The resident male osprey at The Lodge Forest Visitor Centre reserve in the Trossachs, near Aberfoyle, has set up nests with two females.
It is thought both females have laid eggs, and the male now has his work cut out hunting and tending to them.
The RSPB believe it may be the first time this behaviour by an osprey has been caught on camera.
The male, nicknamed Drunkie after a local loch, arrived back at the reserve this spring ahead of his regular mate Katrine, and had already mated with a new female, Arklet, by the time she arrived.
He continued to mate with both females, but when he set up a nest with Katrine, who laid two eggs, it was thought Arklet had moved on.
Wildlife information and education officer Lucy Tozer said staff at the centre were convinced things were back to normal when Arklet disappeared from view.
She said: "We watched Drunkie with both females for several days before Arklet disappeared from view, and we assumed that she'd left the area.
"But when we visited the nest to fix a camera issue, we found a second eyrie, and Arklet incubating what we can only presume is a clutch of eggs, not 200m from the first nest.
"Katrine's eggs have now hatched, but we're not sure what's happening with Arklet's nest at the moment.
"It is difficult to know how the situation will turn out. Male ospreys work extremely hard to feed their families, and they have to provide for the female as well so that the chicks aren't left alone on the nest."
Aberfoyle Ospreys is a partnership between the Royal Society for the Protection of Birds Scotland and Forestry Commission Scotland, based at the Lodge Forsest Visitor Centre in Aberfoyle.
The RSPB's Duncan Orr-Ewing said the situation was "very unusual" for ospreys.
He said: "It may well be the first time that it's been caught on camera like this.
"If there's plenty of fish all season, then the male might manage to get both broods to fledge.
"But if things don't turn out so well, he may have to give up on one of them, or the females may have to fish as well, which can leave the chicks vulnerable.
"We'll just have to wait and see what happens."
An osprey at a Stirlingshire reserve has found itself tending to two nests by getting caught in a "love triangle".