2023 137 ENV

How Does High Ambient Temperature Influence Behavior and Torpor in Anna's Hummingbirds?

By: Leonardo Rodriguez, Rachael Tang, Alexis Ayala, Sierra Ekmalian, Cecilia Doan

Department: Biology

Faculty Advisors: Dr. Derrick Groom, Dr. Ravinder Sehgal, Dr. Jonathon Stillman

Ambient temperatures have consistently exceeded record highs each year in the past decade, and there is an increasing risk of mortality for small bird species. Yet, it remains unclear how high ambient temperature may influence cooling behavior, daily energy expenditure, and torpor. Torpor is a unique physiological state used by many small endotherms to temporarily depress their metabolic rate and internal body temperature. Torpor is used in response to temperature changes, seasonal variation, and other environmental stressors. However, we still do not understand the primary drivers of torpor use. Furthermore, thermoregulatory costs increase substantially in warmer conditions. It is not well known how small birds adjust their cooling behaviors and daily energy intake in warmer conditions. This project investigated how energy intake, mass, and perching behavior are affected under different temperature conditions in Anna’s hummingbirds. Also, how is torpor utilized to mitigate high daytime metabolic activity? We hypothesized that high ambient temperatures caused Anna’s hummingbirds to enter torpor more frequently. In addition, we predicted that birds would perch more often at 35°C to mitigate energetic costs. For the temperature acclimation trials, six Anna’s hummingbirds were monitored with camcorders at 20°C and 35°C to observe how high ambient temperatures influenced perching behavior. Time spent expressing cooling behaviors, such as panting and exposure of feet, at 20°C and 35°C were scored and cross referenced to analyze differences in behavior. Also, time spent perched and off-perch were analyzed between both temperature conditions. Respirometry techniques determined average metabolic rates from CO2, H2O and O2 output. Behavioral recordings and respirometry data were analyzed using general linear models to determine if temperature influenced torpor behavior. Being among the smallest of bird species, understanding how Anna’s hummingbirds withstand high temperature environments will help implement better conservation strategies in California to prevent other bird species from facing extinction.