Wild zebra finches that nest synchronously have long-term stable social ties.

Many animals live and breed in colonies and yet, with just a few exceptions, the value of the social bonds between colony members have rarely been examined. Social ties are important for group coordination at collective tasks and social coordination can facilitate synchronized reproduction among colony members. Synchronized reproduction in turn can amplify the benefits of coloniality, such as social foraging and predator avoidance. We conducted a field study to investigate if synchronized reproduction among individuals in replicated colonies is linked to the strength of their social bond, and whether these strong bonds are maintained beyond the reproductive period. We PIT-tagged wild zebra finches (Taeniopygia guttata), monitoring their reproduction and social foraging over two consecutive years. We then used social network analysis to characterise the strength of social bonds among birds in the population. We show that birds that are more synchronized in their reproductive timing (and breed in the same colony) had significantly stronger social ties both during and after reproduction than expected by chance. Our long-term sampling also revealed that the strong social ties between synchronised breeders were carried over across years. Our study reveals a strong correspondence between synchronised breeding and the social structure of the breeding colony. This suggests that the synchrony between pairs is not a simple process based on opportunity, but a mechanism underpinned by more complex sociality, which could be carried over to other behavioural contexts. The maintenance of cross-contextual social ties across years suggests that social structure could have extensive consequences on the overall life history of individuals in addition to playing a key role for the reproductive dynamics of colonial breeders.