The thieves smashed their way into an area of the visitor centre at Fairburn Ings, taking off with electrical equipment.
The incident happened shortly before 10pm on Saturday, December 28, with CCTV catching the culprits entering the car park of the Newton Road sanctuary, which is run by the Royal Society for the Protection of Birds (RSPB).
Jan Elsworth, the visitor operations manager at the reserve, said: "Because it was so dark we couldn't ID anybody, but there were at least two individuals and they were there for an hour."
The thieves rammed the electric gates at the visitors' car park which will cost thousands to fix, while they stole a large Sharp TV thought to be at least 50" in size, three Phillips computer monitors, several laptops, a mountain bike and a telescope.
And the RSPB looks set to pick up the cost as Mrs Elsworth explained: "The excess of our insurance policies is massive so we can't justify paying it in this case. We will just have to take it on the chin.
"Everybody is really angry about what has happened. Stealing from a charity - is there anything lower?"
Sadly, there has been a steady stream of crime at the nature reserve, including petty vandalism and metal thieves who are using its land to gain access to the railway tracks next to the site.
The car park gates were also rammed during the summer and cost more than £2,750 to fix.
"It feels like we're in the middle of a rural crime wave, but the police have been brilliant, they have helped us every step of the way and we couldn't have asked for more," added Mrs Elsworth.
Fairburn Ings was transformed into a nature reserve in 1957 and has been managed by the Royal Society for the Protection of Birds (RSPB) since the 1970s.
It is home to hundreds of species of birds and is a popular destination for birdwatchers.
Meanwhile, anyone with details about the break-in, should contact police quoting crime reference number 13190662864.