Thermal ecology of Trumpeter Swan Cygnus buccinator incubation

Abstract

Trumpeter Swans Cygnus buccinator, North America’s largest waterfowl, breed at high latitudes where they construct unusually large nest mounds and incubate from the top of the mound while experiencing daily ambient temperature fluctuations of 25°C or more, as well as substantial exposure to solar radiation, high winds, rain and snow. This study investigates how incubating swans accommodate the thermal flux in their environment. Swan incubation dynamics were measured at Red Rock Lakes National Wildlife Refuge in southwestern Montana, USA, a federally protected high-elevation marsh complex where the last breeding population of Trumpeter Swans in the contiguous United States persisted during the early 20th century, a period when they neared extinction. Using around-the-clock digital video imaging, environmental monitoring devices and thermal recording devices, which were placed within clutches and also within and upon nest mound material, thermal dynamics were modelled in relation to swan incubation behaviour. Swans maintained a mean (± s.e.) egg temperature of 35.7 ± 0.27°C during active incubation. Linear mixed models of thermal covariates evaluated using AICc values and model weights (ω) revealed an interactive association between nest attendance and: (1) deviation of ambient environmental temperature from average egg incubation temperature, (2) solar radiation and (3) vapour density. Ambient temperature deviations above and below average egg incubation temperature, elevated solar radiation and declining vapour density were interactively associated with swan presence on the nest. Egg temperature per se was associated with both ambient temperature (positive) and vapour density (positive). Nest mound temperature was positively associated with ambient temperature, solar radiation and vapour density. The nest mound acted as a thermal mass, moderating thermal flux. Nesting success depended on swans shielding their eggs from excessive daytime warming and desiccation, and on contact incubation when ambient temperature was below the average egg incubation temperature.