Paradise Wildlife Park's breeding pair of rare black hornbills have hatched a second pair of chicks, in what is believed to be a 'world-first known' documented event within the species.

Back in April, the breeding pair at the Hertfordshire zoo were announced to be the first pair in Europe to hatch and rear a healthy chick during a two-year period.

The hornbill pair, Mulu and Darwin, have defied all previous known knowledge about the species, having now successfully bred and hatched a pair of chicks for the second time in one year.

Both chicks are currently healthy and growing alongside attentive mum Mulu inside the nest box.

In the early hours of June 23, Mulu, the female black hornbill, once again sealed herself into the nest box, merely 12 days after exiting the nest box with her first chick, Biru.

It was on June 30 that keepers were alerted to eggs being laid on the CCTV cameras in the nest box and they continue to keep an eye on mum and babies.

A Paradise Wildlife Park spokesperson said: "Although Mulu and Darwin are new to parenting, both are doing an amazing job.

"Darwin continues to act as a great partner providing Mulu with plenty of food.

"Both mum and the new chicks still rely on Darwin heavily, whilst he cares for young Biru, their previous chick, outside the nest."

Black hornbills are a species of hornbill native to Asia and are found mainly in lowland forest habitats.

According to the International Union for the Conservation of Nature (IUCN) Red List, the species is considered 'vulnerable' to the risk of extinction in the wild due to high levels of deforestation and hunting.

These chicks mark a major discovery in the documentation of the species as well as in conservation breeding.

Currently only 26 black hornbills are registered in the EEP (EAZA Ex-situ Programme) breeding programme across all European zoos.

The bird team at Paradise Wildlife Park have worked incredibly hard to create a favourable breeding environment.

This includes nest box building, to using a misting system to replicate rainfall, and a carefully managed diet.

If the new chicks successfully make it past early stages towards fledging, keepers expect to see them break out of the nest box with mum Mulu around late September to early October.