DOI: 10.5061/dryad.pnvx0k6xw
title: Data from: Environmental and geographic conditions on the breeding grounds drive Bergmannian clines in nightjars

description: 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.
description: # Data from: Environmental and geographic conditions on the breeding
 grounds drive Bergmannian clines in nightjars ## 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](https://onlinelibrary.wiley.com/doi/abs/10.1111/jbi.15176)\ * [GitHub repo](https://github.com/LezzGitIt/Bergmann-Round2.git) (see `Scripts/Development` for version history) ## Abstract We assembled a cross-continental dataset from 29 researchers to assess: 1. Whether three species of migratory nightjars (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. 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: 1. Start at script `00` to recreate all intermediate files; or 2. Start at script `03` and run analyses using pre-processed data in the `Data/Analysis` folder. **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](https://github.com/ecknight/Bergmann/blob/main/ExtractCovariates.R). 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 data * `Capri_df_combined_07.25.25.xlsx`: Full data set * `Envi_Covs_07.29.25.xlsx`: Processed environmental covariates * `Preprocessed/`: Pre-processed data needed to run script `03` * `capri_analysis07.25.25`: Breeding data set * `capri.fac07.28.25`: Full annual cycle data set * `Intermediates/`: Intermediate data files * `Plots/`: Generated figures * `Rdata/`: `.RData` objects * `Scripts_JBI/`: Analysis scripts (run in order: `00` to `05`). * `00–03`: Data wrangling * `04`: Final analyses * `05`: Main-text figures * `Spatial_files/`: * `Ranges/`: Breeding range shapefiles for Figure 3 * `ETOPO1_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 ID * `Project`: Research project name * `Site.name`: Site name * `Species`: Common nighthawk, European nightjar, or whip-poor-will * `Age`: 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 sunset * `Wing.comb`: Wing length (in mm) averaged across captures within individual and age group * `Mass.combBT`: Mass (in g) averaged (within individual and age group) across captures that have an associated banding time * `Mig.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`: Longitude * `Elev`: 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 names * `Country`: The country where the individual was banded * `TagID`: GPS Tag number (only for birds that have wintering data) * `CP`: Cloacal protuberance * `BP`: Brood patch * `Fat`: Qualitative fat scale 0–5 in North America, 0–6 in Europe * `Wing.Chord`: In millimeters (mm) * `WingFlat`: Was wing flattened upon measurement? 'Yes' in Europe & 'No' in North America * `Tail.Length`: In mm * `Tarsus`: Length in mm * `Mass`: 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`, ``, 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