Early-life environment drives long-term decrease in adult body mass in a wild bird population
Data files
Aug 21, 2026 version files 20.47 MB
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adult_mass_ran_slo.csv
1.10 MB
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code.Rmd
43.88 KB
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data_adults.csv
3.72 MB
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data_nestlings.csv
15.35 MB
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puned_ped.csv
254.61 KB
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README.md
4.94 KB
Abstract
Body mass is a key organismal characteristic that impacts many physiological and ecological processes and is often a strong determinant of fitness. Many recent studies have documented temporal phenotypic changes in this trait in wild animal populations, but identifying the mechanisms underpinning these changes can be difficult. While most research conducted to date has focused on temperature changes as a driver of these trends, the relevance of other environmental variables remains to be analysed, limiting our understanding of the factors driving these changes.
Here, we use 47 years of data to decompose mechanisms behind temporal changes in adult and nestling body mass in a great tit Parus major population in Wytham Woods (UK). Further, we link those changes to temperature, the main driver of temporal trends in mass according to the literature, and to two other environmental variables previously recognised as drivers of body mass: intra- and inter-specific competition and temporal mismatch with a key prey during breeding, winter moth Operophtera brumata caterpillars.
At the population level we report a marked decrease in body mass in adults between 1978 and 2024 (-0.042 Haldanes; -0.020 grams year-1), and show that this results from phenotypic plasticity, driven by a negative between-cohort trend likely reflecting carry-over effects of the early environment. Within cohorts, however, trends were consistently positive, reflecting an age-dependent mass increase.
The temporal change in adults was paralleled by a change in nestling body mass (-0.036 Haldanes). Nestling mass was negatively associated with estimated intensity of intraspecific competition, as well as inter-specific competition from blue tits Cyanistes caeruleus, as quantified by local population density. These effects carried over to adulthood, as shown by a negative association between adult mass and the population density of great tits and blue tits experienced at early life.
Seasonal and developmental temperature and mismatch with the caterpillar food supply, despite being associated with adult and nestling mass, did not explain the observed declines in mass.
Our results illustrate the potential for effects mediated early in development to carry-over into long-term phenotypic change at later life history stages, and emphasise the importance of considering different environmental variables as drivers of phenotypic change in natural populations.
Dataset DOI: 10.5061/dryad.79cnp5jcc
Description of the data and file structure
Data from the long term monitoring population of Wytham Woods, near Oxford.
Files and variables
File: code.Rmd
Description: Code required to replicate the analyses of the paper. This code allows to replicate all the models in the manuscript, including analysing the temporal trends in adult and nestling mass, analysing the links between adult and nestling mass and the environmental conditions (temperature, breeding density and mismatch with the peak of food availability) and run the quantitative genetic models to compute the heritability of adult mass and the correlations at the genetic and year of birth levels between adult and nestling mass (For further details, see manuscript).
File: adult_mass_ran_slo.csv
Description: Data required to replicate the analysis exploring the temporal trends in adult mass within and between cohorts.
Variables
- ring: Individual identity
- Pnum: Brood Identity
- year: Year of measurement
- sex: Sex (males (M) vs females (F))
- weight: Body mass (in grams)
- box: Nestbox identity
- Section: Section identity (9 levels representing the names of the sections)
- year_cont: Year of measurement as a continuous variable
- b_year: year of birth
- est_byear: estimated year of birth for those individuals dispersing to the population (they are assumed to arrive in the population at age=2 years old)
- year_fac: year as a categorical variable
- est_byear_fac: Estimated year of birth as a categorical variable
File: data_adults.csv
Description: Data required to replicate the analysis exploring the temporal trends in adult mass and the links between adult mass and the environmental variables, and the quantitative genetic analyses.
Variables
- ring: Individual identity
- Pnum: Brood identity
- year: Year of measurement
- sex: Sex (males (M) vs females (F))
- weight: Body mass (in grams)
- box: Nestbox identity
- age_cod: Age as juveniles (juv) vs adults (ad)
- Section: Section identity (9 levels representing the names of the sections)
- year_cont: Year as a continuous variable
- animal: Individual identity to link the pedigree
- tmean_breeding: Mean temperature at breeding season (in ºC)
- tmean_winter: Mean temperature in winter (in ºC)
- average_temperature_capture: Mean temperature during capture (in ºC)
- n_days: Period duration
- buffer_Xm_Y: Number of blue tits (Y=b) or great tits (Y=g) pairs breeding in a buffer X meters around each focal nest box where adults breed in a certain year. There are 17 buffers of different sizes (from X=100 meters to X=4000).
- mismatch: Mismatch, in days, with the caterpillar half fall date. Mismatch was not available for all years and rows with no mismatch information have NA (see Linking body mass and mismatch section for further information).
File: puned_ped.csv
Description: Social pedigree of Wytham Woods, pruned to retain informative individuals and describing the relatedness of the individuals in the population. NAs correspond to unknown parents.
Variables
- id: Id of the individual
- dam: Id of the mother
- sire: Id of the father
File: data_nestlings.csv
Description: Data required to replicate the analysis exploring the temporal trends in nestling mass and its links to the environmental variables (breeding density, temperature, and mismatch) and the quantitative genetic analyses
Variables
- pnum: Brood
- bto_ring: Individual ID
- year: Year of measurement
- nest: Nestbox ID
- year_cat: Year of measurement as a categorical variable
- Section: Section ID
- tmean_breeding: Mean temperature during the breeding season (in ºC)
- tmean_winter: Mean temperature in winter (in ºC)
- average_temperature_capture: Mean temperature during development (in ºC)
- Num_Days_laying: Number of developmental days
- buffer_Xm_Y: Number of blue tits (Y=b) or great tits (Y=g) pairs breeding in a buffer X meters around each focal nest box where adults breed in a certain year. There are 17 buffers of different sizes (from X=100 meters to X=4000).
- mismatch: Mismatch, in days, with the caterpillar half fall date. Mismatch was not available for all years and rows with no mismatch information have NA (see Linking body mass and mismatch section for further information).
- recr: Defines whether a nestling was recruited (recr=1) to the breeding population or not (recr=NA)
- mass_m: Nestling mass (in grams)
Code/software
The analyses were conducted using R and the following packages:
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lmertest (v. 3.1-3)
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lme4 (v. 1.1-37)
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ggeffects (v. 2.3.1)
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ggplot2 (v. 4.0.3)
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dplyr (v. 1.1.4)
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brms (v. 2.23.0)
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MCMCglmm (v. 2.36)
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broom.mixed (v. 0.2.9.6)
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splines (v. 4.5.2)