Familiarity, dominance, sex and season shape common waxbill social networks

Abstract

In gregarious animals, social network positions of individuals may influence their life-history and fitness. Although association patterns and the position of individuals in social networks can be shaped by phenotypic differences and by past interactions, few studies have quantified their relative importance. We evaluated how phenotypic differences and familiarity influence social preferences and the position of individuals within the social network. We monitored wild-caught common waxbills (Estrilda astrild) with radio-frequency identifiers in a large mesocosm during the non-breeding and breeding seasons of two consecutive years. We found that social networks were similar, and that the centrality of individuals was repeatable, across seasons and years, indicating a stable social phenotype. Nonetheless, there were seasonal changes in social structure: waxbills associated more strongly with opposite-sex individuals in breeding seasons, whereas in non-breeding seasons they instead assorted according to similarities in social dominance. We also observed stronger assortment between birds that were introduced to the mesocosm at the same time, indicating long-lasting bonds among familiar individuals. Waxbills that had been introduced to the mesocosm more recently occupied more central network positions, especially during breeding seasons, perhaps indicating that these birds had less socially-differentiated associations with flock members. Finally, individual differences in color ornamentation and behavioral assays of personality, inhibitory control, and stress were not related to network centrality or association patterns. Together, these results suggest that, in gregarious species like the common waxbill, social networks may be more strongly shaped by long-lasting associations with familiar individuals than by phenotypic differences among group members.

Bullying as an advertisement of social dominance in common waxbills

“…This indicates that, rather than attacking directly potentially dangerous opponents, like those socially distant bystanders, waxbills instead show-off aggressiveness in their presence, mostly by bullying subordinate individuals. In gregarious species like the common waxbill, with long-term social associations [40], it should be advantageous for individuals to recognize group members individually [21,41] and to monitor and memorize their dominance status, to avoid conflict. However, by definition the frequency of interactions declines with social distance, which diminishes the opportunities to monitor dominance status and could also affect memory reliability [26,65].…”

“…Four months of RFID data in this system provides abundant information to robustly infer the social network [42] and dominance hierarchy ( [38]; see also the results, below). It is also useful that these four months are in the non-breeding season [44], when sex roles should be more similar [40], because we can analyse social behaviour and group structure without confounding effects of reproduction. The RFID data went through an automated curation process, described in [42], to correct reading redundancies or failures of the RFID antennae.…”

“…Previous social network analyses comprising this time period in the mesocosm, as well as data from three additional seasons, tested different proximity-based criteria to identify associations, and found that the criterion better able to reveal network structure is based on the time that two individuals are at the same time in antennae less than 40 cm from each other (electronic supplementary material, figure S1 in [40]). Therefore, similarly to [40], we used as index of association the total duration of time that a pair of individuals was together at up to 40 cm from each other, divided by the sum of durations that each of the two individuals was in the RFID datastream. This index ranges from 0 to 0.5 and is otherwise similar to the occurrence-based simple ratio index [51].…”

“…In this and the remaining analyses, we used both permutation-based and parametric statistical tests, following [40]. We report results from the parametric regression models in the main text, but we also report methods and results for the permutation-based approach in the electronic supplementary material.…”

“…where sometimes subordinates can win over more dominant individuals) that are stable through time, even across seasons and between consecutive years [38,39]. Waxbills also form stable social networks, with non-random and long-lasting associations among individuals [40], suggesting good individual recognition abilities that could help avoid conflicts [41]. We studied waxbills with passive-integrated transponder (PIT) tags, living in a large open-air mesocosm equipped with an array of radio-frequency identification (RFID) antennae in perches and feeders (figure 1).…”