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/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/Preprocessed 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. 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

• 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_analysis07.28.25: 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/Preprocessed 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: Species common name - 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 files within the Data/Preprocessed, as well as Envi_Covs_07.29.25:

Note: All variable names are prefixed with B (breeding) or W (wintering).

• Lat: Latitude (decimal degrees)
• Long: Longitude (decimal degrees)
• 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, <NA>, or an empty cell " " . These are standardized to NA in script 00_DW_njdf_JBI.R (line 46). Thus, in the Data/Preprocessed 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