Dataset DOI: 10.5061/dryad.1ns1rn975

Description of the data and file structure

The study was carried out in Southern Sweden in a nest box population of blue tits (Cyanistes caeruleus). We measured the maximum exercise-induced metabolic rate (MMR), endurance, and pre- and post- exercise resting metabolic rate (RMRpre and RMRpost) of male blue tits both early on and at the peak of the nestling feeding (day 5 and day 14). We compared blue tits with enlarged and control brood sizes. We also measured nestling biometrics and feeding frequency.

Files and variables

File: all_nest_data.csv

Description: Exercise capacity data for all birds measured in the study.

Variables

• year: year when data was collected
• ID: nest box ID
• day: nestling age (day 0 = hatching date)
• endurance: Duration of exercise test (min)
• MMR: maximum exercise-induced metabolic rate

File: analysis.R

Description: R code for reproducing analyses, tables and plots in the manuscript.

File: D14.csv

Description: Exercise capacity data for all birds measured when nestlings were 14 days old.

Variables

• year: year when data was collected
• ID: nest box ID
• ringnr: bird individual ID
• age: age determined from plumage characteristics (2CY = 2nd calendar year, 3CY+ = 3rd calendar year or later)
• tx: brood size category (E = enlarged, C = control)
• brood_size: number of nestlings after hatching
• brood_size_manip: number of nestlings after brood size manipulation
• aprild: date in April day (days after March 31)
• day: nestling age (day 0 = hatching date)
• mass: body mass (g)
• date: date (M/D/YYYY)
• time: time at the start of metabolic measurements (HH:MM:SS)
• RMRpre: resting metabolic rate before exercise capacity test (W)
• RMRpost: resting metabolic rate after exercise capacity test (W)
• endurance: duration of exercise capacity test (min)
• MMR: maximum exercise-induced metabolic rate (W)
• holding_time: time between capture and start of metabolic measurements (hours)
• nestling_mass_D14: mean nestling mass when nestlings were 14 days old (g)
• nestling_mass_D5: mean nestling mass when nestlings were 5 days old (g)
• nestling_wing_D14: mean nestling wing length when nestlings were 14 days old (mm)
• nestling_tars_D14: mean nestling tarsus length when nestlings were 14 days old (mm)
• accelerometer: individual had an accelerometer mounted with a leg-loop harness on days 12 - 14 (n = no, y = yes)

File: paired.csv

Description: 

Variables

• year: year when data was collected
• ID: nest box ID
• ringnr: bird individual ID
• age: age determined from plumage characteristics (2CY = 2nd calendar year, 3CY+ = 3rd calendar year or later)
• tx: brood size category (Enlarged or Control)
• day: nestling age (day 0 = hatching date)
• date: date (M/D/YYYY)
• time: time at the start of metabolic measurements (HH:MM:SS)
• RMRpre: resting metabolic rate before exercise capacity test (W)
• RMRpost: resting metabolic rate after exercise capacity test (W)
• endurance: duration of exercise capacity test (min)
• MMR: maximum exercise-induced metabolic rate (W)
• mass: body mass (g)
• holding_time: time between capture and start of metabolic measurements (hours)
• ff: feeding frequency (visits per hour)

File: population_nestlings.csv

Description: Nestling biometrics data from 2023 and 2024 in the study population

Variables

• year: year when data was collected
• Nestbox: Nestbox ID
• mass: mean nestling mass (g) when nestlings were 14 days old
• wing: mean nestling wing length (mm) when nestlings were 14 days old
• tars: mean nestling tarsus length (mm) when nestlings were 14 days old
• bsize: brood size when nestlings were 14 days old

Code/software

We used R version 4.5.2 (2025-10-31 ucrt) for the analysis

We used these packages in the R script for data analysis:

library(ggplot2)

library(dplyr)

library(car)

library(broom)

library(emmeans)

library(lme4)

library(lmerTest)

library(rptR)

library(ggpubr)

library(here)