DOI: 10.5061/dryad.dbrv15fdk
title: Migratory strategies, fat loads and torpor use in migrating hummingbirds

description: North American bee hummingbirds frequently enter torpor during autumn
 migrations to save energy. The frequency with which they use torpor is
 mediated by fat loads. To estimate frequencies and temporal patterns of
 torpor use, we measured lipid loads and feather hydrogen stable isotope
 ratios (δ2H) in 942 individuals of four species of migrating bee
 hummingbirds: Selasphorus rufus (Rufous Hummingbird), Selasphorus calliope
 (Calliope Hummingbird), Selasphorus platycercus  (Broad-tailed
 Hummingbird), and Archilocus alexandri (Black-chinned Hummingbird), at a
 stopover site in Southwestern New Mexico. In a subsample of juvenile
 individuals of each species, we constructed functions relating lipid loads
 and feather δ2H values to torpor incidence, using respirometry. We used
 these functions, along with lipid load and δ2H data from the remaining
 captured juvenile birds to estimate their temporal patterns of torpor use.
 Lipids were significantly higher in adult S. rufus and S. platycercus than
 in juveniles but did not differ with age in S. calliope and A. alexandri.
 In S. rufus, lipid loads decreased with capture date, while in A.
 alexandri they increased. There was no effect of capture date on lipid
 loads in S. platycercus and S. calliope. In all species torpor incidence
 decreased with increasing fat loads in a predictable fashion. A large
 fraction (70%) of S. calliope and a smaller fraction (30%) of S. rufus
 individuals entered torpor, but these fractions remained constant over
 time. In contrast, the fraction of S. playtcercus individuals that entered
 torpor increased with capture date, whereas that of A. alexandri
 decreased. Our results revealed similarities, but also unexpected
 interspecific differences in fat loads and torpor incidence in migratory
 bee hummingbirds. Some of the correlates of these differences remain
 unexplained but represent an opportunity to better understand the causes
 and consequences of torpor-mediated migration.
description: We captured 942 adult and juvenile hummingbirds of four species
 at a stopover site near Mimbres, NM, USA, between July and September 2022.
 A Quantitative Magnetic Resonance scanner was used to non-invasively
 measure lipid content in each bird. A Push Flow-through Respirometry
 System was used to measure overnight metabolic parameters and use of
 nocturnal torpor in 94 adult and juvenile birds. Respirometry metabolic
 data were recorded using Sable Systems ExpeData version 1.9.27 software. A
 retrix (R1) feather was collected from each bird, and feather hydrogen
 stable isotope ratios were measured on 942 specimens, at the University of
 New Mexico Center for Stable Isotopes. Results for each species were
 compiled into analyses of capture date, body mass, fat-free mass, fat
 mass, molt/natal origin, torpor probability, and torpor fat thresholds.
 Results of a subset of 63 juvenile birds tested in respirometry were used
 to predict torpor probabilities in the larger dataset of 573 juvenile
 birds, in which we measured both lipids and feather hydrogen stable
 isotopes, but not metabolic parameters. All birds were fed and released at
 the capture site following measurements. Microsoft Excel and JMP Pro 18
 Statistical Software were used to perform data analyses. We conducted
 statistical analyses to determine how lipid-loads and torpor patterns are
 related to natal origins and migration patterns.
description: **Migratory strategies, fat loads and torpor use in migrating
 hummingbirds** Dataset DOI:
 [10.5061/dryad.dbrv15fdk](10.5061/dryad.dbrv15fdk) ## Description of the
 data and file structure This supplemental data (collected in the field by
 the authors) was used to analyze energy management strategies in migrating
 *Selasphorus rufus*, *Selasphorus calliope*, *Selasphorus platycercus*,
 and *Archilochus alexandri* hummingbirds. We captured 942 birds and
 performed respirometry trials on 94 adult and juvenile individuals. We
 measured both body lipid and feather hydrogen stable isotope ratios on all
 birds. We used results from respirometry trials of 63 juvenile birds to
 predict torpor frequency as a function of body fat and latitudinal origin
 on the larger subset of 573 juvenile birds. Measured variables and units
 are as follows: ### Files and variables #### File:
 Supplemental_Material_S1_Fat_and_Metabolic_Parameters.csv 
 **Description:** Full dataset of Metabolic parameters used in lipid (fat),
 hydrogen stable isotope, and torpor analyses in four species of migrating
 *Selasphorus rufus*, *Selasphorus calliope*, *Selasphorus platycercus*,
 and *Archilochus alexandri* hummingbirds. ##### Variables * Bird ID:
 Banding code and Id# - Capture date: calendar date * Day of Year (DOY):
 sequential date - Capture time: clock time AM/PM * Age: AHY = after
 hatch-year (adult), HY = hatch-year (juvenile) - Sex: M = male, F = female
 * δ2Hf: feather hydrogen stable isotope ratio (‰) - δ2Hcor: discrimination
 factor-corrected hydrogen stable isotope ratio (‰) * Mb (g): body mass
 (grams) - Mf (mg): fat mass (milligrams) * Mf (%): fat mass as percent of
 body mass - Wing Chord (mm): direct distance from leading edge to trailing
 edge of wing (millimeters) * Fat-free mass (g): Mb - (Mf / 1000) (grams) -
 Log Torpor: 0 = no torpor, 1 = torpor, n/a = not applicable Note: cells
 marked with "n/a" represent birds that were not measured in
 respirometry trials. #### File:
 Supplemental_Material_S2_Summary_of_Body_Mass_Components.csv
 **Description:**   Tabular summary of body mass components in the four
 species of hummingbirds we examined. **Variables** * Species * Age - Sex
 (M, F = adult, m, f = juvenile) * Sample size (N) * Average body mass in
 grams (Mb) * Average fat-free mass in grams (Mff) * Average Body fat in
 grams (Mf) * Average body fat as percent of Mb **Table Caption** This
 table summarizes diverse patterns in mean Body Mass (M~b~), Fat-free Mass
 (M~ff~), Fat Mass (M~f~), and % Fat by age and sex, in the four study
 species. Adult *S. rufus* hummingbirds had higher M~b~ than juveniles
 (F(2, 376)=27.3, p&lt;0.0001), and female *S. rufus* had higher M~b~ than
 males (F(2, 376)=5.5, p=0.0199). Adult *S. calliope* had higher M~b~ than
 juveniles (F(2, 67)=6.9, p=0.0109), and female *S. calliope* had higher
 M~b~ than males (F(2, 67)=4.2, p=0.0444). In *S. platycercus*, adults had
 higher M~b~ than juveniles (F(2, 202)=5.4, p=0.0212), and there was no sex
 difference (p=0.1640). In *A. alexandri*, M~b~ did not differ with age
 (p=0.8771), and females had higher M~b~ than males (F(285)=24.9,
 p&lt;0.0001. *S. rufus* adults had higher M~ff~ than juveniles
 (F(2,376)=42.7, p&lt;0.0001), and there was no sex difference (p=0.1507).
 M~ff~ did not differ with age or sex in *S. calliope* or *S. platycercus*
 (p&gt;0.05 in all variables). In *A. alexandri*, there was an interaction
 with age and sex, where females had higher M~ff~ than males (F(3,
 284)=73.9, p&lt;0.0001), and adult females had higher M~ff~ than any other
 age/sex class (F(3, 284)=10.1, p=0.0017). In both *S. rufus* and *S.
 platycercus*, M~f~ was higher in adults (F(2, 376)=42.7, p&lt;0.0001 and
 F(2, 202)=14.3, p=0.0002) respectively, with no effects of sex
 (p&gt;0.05). No effects of age or sex were observed in *S. calliope* or
 *A. alexandri* (p&gt;0.05). Results of % Fat versus age and sex mirrored
 those of M~f~ in grams. Upper case (M, F) represents adult males and
 females, and lower case (m, f) represents juvenile males and females.
 Values are averages ± standard error. #### File:
 Supplemental_Figure_S3_Adult_d2H_vs_Date_and_Sex.pdf **Description:**
 Corrected feather hydrogen stable isotope ratios (δ^2^H~cor~) in adult
 hummingbirds did not change with capture Day of Year (DOY). Male *S.
 rufus* (A) had significantly depleted feathers compared to females (-4.0 ±
 1.5 ‰). Blue and red lines (A) represent male and female values in *S.
 rufus*, and purple lines in (B), (C), and (D) are the output of model
 averaging, where the effect size of sex was too small to detect
 graphically. Violin plots depict distributions of δ^2^H~cor~ in each
 species. ### Code/software No free or open software was used in this
 study. ### Access information Other publicly accessible locations of the
 data: * None Data was derived from the following sources: * Data
 collection by the author in the field