Data for: Social network data in wild great tits during ontogeny

Sociality impacts many biological processes and is tightly linked to an individual’s fitness. To maximize advantages of group living, many social animals exhibit preferences for associating with individuals that provide the most benefits, such as kin, familiar individuals or those of similar phenotype. Such social strategies are not necessarily stable over time but can vary with changing selection pressures. In particular, young individuals transitioning to independence should continuously adjust their social behavior in light of developmental changes. However, social strategies exhibited during adolescence in animals are understudied and the factors underlying social network formation during ontogeny remain elusive. Here, we tracked associations of wild great tits (Parus major) during transition to independence and across their first year of life. Both spatial and social factors were predictors of dyadic associations, with fledglings initially preferring to associate with siblings and peers over non-parent adults. We found no evidence for preferred associations among juveniles of similar age or fledge weight, but juveniles were marginally more likely to increase associations if their parents were also associated. Similarly, they showed a weak preference for associating with their parents’ adult associates. By autumn, juvenile birds exhibited social strategies similar to those of adults, establishing stable social ties that persisted into next spring. Overall, this research demonstrates dynamic changes in social networks during ontogeny in a species with fast life history and limited parental care, which likely reflect changes in selective pressures. It further highlights the importance of long-term social bonds based on familiarity in this species. 

Methods
The study took place in the woodland around the Max Planck Institute of Animal Behavior in Radolfzell, Germany (47.76811, 8.99652 [WGS84]) as part of a long-term project on social behavior in wild great tits. Tits were caught in mist nets throughout the year and equipped with a metal leg ring issued by the Radolfzell Bird Observatory and a unique passive integrated transponder (PIT) tag (Eccel Technology, EM4102) for long-term identification, then sexed and aged based on plumage (Svensson 1992). To study their breeding behavior we provided 207 nest boxes (Schwegler type 1B, 2M, 3SV) across the woodland. During the breeding season (early April until June 2020), nest boxes were monitored at least twice per week, recording nest stages, clutch sizes, start of incubation, and hatch and fledge dates of nestlings. Breeding pairs were identified four days after chick hatching with a built-in RFID antenna around the entrance hole of a faceplate logger (Naturecounters Ltd.) that recorded PIT tags of adult birds entering the nest box. Any untagged adult individuals were subsequently caught in the nest box and equipped with a metal leg ring and a PIT tag. On day 15 after hatching, nestlings were temporarily removed from their nest box for weighing and were equipped with a metal leg ring and a PIT tag before being returned to their nest. Note that sex of nestlings cannot be determined based on plumage (Svensson 1992) – sex was therefore not included in any of the analyses. To determine the exact date of fledging, we returned to the nest daily from 22 days after hatching onwards until all nestlings had fledged.

To record foraging associations among PIT-tagged birds, we deployed six feeders across the woodland at a distance of approximately 150-200 m from each other, each fitted with two RFID antennae to record visiting birds’ PIT tags. The feeders were installed simultaneously for one 48-hour period per week during active data collection, following protocol established in previous studies (Farine et al. 2012). We collected data all throughout the breeding season into summer to record associations of juveniles during transition to independence. Furthermore, to investigate social network dynamics throughout birds’ first year of life, we collected data during three consecutive weeks each the following autumn, winter and spring, with 48-hours of data collection in each week.