Hummingbirds budget energy flexibly in response to changing resources

Abstract

A key component of individual fitness is the ability to manage energy stores in response to variable resource availability, but because directly measuring energy budgets is difficult, daily energy management is rarely measured. Hummingbirds' energy management is relatively simple to model compared to other endotherms because they have high mass‐specific metabolic rates and store little fat. We determined which aspects of the hummingbird daily energy budget (i.e. thermoregulation, daytime activity costs, night‐time costs) change at the individual level in response to environmental variation. We found that daily energy expenditure varied threefold in two populations of broad‐billed hummingbirds (Cynanthus latirostris). Our model indicated the energy budget was distributed in the following proportions: daytime activity, 59% (range 22%–84%); thermoregulation, 23% (11%–32%); basal metabolism, 7% (3%–16%); and night‐time energy, 17% (6%–37%). Activity costs were higher at the hotter, homogeneous site and during the early‐wet season at both sites. Increased daily energy expenditure was related to decreased nectar availability and not significantly related to temperature or bird mass. With climate change, the indirect energetic costs of shifting resources could have greater impacts on endotherm energy budgets than direct costs such as thermoregulation. Increased foraging and activity costs could decrease the energy available to birds for somatic repair and reproduction, potentially causing differential fitness across seasons and sites. A free Plain Language Summary can be found within the Supporting Information of this article.

The allometry of daily energy expenditure in hummingbirds: An energy budget approach

“…While BMR is often used in global analyses as a proxy for DEE, few endotherm studies have verified whether their relationship is consistent across taxa (Mathot & Dingemanse, 2015). Individuallevel studies indicate that BMR and DEE are often uncorrelated (especially in birds; Koteja, 1991;Portugal et al, 2016;Shankar et al, 2019). Since BMR is measured under often unrealistic controlled conditions, DEE is likely more ecologically relevant and under stronger selection constraints than BMR, because DEE is a more direct measure of how organisms live in their environment (Hudson, Isaac, & Reuman, 2013).…”

“…Following previous hummingbird energy budget studies (López-Calleja & Bozinovic, 2003;Shankar et al, 2019;Wolf, Hainsworth, & Gill, 1975 Flying metabolic rate (FLMR) 0.72 0.90 Butler (1995, 2015) and Castro and Myers (1988) Torpor metabolic rate (TMR)…”

“…We collected data on DEE using DLW. We either followed the standard DLW protocol (Speakman, 1998) We determined the exact dose (0.1-0.5 g) by precise mass measurements (nearest 0.001 g) before and after feeding, and calibrated these measurements using a control (see Shankar et al, 2019 for details). In both cases, we collected urine samples both after equilibration (half an hour), and approximately 24 hr after the bird was released.…”

Metabolic partitioning of sucrose and seasonal changes in fat turnover rate in ruby-throated hummingbirds (Archilochus colubris)

“…Seasonal and environmental differences may also influence daily energy expenditure in wild hummingbirds. In broad-tailed hummingbirds, daily energy expenditure can range from 12.6 kJ during the dry season to 39.8 kJ in the wet season, despite body mass remaining constant (Shankar et al, 2019). In that study, variability in daily energy expenditure was largely attributed to changes in daily activity (time spent perching, flying and hovering).…”

“…Thus, changes in body mass, fat reserves, energy expenditure and life history traits may influence the rate of fat turnover in animals (Brown, 2000;Keicher et al, 2017). Seasonal and environmental differences contributed to a 3-fold difference in daily energy expenditure in broad-tailed hummingbirds (Shankar et al, 2019). This suggests that seasonal changes in fat turnover rates could occur in hummingbirds in coordination with seasonal changes in body mass and daily energy expenditure.…”