Selection on relationship strength in wild social networks

Parents are in conflict over the level of care that each should provide, but must also cooperate in the shared venture of raising a family. The ability of an individual to fo rm and maintain a pair bond is therefore expected to play a vital role in maintaining its fitness. We used large-scale pre-breeding social network data to test the relationship between pairwise association pre-breeding and reproductive behaviour and success in great tits. Winter associations between partners for 458 pairs were determined over five winters from over 2 million visits by tagged birds to PIT tag loggers at temporary feeding sites. For each individual, the strength of its association in the wider social network, as well as the duration and strength of association with its partner was determined. The strength of an individual’s association with the wider network, as well as the length and strength of association with its future breeding partner were important predictors of reproductive success. In particular, pairs that formed earlier in the winter bred earlier in Spring and had larger clutch sizes, and pairs with a stronger association fledged heavier young. Additionally, the length of the pair bond predicted the level of coordination between partners over parental care. This suggests that long-term pre-breeding relationships play an important role in determining individual fitness components, and should be selected for in addition to partner quality.