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 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.

Methods
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 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 1st 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.