An experiment on the function of the long-term pair bond period in the socially monogamous bearded reedling

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Pair bond duration and age are suggested to be important determinants for reproductive success in long-term monogamous pair bonding. These two effects are generally difficult to separate, however, and experimental approaches are scarce. We experimentally examined the role of the extended pair bond period, controlling for age and experience, in the bearded reedling, Panurus biarmicus. In this species pairs form in juvenile flocks and usually remain stable for life. We experimentally manipulated the pair-formation period in aviary pairs prior to their first breeding season and observed the effect of varying pair bond duration on breeding success, male parental effort and breeding coordination. This was done for pairs with long (6.5 months), medium (4 months) and short (2 weeks) formation periods. Our results suggest that the length of the bonding period does not influence male parental effort, as males with medium and short formation periods did not reduce their parental effort in nest building, during incubation or in nestling provisioning. Nevertheless, coordination of pair partners (in terms of simultaneous start of nest building) occurred more often when the pair bond period was long, and these pairs also started breeding earlier. Pairs with long bonds succeeded more often in breeding attempts and had higher hatching and fledging success than pairs with medium and short formation periods. Thus, our results support the hypothesis that the duration of the pair bond may improve coordination within a pair before the start of breeding and thereby may result in enhanced reproductive success.