Hummingbirds use insulated nests to save energy for the clutch

Abstract

All wild animals need to perform a balancing act with the energy they take in and the energy they burn. To save energy, hummingbirds can use torpor, a process where they decrease their body temperature and metabolism. However, mothers only use torpor in an emergency, as their eggs and chicks are sensitive to low temperatures. Erich Eberts, from Loyola Marymount University, USA, and an international team of colleagues from various US and Canadian universities set out to determine how female Allen’s hummingbirds (Selasphorus sasin) manage their energy budget at night-time while incubating chicks and, importantly, whether their nests provide energy savings.From January to May in 2017 and 2018, the team located more than 60 Allen’s hummingbird nests with eggs or chicks in the suburbs around Loyola Marymount University in Los Angeles, CA, USA. The researchers then monitored 56 of the nests until the chicks fledged (40 nests), were killed by predators (12 nests) or failed for some other unknown reason (4 nests). To measure the temperatures of the hummingbird mothers, the researchers placed thermal cameras 0.5-1 m away from nests, programmed to capture one image every minute, successfully collecting thermal images on 108 nights from 14 of the 56 nests they were monitoring. The researchers then used the thermal images to determine the mother’s body temperature, and the temperature of a nearby branch or leaf to measure the local environmental temperature.For their analysis, the team then defined the thermal states of the hummingbirds as either normal body temperature, deep torpor (low body temperature), shallow torpor (moderately low body temperature) or the transition to or from torpor. Then, in a separate series of theoretical models, the researchers estimated the energetic costs of the hummingbirds under a range of conditions, with either torpid or normal body temperatures for 0, 2, 6, 10 or 12 h, and either nesting or completely exposed to the surrounding air.The researchers found that on 103 nights (of the total 108), the hummingbird mothers maintained normal body temperatures, and failed to enter torpor to conserve energy. However, one hummingbird did enter deep torpor on two nights in February, decreasing its skin temperature to 14°C, and two other hummingbirds used shallow torpor on three nights, decreasing skin temperatures to approximately 19°C. However, the chicks of the mothers that sporadically used torpor still developed successfully and took flight when they were old enough. The researchers concluded that hummingbird mothers rarely enter torpor to protect their chicks from nocturnal low temperatures, although they do resort to using torpor in exceptional circumstances.Nevertheless, the team discovered that the temperatures within the nest remained approximately 7.5°C higher than the surrounding air, thanks to the insulation they provided, which allowed the roosting mothers to conserve energy. In one of the teams’ energetic models, a nesting mother maintaining a normal body temperature for 12 h could save around 24% of her energy thanks to her nest compared with a mother exposed to the environment. Yet, if nesting hummingbirds used torpor for 6 h, they could save 41% of the energy they would expend maintaining a normal body temperature in the open.Overall, Eberts and colleagues successfully used a non-invasive method to determine that mother Allen’s hummingbirds almost always forgo torpor when nesting. The mothers prioritize the health of their chicks, instead of saving energy for themselves. In addition, hummingbird mothers can depend on their well-constructed, insulated nests to provide a warm home and save energy when raising their young.