One of the three osprey chicks at the Glaslyn osprey project in north Wales has died unexpectedly.
One of the three osprey chicks died of natural causes
The chick had been standing in the nest strengthening its wings when it fell backwards and was not strong enough to correct itself.
The RSPB said the chick died of natural causes but that the other two youngsters remained healthy.
It is uncommon for ospreys to raise more than two chicks so the death was not unusual, the charity said.
All three chicks, which hatched in May, had been developing normally and were very similar in size so the RSPB were unsure as to why the chick died on Friday lunchtime.
Emyr Evans, from RSPB Cymru said the ospreys' behaviour in the days leading up to the death had been normal.
"All three chicks had been active and eating well so there was no indication that this would happen," he said.
"However, it is most unusual for ospreys to raise more than two chicks so there was always a chance that one of these youngsters would not make it.
"It is sad to lose one of the chicks at this stage but it is the way nature works and we will continue to show nest-cam footage at the viewing site and explain the situation openly to our visitors."
Mr Evans said the adult ospreys may expel the dead chick from the nest and if circumstances allowed, the body would then be retrieved by officers and sent for testing.
As well as establishing the cause of death, the tests would also determine which chick had died.
A spokeswoman for RSPB Cymru said some birds of prey ate their dead chicks but it was unlikely the ospreys would because their diet consisted solely of fish.
"The other chicks are fine," she said. "They do not seem concerned they have a dead bird in the nest."
Their father has nested in north Wales for three consecutive years, identified by the orange ring on his right leg. It is not known if the mother is a returning Glaslyn osprey.
It is expected the remaining chicks, who will also be ringed, will soon be learning to fish with their parents before leaving for Africa.
The dead chick remains in the nest with the two healthy youngsters
The viewing site at Pont Croesor near Porthmadog is manned by RSPB staff and volunteers and more than 34,000 have visited since April to see the ospreys.
The rare breed, hunted almost to extinction in Victorian times, remains an amber list species, which means it still needs protection for numbers to recover.
Thanks to a breeding scheme which took osprey chicks from their Scottish stronghold to other parts of the country, the Glaslyn site has hosted breeding ospreys since 2004.
Last year, an osprey pair hatched two chicks from three eggs.
The Glaslyn Osprey Project is partly funded by the European Union's Objective One programme.