Keepers at a bird sanctuary in West Sussex hoped that the last remaining female Blue Duck in the country - called Cherry - might mate with either of the drakes, Ben or Jerry.
But neither male duck appeared interested and are now inseparable at the Arundel Wetland Centre, leaving Cherry to her own devices. Centre warden Paul Stevens said he was disappointed that efforts to produce new Blue Duck offspring had failed but said the two male birds made "a lovely couple".
"They stay together all the time, parading up and down their enclosure and whistling to each other as a male might do with a female he wants to mate with," he said.
"People who visit the centre think they're a fantastic couple, without really coming around to the idea that they are two males. They both have very big personalities and people come from all over the country to come and see them. Cherry doesn't seem bothered by it, she's just happy to keep herself to herself."
Blue ducks originate from New Zealand but there were thought to be just three birds in the UK. Keepers initially introduced Ben to Cherry, but neither seemed keen. They then brought Jerry down from a sanctuary in London.
Mr Stevens said: "Cherry showed some interest in him. She displayed typical mating behaviour - she approached him and called to him, she even looked like she was nesting.
"We thought it was great and it was all going to happen but nothing ever did."
Mr Stevens said the male ducks were then placed in the same enclosure: "To our surprise the two males really took to each other and it was obvious that they really liked each other.