DOI: 10.5061/dryad.nvx0k6f31
title: Data from: Drivers of variation in egg size in a cooperative breeder with a redirected helping system

description: Females are expected to balance the benefits of current reproductive
 investment against the costs of that investment for future reproduction.
 Egg size may be subject to this trade-off, the outcome of which may depend
 on the intrinsic characteristics of the laying female or the environmental
 conditions that she encounters, such as weather and food supply. In
 addition, a female’s social environment may affect egg investment: in
 several cooperatively breeding species, females adjust egg investment
 according to the availability of help at the nest. In this study, we used
 long-term data and a field experiment to investigate the factors
 influencing egg size in the long-tailed tit Aegithalos caudatus, a
 cooperative breeder with a redirected helping system and relatively
 variable egg size. We show that females laid eggs of a consistent size
 within and across clutches, and that skeletally larger females laid larger
 eggs. However, we found no evidence that environmental conditions or
 social environment influenced egg investment. Therefore, egg size appears
 largely to be an intrinsic characteristic of individual females. We
 discuss the importance of the predictability of future conditions for
 females when making investment decisions during egg-laying and stress the
 need for further studies of pre-laying investment in a wider range of
 cooperative breeding systems.
description: These data come from a study conducted on a population of
 long-tailed tits (<em>Aegithalos</em>
 <em>caudatus</em>) in the Rivelin Valley, Sheffield, UK
 (53°23’N, 1°34’ W) between 2011 and 2024. The key variable we were
 investigating was egg mass, used as a measure of females' per-egg
 reproductive investment. Individuals were ringed with
 unique colour combinations under British Trust for Ornithology licence,
 allowing them to be identified and their reproductive attempts monitored.
 During each year’s breeding season (late February-early June), nests were
 located by following adult birds and monitored every 1-3 days. Once lined,
 nests were checked manually for eggs and the laying date of the first egg
 was recorded. Approximately eleven days later (mean ± SD = 11.03 ± 0.95
 days after clutch initiation, <em>n</em> = 152 clutches),
 following clutch completion, the size of the clutch was recorded, and all
 eggs were temporarily extracted from the nest using a spoon and weighed to
 the nearest 0.001g with a Tanita 1230 digital scale. In
 addition to egg masses, other characteristics of females and their
 breeding attempts were recorded. Relative lay date for each clutch was
 calculated as the number of days after the 1<sup>st</sup>
 March of that year that the first egg in that clutch was laid. The tarsus
 length of breeding females was measured to the nearest 0.1mm using
 callipers, to index their skeletal body size. Females’ ages were known
 precisely if they had hatched within the study area and been ringed as
 chicks. Immigrant adult females were assumed to be one year old at the
 time of ringing because juveniles disperse during their first winter.
 Finally, the number of helpers at each nest was counted during routine
 nest monitoring and 1 hour provisioning watches at two-day intervals
 throughout the nestling stage. Daily minimum temperatures and rainfall
 measurements were obtained from Weston Park Weather Station, ~5km from the
 study site (53°38′N, 1°49′W). These daily temperature and rainfall
 measurements were averaged across the pre-laying period for each clutch,
 arbitrarily judged to begin 20, 10, or 5 days before the first egg was
 laid and to end the day before the last egg was laid.
 During the breeding seasons of 2022 and 2023, experiments were
 conducted to test whether supplementing female long-tailed tits with food
 influenced the size of eggs they lay. As nests were discovered, they were
 assigned to one of two groups: (i) a fed treatment, in which pairs
 received supplementary food in the form of suet blocks; or (ii) a control
 treatment, in which pairs were not provided with feeders. The size of
 their eggs was recorded as above.
description: # Data from: Drivers of variation in egg size in a cooperative breeder
 with a redirected helping system
 [https://doi.org/10.5061/dryad.nvx0k6f31](https://doi.org/10.5061/dryad.nvx0k6f31) ## Description of the data and file structure Our study explores the factors driving variation in egg mass in long-tailed tits (*Aegithalos caudatus*), a cooperative breeder with an atypical redirected helping system. We include an analysis of a long-term dataset spanning 2011-2024 and a supplementary feeding experiment between 2022-2023. ### Files and variables #### File: main\_dataset.csv **Description:** For each breeding attempt, every row is identical except in the egg_mass column, which represents the different masses of each egg in that particular clutch. Missing data: NA **Variables:** * year = the year that the egg was laid in * breeding_attempt = An unique identifier for each nesting attempt (the year of the nest/the number of the nest in that year). Nests were numbered chronologically according to the order in which they were found. * Treatment = whether females were fed or did they receive the control treatment - only relevant for the supplementary feeding experiment * egg_mass = the mass of each egg in grams * clutch_mean = the mean mass of all eggs in that clutch * weighing_date = The date on which the egg was weighed * weighing_day = number of days after the 1st of march of that year that eggs were weighed (so it can be compared among years as a continuous variable) * failed = did the breeding attempt fail (e.g. due to predation)? (yes or no) * female_ring_ID = the colour ring combination of the female that laid the egg. At any one point in time, each colour combination is unique to that individual female, but combinations are reused over time. * female_BTO = the unique BTO ring for the female that laid the egg. This is unique to that individual female and is never reused for another individual * first_egg_date = the date that the first egg in the clutch was laid * first_egg_day = the number of days after the 1st of march of that year that the first egg in the clutch was laid (so it can be compared among years as a continuous variable) * clutch_size = the number of eggs in the clutch from which the egg was taken * last_egg_date = the date that the final egg in the clutch was laid * last_egg_day = the number of days after the 1st of march of that year that the last egg in the clutch was laid (so it can be compared among years as a continuous variable) * days_incubating = the number of days the eggs in that clutch were incubated for before they were weighed. Incubation was assumed to start on the day the last egg was laid. * hatch_date = the date the egg hatched * hatch_day = the number of days after the 1st of march of that year that the egg hatched (so it can be compared among years as a continuous variable) * ringing_date = the date that the chick that hatched from the egg was ringed * fledge_date = the date that the chick that hatched from the egg fledged * fledge_day = the number of days after the 1st of march of that year that the chick that hatched from the egg fledged (so it can be compared among years as a continuous variable) * helper_presence = were the chicks at that nest fed by helpers (i.e. non-parents) yes/no * helper_number = how many individuals were helpers at that nest * female_tarsus = what was the tarsus length of the female that laid the egg (in mm) * F_cohort_strict = the cohort year (i.e. year they hatched) of only those females who were ringed as chicks, and therefore for whom their cohort year is known * F_cohort_relaxed = the cohort year of those females who were ringed as chicks AND those who were immigrants estimated to have been 1 year old when they arrived at the field site (as juveniles disperse during their first winter) * F_age_strict = the age of only those females who were ringed as chicks, and therefore for whom their age is known * F_age_relaxed = the age of those females who were ringed as chicks AND those who were immigrants estimated to have been 1 year old when they arrived at the field site (as juveniles disperse during their first winter) * All weather variables were averaged across the pre-laying period windows, which started either 20, 10, or 5 days prior to the laying of the first egg, and ended the day before the laying of the last egg. * mean_rainfall_x is the average daily rainfall during the pre-laying period window * min_temp_x is the mean daily minimum temperature during the pre-laying period window * mean_temp_x is the mean daily average temperature during the pre-laying period window * average temperature in a given day was calculated as the minimum + maximum temp that day/2 * tarsus_mean = the mean tarsus length of the chicks that hatched from that clutch (mm) * nestling_mass_mass = the mean mass of chicks that hatched from that clutch (g) #### File: female\_repeats.csv **Description:**  Average within-clutch egg masses from females that had 2 breeding attempts in the dataset. ##### Variables * female_BTO = the unique BTO ring for the female that laid the egg. This is unique to that individual female and is never reused for another individual * attempt_1 = mean within-clutch mass from the female's first breeding attempt in the dataset * attempt_2 = mean within-clutch mass from the female's second breeding attempt in the dataset ## Code/software File "LTT_Egg_Code.R" We used R version 4.4.1 in RStudio with the following packages: * tidyverse * e1071 * ggfortify * lme4 * pbkrtest * car * effects * viridis * rsq * patchwork * rptR * emmeans * glmmTMB * DHARMa