Research
My research interests include hummingbirds, torpor, physiological ecology, thermal biology, energetics, urban ecology, trees, corvids, animal behavior, remote fieldwork, citizen science, and lots more!
Hummingbird torpor: A extreme and versitile energy saving strategy
Sleeping normothermic hummingbird
Torpid hummingbird
Metabolic rate of a hummingbird on a night it used torpor
Animation by Denise Kaya:
Full 7 minute video at vimeo.com/446351256
Designing and testing hyper-realistic artificial feeders to analyze hummingbird feeding preference and behavior
Madison Giles, Peter Auger, Maria Curely, Erich Eberts​
In Preparation
GSU 2023 Conference Poster
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Hummingbirds are charismatic and important pollinators that promote native plant diversity and often live close to humans. Their main food source, high-energy floral nectar, fuels their metabolically intense activity of hovering between flowers, pollinating as they go. Humans commonly attract hummingbirds to their windows and gardens with artificial feeders filled with sugar water. These anthropogenic food sources could potentially alter natural hummingbird feeding and pollination behaviors by lowering visitation to natural flowers. We designed and constructed a hyper-realistic artificial feeder (PF) to visually resembling a red passionflower (Passiflora vitifolia), a common nectar source for hummingbirds. Internet protocol video cameras were used to remotely monitor and record the frequency and duration of hummingbird visits to the PF and a classic saucer-shaped (CS) feeder during peak visitation times (two hours after sunrise and before sunset) at three locations in California. We predicted that hummingbirds would prefer and exhibit more natural behaviors at the hyper-realistic PF feeder in comparison to the CS feeder. We recorded 4064 visitations across all locations and feeders observed. At all locations, hummingbirds spent more time at the CS feeder, which unlike the PF feeder featured a perch. The PF feeder elicited shorter visits, which is consistent with more natural feeding behavior (i.e. hovering and quickly feeding). Future work aims to investigate whether these results are consistent across species and sex, sites with different feeder history, adjusted feeder designs, and in comparison with visitations to real flowers.
Free-living hummingbirds rarely use torpor while nesting
Erich Eberts, ​Glenn J. Tattersall, Peter J. Auger, Maria Curley, Melissa I. Morado, Eric G. Strauss, Donald R. Powers, Noemi M. Soveral, Bret W. Tobalske, Anusha Shankar
​Journal of Thermal Biology, 2022
www.sciencedirect.com/science/article/abs/pii/S0306456522002054
journals.biologists.com/jeb/article/226/5/JEB244994/293518/Hummingbirds-use-insulated-nests-to-save-energy
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For reproducing animals, maintaining energy balance despite thermoregulatory challenges is important for surviving and successfully raising offspring. This is especially apparent in small endotherms that exhibit high mass-specific metabolic rates and live in unpredictable environments. Many of these animals use torpor, substantially reducing their metabolic rate and often body temperature to cope with high energetic demands during non-foraging periods. In birds, when the incubating parent uses torpor, the lowered temperatures that thermally sensitive offspring experience could delay development or increase mortality risk. We used thermal imaging to noninvasively explore how nesting female hummingbirds sustain their own energy balance while effectively incubating their eggs and brooding their chicks. We located 67 active Allen’s hummingbird (Selasphorus sasin) nests in Los Angeles, California and recorded nightly time-lapse thermal images at 14 of these nests for 108 nights using thermal cameras. We found that nesting females usually avoided entering torpor, with one bird entering deep torpor on two nights (2% of nights), and two other birds possibly using shallow torpor on three nights (3% of nights). We also modeled nightly energetic requirements of a bird experiencing nest temperatures vs. ambient temperature and using torpor or remaining normothermic, using data from similarly-sized broad-billed hummingbirds. Overall, we suggest that the warm environment of the nest, and possibly shallow torpor, help brooding female hummingbirds reduce their own energy requirements while prioritizing the energetic demands of their offspring.
Sleeping nesting hummingbird
Torpid nesting hummingbird, a rare occurance
Reversal of the adipostat control of torpor during migration in hummingbirds
​Erich Eberts, Chris Guglielmo, Ken Welch
eLife, 2021
elifesciences.org/articles/70062
​​Many small endotherms use torpor to reduce metabolic rate and manage daily energy balance. However, the physiological 'rules' that govern torpor use are unclear. We tracked torpor use and body composition in ruby-throated hummingbirds (Archilochus colubris), a long-distance migrant, throughout the summer using respirometry and quantitative magnetic resonance. During the mid-summer, birds entered torpor at consistently low fat stores (~5% of body mass), and torpor duration was negatively related to evening fat load. Remarkably, this energy-emergency strategy was abandoned in the late summer when birds accumulated fat for migration. During the migration period, birds were more likely to enter torpor on nights when they had higher fat stores, and fat gain was positively correlated with the amount of torpor used. These findings demonstrate the versatility of torpor throughout the annual cycle and suggest a fundamental change in physiological feedback between adiposity and torpor during migration. Moreover, this study highlights the underappreciated importance of facultative heterothermy in migratory ecology.
Threshold for Torpor Initiation is Modulated by Perceived Nighttime Energy Demand
Erich Eberts, Chris Guglielmo, Ken Welch
In revision
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Environmental temperature shapes the energy management strategies animals use to balance their energy budgets. Many small endotherms can use torpor to reduce their metabolic rate and avoid the high energetic costs of normothermy at cold temperatures. However, the ways in which environmental temperature interacts with physiological signals for torpor initiation throughout the annual cycle remain unclear. We explored relationships among air temperature, energy status, and torpor use in captive ruby-throated hummingbirds (Archilochus colubris) during the summer and winter using respirometry and quantitative magnetic resonance. We repeatedly evaluated torpor use of individual birds experiencing nighttime air temperatures of 10°C, 20°C, or 30°C. We found that no summer birds, and only one winter bird used torpor at 30°C. In the summer, birds used torpor more frequently and for longer on colder nights and when they had lower evening fat stores. Furthermore, birds entered torpor at higher fat thresholds on 10°C nights, reserving excess fat in anticipation of higher energy costs incurred after torpor entry. In the winter, birds entered torpor earlier and remained in torpor longer at 10°C, but used torpor at similar frequencies and fat levels on 10°C and 20°C nights, and irrespective of evening fat stores. The summer energy emergency torpor strategy allows birds to survive while maintaining advantageous lean body masses. In contrast, the winter routine strategy could allow free-living birds, who are unconstrained by the low body mass requirements of the breeding season, to spare extra fat stores that confer resilience to interspecific competition and cold nighttime temperatures experienced at their wintering grounds. These seasonal changes demonstrate how torpor allows hummingbirds to fine-tune their energy budgets and maintain advantageous body compositions throughout the annual cycle.
Energy-Emergency Torpor Use Among Age-Sex Classes of Wild-Caught Ruby-throated Hummingbirds (Archilochus colubris)
​Erich Eberts, Ken Welch
In Revision
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Animals of different ages and sexes often exhibit different morphological, behavioral, and physiological characteristics that underly variation in the strategies they use to balance energy intake and expenditure. For small endotherms with limited endogenous energy stores, torpor can confer resilience when food intake is limited; however, torpor use can be constrained in certain life history stages and environments. We investigated torpor use among age-sex classes of ruby-throated hummingbirds (Archilochus colubris) that experienced natural daytime foraging conditions in the summer breeding period. We quantified relationships between endogenous energy stores and torpor use by using empirically-informed body composition estimates and measuring metabolic rate. We found that all birds entered torpor in energy emergencies; using torpor more often and for longer on nights they started with lower fat levels. We found that juveniles were less likely to enter torpor than adults, possibly because the inability to use an evening hyperphagia strategy impacted adults, who prioritize lean daytime weights, more than juveniles, who maximize daytime energy storage. Furthermore, juvenile males used torpor less often than adult and juvenile females, possibly because their superior competitive ability allowed them to achieve greater daytime energy stores. Additionally, we did not detect a seasonal switch to routine torpor strategy, as documented in captive adult males and inferred in wild adult females. Rather, juveniles continued to exhibit a clear energy emergency pattern into the late summer premigratory period. These results suggest that competition may not be a determining factor in torpor use in ruby-throated hummingbirds in the context of this study, and that seasonal flexibility in torpor use could be constrained by development and prior migratory experience.
Effects of Artificial Feeders and Habitat Restoration on Vertebrate Pollinators and Pollination: A Metanalysis and Literature Review
Erich Eberts​, Nicholas Sookhan Scott MacIvor
In Preparation
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Urbanization and anthropogenic activity in natural landscapes often lead to changes in wildlife food resource availability. While habitat fragmentation and destruction reduce availability and diversity of natural food resources for wildlife, wildlife managers and nature enthusiasts often aim to counteract these negative impacts by supplying wildlife with artificial food resources and restoring or enhancing habitat. However, supplying anthropogenic food resources can also have consequences for animal behavior, individual and population health, and ecosystem services such as pollination. I conducted a literature search and metanalysis to investigate the effectiveness of artificial feeding and habitat restoration for promoting vertebrate pollination. I identified 10 studies that contained relevant and suitable data for a metanalysis, and I extracted 25 quantitative comparisons between treatment and control groups within these studies. I calculated effect sizes and evaluated trends with respect to taxa, type of conservation action, and type of measured biological response using metanalytical mixed effects models. This metanalysis suggests that sugar water feeders and improving habitat tend to positively impact pollinator abundance and richness, but effects on pollination activity and plant reproductive success remain unclear. Additional qualitative studies generally support these findings, and emphasize the potential for a combined strategy to benefit community structure and plant-pollinator network connectivity. Thus, we suggest that the context of artificial feeding should be carefully considered, and optimally be used in proximity to natural, or restored native habitat.
Foraging behaviors of Archilochus colubris in the presence and absence of aggressive insect competitors
Shawna Moore, Megan Bontrager, Art Weis, Emma Walker, Erich Eberts​
In Preparation
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When multiple species share a common resource, they are often forced to either temporally or spatially partition resources in order to reduce competitive interactions. Resource partitioning has been observed in plant-pollinator communities between hummingbirds and insects who compete for nectar. These behaviors may restrict the feeding potential of one or more species, which in turn affects the pollination success of the corresponding plant species. In this study, we look at whether hummingbirds avoid aggressive pollinator insects such as bees and wasps, and how their feeding behavior changes in the presence and absence of these competitors. By observing hummingbird visitations to feeders both with and without insects present, we found that hummingbirds spent more time feeding when insects were absent, but they did not have a significant preference between sites that contained insects and sites that did not. Additionally, hummingbirds were shown to be attracted to inflorescence where bees and wasps were actively feeding. These findings are important in determining how plant-pollinator community interactions affect the foraging success of nectivores and the pollination success of the primary producers that set the foundation for all ecosystem trophic levels.
Metabolic Fates of Evening Crop-Stored Sugar in Ruby-Throated Hummingbirds (Archilochus colubris)
​Erich Eberts, Morag Dick, Ken Welch
Diversity, 2019
www.mdpi.com/1424-2818/11/1/9
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​During the day, hummingbirds quickly metabolize floral nectar to fuel high metabolic demands, but are unable to feed at night. Though stored fat is the primary nocturnal metabolic fuel, it has been suggested that hummingbirds store nectar in their crop to offset fat expenditure in the night or to directly fuel their first foraging trip in the morning. We examine the use of crop-stored sugar in the nocturnal energy budget of ruby-throated hummingbirds (Archilochus colubris) using respirometry and 13C stable isotope analysis. Hummingbirds were fed a 13C-enriched sugar solution before lights-out and held in respirometry chambers overnight without food. Respirometry results indicate that the hummingbirds metabolized the sugar in the evening meal in less than 2 h, and subsequently primarily catabolized fat. Breath stable isotope signatures provide the key insight that the hummingbirds converted a substantial portion of an evening meal to fats, which they later catabolized to support their overnight metabolism and spare endogenous energy stores. These results show that the value of a hummingbird’s evening meal depends on how much of this energy was converted to fat. Furthermore, this suggests that evening hyperphagia is an important energy maximization strategy, especially during energetically expensive periods such as migration or incubation.
Crow and Least Tern Conservation Research
​Erich Eberts, Pete Auger, Eric Strauss
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[abstract]
