An effective conservation strategy for a species requires knowledge of its biology and life history. This applies to the endangered Ma’oma’o Gymnomyza samoensis, a honeyeater endemic to the Samoan archipelago. Now locally extinct in American Samoa, this species is currently found only in declining numbers on the islands of Upolu and Savai’i in Samoa. Despite being endangered, the life history and breeding behaviour of the Ma’oma’o has not been documented previously. Here we examine Ma’oma’o nesting and breeding biology, which are unique among studied honeyeaters and unusual for passerines in general. Ma’oma’o lay only a single egg per clutch and have an extended breeding season that occurs outside the rainy season and peaks during budburst. Allometric analysis of the length of the nesting period of different honeyeaters versus adult body weight showed that Ma’oma’o remain in the nest for a longer period than expected for their body size. The post-fledging dependency period of 2.5–3 months was also extended compared to other honeyeater species. No Ma’oma’o were observed re-nesting after successfully raising a chick, though pairs attempted to re-nest following breeding failure. Despite the extended breeding season, the maximum annual reproductive capacity of Ma’oma’o is limited by their one-egg clutch and failure to nest again after fledging one chick. We discuss how these slow life history traits can influence conservation strategies, affect monitoring and limit recovery.