Drivers of variation in egg size in a cooperative breeder with a redirected helping system

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 envir..., These data come from a study conducted on a population of long-tailed tits (Aegithalos caudatus) 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, n = 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 123..., , # 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 chronologicall...,