water to cool their eggs.

Incubation requires a balance between sitting on the eggs to maintain their temperature and leaving the nest to refuel by foraging. Different bird species cope with this conflict in various ways.

Male Emperor Penguins fast for 64 days, living off stored fat while incubating eggs. In other species, including many gulls, shorebirds, and songbirds, the male and female take turns incubating the clutch. Females of many songbirds and hummingbirds incubate the eggs alone. In some species, such as goldfinches and crossbills, the male will supplement the female with food while she sits on the nest. But in other species, the female must leave the eggs unattended when she goes out to forage. For a hummingbird, that's at least 140 times each day.

Incubation thus involves a series of trade-offs: a female gains energy by leaving the nest to forage, but she must expend energy to rewarm or cool the clutch after returning. The way that birds allocate their incubation and feeding times can affect reproductive success.
Recent studies have shown that energy requirements during incubation are so demanding that they can limit clutch size and therefore the number of young that a female can produce. Ambient temperatures may also play a role in determining clutch size and determining how hard birds must work to keep the eggs at the proper temperature. Large clutches require more energy to warm, but they also retain heat better while the female is foraging. Thus, large clutches may be easier to incubate in cooler environments.

Data from The Birdhouse Network have shown that clutch sizes of Eastern Bluebirds increase from south to north and from east to west (Birdscope, Spring 2000). Do incubation constraints factor in? By collecting data on incubation and ambient temperatures at Eastern Bluebird nests, participants in our pilot study are helping us develop protocols for a study investigating the factors limiting clutch size and reproductive success. Stay tuned.