Mutual preening is a behaviour frequently observed in pairs of monogamous birds, such as these Common Moorhens.
Monogamy is a reproductive strategy in which an individual has only one partner at a time.
The vast majority of birds are socially monogamous, meaning that they form pairs and commit to a single partner, but may engage in extra-pair copulation. This is because most young birds are helpless when they are born and require bi-parental care to survive.
In most species, the birds form new pairs every breeding season, whilst in others, the pairs may remain united for years or even for life. This is especially common in long-lived species that re-utilize the same nesting site every year, such as seabirds and large birds of prey.