Staff at a bird of prey sanctuary got a shock when Harold the griffon vulture, thought to be a male bird for the last 20 years, laid an egg.

The surprise discovery was posted on Facebook yesterday following the revelation at Eagle Heights in Eynsford, with the sanctuary declaring:

"***BREAKING NEWS*** Today Eagle Heights staff are FLABAGASTED!!! (sic) 20 years ago we bought HAROLD our MALE Griffon Vulture only to find this morning that he is actually a SHE and has laid her 1st ever egg!!! We will now be searching for a husband for her so watch this space..."

The unlikely discovery has been questioned by some followers of the sanctuary, who suggested wildlife experts should have been capable of identifying Harold's gender, and wondered why he hadn't laid an egg previously.

"Is this a hoax?" asked one.

But Eagle Heights insisted it was no joke, explaining: "20 odd years ago when we got Harold he had paper work telling us he was a male. So we took their word for it, as sexing a bird can be rather stressful, as you have to take blood or pull out a blood feather and get a DNA test done.

"Males and females look exactly the same, so unless you DNA there is no way of telling, especially in vultures.

"Sometimes with eagles you can make a guess regarding their size as to whether they are male or female, but vultures are not that easy."

And Sanctuary staff also had an explanation as to why Harold hadn't laid an egg previously, adding: "In the wild they start to breed as soon as they reach maturity because their survival rate is so low, so they start as soon as possible in an attempt to increase population.

"In captivity it's a bit older at around 10 years old on average. So Harold is still a bit older, but up until recently she has been tethered in our main mews (clipped to a perch) which obviously is not suitable for nesting. Now she's been given a big aviary she clearly thought the time was right!??"

But while some suggested a name change, it looks like the name Harold is likely to remain.

"We've had her for so long," added the sanctuary. "It will always be a habit to call her Harold, so it may just stay as her name!"