A swooping mass of starlings above Strumpshaw Fen nature reserve enthralled wildlife lovers who witnessed the aerial spectacular recently.
RSPB communications officer Rupert Masefield said the murmuration of starlings had been an amazing sight and urged anyone with an interest in nature and wildlife to try and see the phenomenon.
He said: 'There were an estimated 21,000 birds at the reserve last Saturday, 15,000 on Monday evening and between 15,000 and 20,000 on Tuesday.'
As the autumn progresses starlings arrive from breeding areas to the north, some from as far as Scandanavia.
Mr Masefield said they gathered in large roosts over the winter for safety and warmth.
'In Strumpshaw they roost in the reed beds and trees but they are also known to roost in buildings in urban areas.'
He said the birds were looking for areas where they were protected from predators.
During the day they disperse as they go in search of food, but return in the late afternoon to their roosting areas when murmurations are sometimes seen.
'The phenomenon doesn't always occur as the birds often arrive in small groups and settle down,' said Mr Masefield. 'But sometimes when you get lots of birds together in the sky at once or if they are flushed out by a predator it happens.'
He said it was a temporary phenomenon that occurred as the birds moved south.
'They don't stay here permanently so you have this constant movement of birds coming and going. Their numbers peak at some point before they start decreasing again.
'It's not uncommon for a murmuration to take place, but its not easy to predict when or where they will happen, although some places are more reliable than others. In Norfolk it varies from year to year where you can see it.'
He said when in the sky the birds flocked together much like a school of fish swimming together.
'There are various reasons given for this - it's more confusing for predators and there is safety in numbers while some theories have it they indicate safe roosting places to other birds.'
According to the British Trust for Ornithology starling numbers have fallen by 66pc since the mid 1970s in the UK and are a red listed species.
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