Data from: Environmental and geographic conditions on the breeding grounds drive Bergmannian clines in nightjars
Data files
Aug 11, 2025 version files 34.89 MB
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README.md
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Repository_Bergs_rule_nightjars.zip
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Abstract
Aim: To evaluate (1) whether three migratory nightjar species (Family Caprimulgidae) adhere to Bergmann's rule, (2) whether environmental factors on the breeding or wintering grounds determine body size, and (3) which mechanistic hypotheses best explain Bergmannian patterns in body size.
Location: North and South America; Europe, and Africa.
Taxon: Eastern whip‐poor‐will (Antrostomus vociferus), Common nighthawk (Chordeiles minor), and European nightjar (Caprimulgus europaeus).
Methods: We used GPS tracking and morphometric data to assess competing hypotheses explaining variation in body size for each species, based on their breeding (n = 3388) and wintering (n = 189) locations.
Results: All three species exhibited Bergmannian patterns in body size, providing the first evidence that nightjars conform to Bergmann's rule despite adaptations to severe environmental conditions. Environmental and geographic variables at breeding sites were stronger predictors of body size than wintering‐site variables. Although we found partial support for Bergmann's temperature regulation hypothesis, geographic variables, rather than specific environmental factors, emerged as the strongest predictors of body size variation.
Main conclusions: Latitude and longitude correlated strongly with environmental variables and migratory distance; thus, these geographical variables likely encompass many factors that influence body size in nightjars. The present study is among the first to use tracking data from individual birds to understand how environmental pressures across the annual cycle are related to body size. Our findings highlight the critical role of geographic breeding‐ground factors in shaping Bergmannian patterns, offering robust evidence to support nearly two centuries of research since Bergmann's rule was first described in 1847.
Manuscript Information
Journal: Journal of Biogeography
Title: Environmental and geographic conditions on the breeding grounds drive Bergmannian clines in Nightjars
Authors: Aaron A Skinner, Alicia M Korpach, Susanne Åkesson, Marja H Bakermans, Erin M Bayne, Thomas J. Benson, Giovanni Boano, R Mark Brigham, Simon S Christiansen, Greg J Conway, Christina M Davy, Ruben Evens, Kevin C Fraser, Autumn-Lynn Harrison, Anders Hedenström, Ian G Henderson, Juha Honkala, Lars B Jacobsen, Michiel Lathouwers, Peter P Marra, Janet W Ng, Gabriel Norevik, Amy L Scarpignato, Kasper Thorup, Christopher Tonra, Steven L Van Wilgenburg, Andrew C Vitz, Michael Ward, and Elly Knight
Associated Files:
- Manuscript: https://onlinelibrary.wiley.com/doi/abs/10.1111/jbi.15176\
- GitHub repo (see
Scripts/Developmentfor version history)
Abstract
We assembled a cross-continental dataset from 29 researchers to assess:
- Whether three species of migratory nightjars (Family Caprimulgidae) adhere to Bergmann's rule;
- Whether environmental factors on the breeding or wintering grounds determine body size; and
- Which mechanistic hypotheses best explain Bergmannian patterns in body size.
We captured 7,318 Caprimulgids (3,388 used in final analysis) using mist nets and conspecific playback across North America and Europe, a subset of 215 of which were equipped with telemetry tags to obtain winter locations. Birds have morphological measurements (mass and wing length), migratory distance, and environmental variables associated with breeding and winter locations. See Methods and Supporting Information in the manuscript for full details.
Species
Eastern whip-poor-will (Antrostomus vociferus), Common nighthawk (Chordeiles minor), and European nightjar (Caprimulgus europaeus)
Usage
Instructions
All necessary data and code are provided to reproduce the analyses, figures, and tables in the main text. If you open the .Rproj file, everything will run as-is. Otherwise, simply set your working directory to the path where you saved the repository folder.
There are two options:
- Start at script
00to recreate all intermediate files; or - Start at script
03and run analyses using pre-processed data in theData/Analysisfolder.
NOTE: Due to Google's data-sharing policies, we do not include the raw environmental data downloaded from Google Earth Engine. However, the associated script is available here. Script 01_DW_EnviCovs_JBI.R will not run without these raw inputs. The processed output (Envi_Covs_07.29.25.xlsx) is included in the Data folder and is required input for script 02.
Requirements
- R (version 4.0 or higher)
- Required R packages (called at the top of each script)
File Structure of Repository
File: Repository_Bergs_rule_nightjars.zip
Data/: Raw input dataCapri_df_combined_07.25.25.xlsx: Full data setEnvi_Covs_07.29.25.xlsx: Processed environmental covariatesPreprocessed/: Pre-processed data needed to run script03capri_analysis07.25.25: Breeding data setcapri.fac07.28.25: Full annual cycle data set
Intermediates/: Intermediate data filesPlots/: Generated figuresRdata/:.RDataobjectsScripts_JBI/: Analysis scripts (run in order:00to05).00–03: Data wrangling04: Final analyses05: Main-text figures
Spatial_files/:Ranges/: Breeding range shapefiles for Figure 3ETOPO1_coni.tif: Elevation raster (required for Common nighthawk models)
Tables/: Output tables
Column Titles & Descriptions
In the Data/Analysis folder, two files (capri_analysis07.25.25 and capri.fac07.28.25) include:
Band.Number: Unique IDProject: Research project nameSite.name: Site nameSpecies: Common nighthawk, European nightjar, or whip-poor-willAge: Adult, Young, or Unknown (Unk)Sex: Male (M), Female (F)tsss.comb: Time since sunset (hours) averaged across captures within individual and age group. Tsss = 0 would be a bird captured at sunsetWing.comb: Wing length (in mm) averaged across captures within individual and age groupMass.combBT: Mass (in g) averaged (within individual and age group) across captures that have an associated banding timeMig.dist: Total fall migration distance ('great-circle-distance' or 'as the crow flies') in kilometers between breeding grounds, all consecutive migration points, and wintering grounds
Environmental Covariates
Found in the two Data/Analysis files and Envi_Covs_07.29.25:
Note: All variable names are prefixed with B (breeding) or W (wintering).
Lat: Latitude (decimal degrees)Long: LongitudeElev: Elevation (m)Srad: Solar radiation (kJ m⁻² day⁻¹)Tavg: Average temperature (°C × 10)EVI: Enhanced Vegetation Index (unitless; scaled from -1 to +1, typically 0–1 in vegetated areas)Prec: Precipitation (mm)EviCV: The coefficient of variation (CV) of breeding EVI (%)CVprec: CV of precipitation (%)Tcv: CV of temperature (%)
Additional Columns in Full Dataset
The file Capri_df_combined_07.25.25 (in Data/) includes:
Year: Year of capture (YYYY)Banding.Date: Date of capture (m/d/yy)Banding.Time: Time of capture (hh:mm)Site.name: Researcher provided site namesCountry: The country where the individual was bandedTagID: GPS Tag number (only for birds that have wintering data)CP: Cloacal protuberanceBP: Brood patchFat: Qualitative fat scale 0–5 in North America, 0–6 in EuropeWing.Chord: In millimeters (mm)WingFlat: Was wing flattened upon measurement? 'Yes' in Europe & 'No' in North AmericaTail.Length: In mmTarsus: Length in mmMass: Grams (g)B.dep: The last date that individual was observed on the breeding grounds (yyyy-mm-dd)W.arr: The first date individual was observed on the wintering grounds (yyyy-mm-dd)Mig.n: The # migration segments (i.e., datapoints - 1) that the migration distance is calculated from. I.e., if you calculate mig.dist from the last breeding grounds point, 7 migration points, and 1 winter point, Mig.n = 8.Temp.res.mig: The temporal resolution of points collected on migration (i.e., every X days the tag took a fix).
Missing Data
Missing values were reported in a variety of formats: -99, -990, 9999, ., -, NONABAND, na, <NA>, or an empty cell " " . These are standardized to NA in script 00_DW_njdf_JBI.R (line 46). Thus, in the Data/Analysis folder, the capri_ files show missing values as blank cells and will be read in as NA by default in R.
License
The data is licensed under a CC0 waiver, and thus has no restrictions. The authors request that the data and manuscript be cited where appropriate.
Acknowledgments
Thanks to all researchers who contributed data, field assistants who made the work possible, and especially Elly Knight and Alicia Korpach for their contributions to the analyses and manuscript.
Contact
Questions? Contact: skinnerayayron93 [at] gmail [dot] com