This study aimed to discover whether the Hill Mynah Gracula religiosa forms pairs for a single breeding season or engages in long-term pair bonding, and to investigate the role of female aggression in maintaining pair bonds. Experiments were conducted on captive birds in Thailand between 2006 and 2013, and pair behaviours such as allopreening and aggression were observed four times every day. In the first experiment, when each breeding pair was preparing to rear a new clutch, pairs were placed in a new aviary with unfamiliar males and females with which they could potentially form relationships. Results indicated that breeding pairs stayed with the same mates over four continuous breeding seasons (2006–2009) without changing partners or taking additional mates. In a second experiment, the role of female aggression in maintaining long-term pair bonding was examined. Each breeding pair, when preparing to rear a new clutch, was placed in a new aviary with three unfamiliar females. The results showed that female breeders displayed vigorous aggression towards the other three females, forcing them to retreat from the pair; this behaviour was sustained over four continuous breeding seasons (2010–2013). Thus, it is suggested that the Hill Mynah engages in long-term pair bonding and that the aggression of the breeding female is an essential component in the maintenance of pair-bonds.
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