The Javan green magpie chicks were bred at Chester Zoo in what its experts said was a UK first, taking the total number of the birds at the centre to 11.
The species is native to the forests of western Java, in Indonesia, and is listed as critically endangered by the International Union for the Conservation of Nature - which says the population in the wild is decreasing.
Experts at Chester Zoo warned the birds are facing extinction in the wild as a result of a loss of their natural habitat and also the illegal bird trade.
Six pairs of the magpies were flown from the Cikananga Wildlife Centre in Java to the UK in late 2015 to establish a breeding programme, before they can be returned to the Indonesian forests.
Andrew Owen, the zoo’s curator of birds, said: “In that time we’ve seen Javan green magpies disappear almost completely from the wild as they are captured for the illegal bird trade.
“Huge areas of forests that were once filled with beautiful songbirds, are falling silent.
“Knowing that our first pair had nested was a momentous occasion for us - seeing the first chick was even more special.
“All four chicks have now fledged and are currently sporting blue feathers, which will eventually turn apple green as they mature.
“So far we have successfully bred from two adult pairs and these four chicks are a vital addition to the worldwide population.
“Every individual we breed here could help save the species as the clock is ticking and time is running out.”