One of volunteers who helps look after the Osprey nests at Llyn Brenig says there is "a lot of anger and also a little bit of grief" among his team.
It is after one of the nesting platforms was cut down with a chainsaw on Friday evening.
Nick Kite from the Brenig Osprey Project says they are trying to encourage the breeding pair of Ospreys to take on a new nest nearby, after their one newly laid egg was destroyed by the vandalism.
On Sunday morning, North Wales Wildlife Trust shared a poignant picture of the breeding pair looking down at where their nest once was.
"There's a lot of anger there. But it's mixed with other emotions too: Frustration and even a little bit of grief", Nick Kite told ITV Cymru Wales.
"Overall I think everyone can't understand why anyone would do something so barbaric to birds that bring people so much joy"
North Wales Police said it is "truly appalled by last nights disgraceful act at Llyn Brenig" and was working on various lines of enquiry. The force says it believes the vandals may have approached by boat.
On Twitter, the rural crime teams said: "Wildlife crimes always have a motive and we believe the person who did this had a strong motive to rid the area of ospreys and particularly the lake".
Rob Taylor, Rural Crime Team manager at North Wales Police told ITV Cymru Wales it's the worst incident he has seen in 14 years on the team.
"It's one of the most despicable and selfish acts I've ever seen", he said.
"We see some horrendous sights investigating wildlife crime but for someone to purposely go there and cut down the nest probably for their own gain is just absolutely disgraceful", Rob Taylor added.
The Osprey is a fish-eating bird of prey and is an Amber List species because of its historical decline.
The RSPB says that's due to illegal killing and low breeding numbers.
Ospreys are listed as a Schedule 1 species on The Wildlife and Countryside Act.