Black-tailed godwits usually stick with the same mate for life, but a 12-year-old male at Welney Wetland Centre on the Norfolk-Cambridgeshire border obviously has other ideas.
An RSPB spokesman said the godwit's infidelity was "unique in our experience and fascinating behaviour".
Several chicks have hatched but it is not yet known which of the bird's mates is the mother.
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There are only two other breeding pairs of black-tailed godwits at the reserve, both of whom have also hatched chicks in a more traditional family unit.
Experts at the wetland reserve described the situation as "precarious" and speculated that "these normally monogamous birds [might be] trying a new tactic this summer".
Dr Viola Ross-Smith from the British Trust for Ornithology (BTO), said: "Black-tailed godwits generally pair with the same partner for life, but will divorce if their partner does not arrive back on the breeding grounds at the right time, or look for a new partner if their old one dies, so it could very well be the case that bigamy is new to Welney."
The majority of other 60 breeding pairs in the UK spend the summer at the RSPB's Nene Washes reserve near Peterborough before migrating to Africa for the winter.
Black-tailed godwits are classified as "near threatened" on the IUCN Red List of Threatened Species.
A usually faithful bird has surprised experts by finding a second "partner".