Data from manuscript:

Abstract

Reproduction is a trade-off between short-term energetic costs and long-term fitness benefits. This is especially apparent in small endotherms that exhibit high mass-specific metabolic rates and live in unpredictable environments. Many of these animals use torpor, substantially reducing their metabolic rate and body temperature to cope with high energetic demands during non-foraging periods. In birds, when the incubating parent uses torpor, the lowered temperatures that thermally sensitive offspring experience could delay development or increase mortality risk. We used thermal imaging to non-invasively explore how nesting female hummingbirds sustain their own energy balance while effectively incubating their offspring. We located 67 active Allen’s hummingbird (Selasphorus sasin) nests in Los Angeles, California and recorded nightly time-lapse thermal images at 14 of these nests for 108 nights using thermal cameras. We found that nesting females usually avoided entering torpor, with exceptions on two nights. We also modeled the energetic trade-offs of females using vs. avoiding torpor. We confirmed both through the field data and modeling approaches that nest insulation allowed nesting hummingbirds to avoid entering torpor on a regular basis. Nesting female hummingbirds seem to prioritize the nighttime energetic demands of their offspring, within the helpful environment of a warm nest.