Hovering in the heat: effects of environmental temperature on heat regulation in foraging hummingbirds.

Donald R Powers,Susan M Wethington,Kathleen M Langland,Catherine H Graham,Sean D Powers,Bret W Tobalske

doi:10.1098/rsos.171056

Abstract

At high temperature (greater than 40°C) endotherms experience reduced passive heat dissipation (radiation, conduction and convection) and increased reliance on evaporative heat loss. High temperatures challenge flying birds due to heat produced by wing muscles. Hummingbirds depend on flight for foraging, yet inhabit hot regions. We used infrared thermography to explore how lower passive heat dissipation during flight impacts body-heat management in broad-billed (Cynanthus latirostris, 3.0 g), black-chinned (Archilochus alexandri, 3.0 g), Rivoli's (Eugenes fulgens, 7.5 g) and blue-throated (Lampornis clemenciae, 8.0 g) hummingbirds in southeastern Arizona and calliope hummingbirds (Selasphorus calliope, 2.6 g) in Montana. Thermal gradients driving passive heat dissipation through eye, shoulder and feet dissipation areas are eliminated between 36 and 40°C. Thermal gradients persisted at higher temperatures in smaller species, possibly allowing them to inhabit warmer sites. All species experienced extended daytime periods lacking thermal gradients. Broad-billed hummingbirds lacking thermal gradients regulated the mean total-body surface temperature at approximately 38°C, suggesting behavioural thermoregulation. Blue-throated hummingbirds were inactive when lacking passive heat dissipation and hence might have the lowest temperature tolerance of the four species. Use of thermal refugia permitted hummingbirds to tolerate higher temperatures, but climate change could eliminate refugia, forcing distributional shifts in hummingbird populations.

Highlights

Increases in daily and yearly temperature variation associated with climate change might make bodytemperature maintenance increasingly difficult for birds
At Harshaw Creek (HC) and SC, we studied captive and free-living broad-billed hummingbird (Cynanthus latirostris, 3.3 g; BBLH) and black-chinned hummingbird (Archilochus alexandri, 3.0 g; BCHU) June–July 2013
Over the same Ta range the integrated mean THDA showed no relationship with Ta

Summary

Introduction

Increases in daily and yearly temperature variation associated with climate change might make bodytemperature maintenance increasingly difficult for birds. Dissipating excess heat produced during flight is challenging for birds because feathers provide an insulative layer that restricts heat loss, at slow flight speeds when forced convection is low [3,4,5,6]. We know that birds passively dissipate heat (radiation, conduction and convection) through specific heat dissipation areas (HDAs) around the eyes, shoulder and feet/legs where plumage density is low, thereby exposing the skin [3,4,6]. In calliope hummingbirds (Selasphorus calliope), these HDAs allow sufficient passive heat dissipation for maintenance of heat balance at a moderate environmental temperature (21°C) across flight speeds in the range 0–12 m s−1 [3]. The thermal gradient that drives passive heat dissipation will become smaller or be eliminated, making heat dissipation difficult

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