Officials are offering a $1,000 (£646) reward for information on a missing bird in Palmyra that holds the key to ensuring that the species does not go extinct.

With the city falling into the hands of Islamic State (Isis) militants, authorities fear the worst for the region's critically endangered bird.

The female northern bald ibis alone knows the migration routes to winter grounds in Ethiopia.

Without her, other captive birds cannot be released and the species could go extinct, the Society for the Protection of Nature in Lebanon told BBC.

Three other of the ibis species were abandoned after guards fled following advances of Islamic State militants into the Syrian city. The fate of three more captive birds is not known.

The species was thought to have gone extinct in the region until a small breeding colony of the northern bald ibis was found near the city in 2002.

Since then their number has fallen to four.

A migratory bird found in barren, semi-desert or rocky habitats, the northern bald ibis became extinct in Europe 300 years ago due to hunting, loss of habitat and the use of pesticides.

Following a long-term decline, it now has an extremely small population, with over 95% of truly wild birds concentrated in one sub-population in Morocco, says BirdLife.

In Syria its wild population declined dramatically in the past 30 years to show a small spurt, but then declined with just a single individual returning to the colony in 2013.

The bird is listed as critically endangered despite scientific efforts to establish a colony in Europe.

The Syrian population is behaviourally distinct from the Moroccan one, with which it is thought to have separated long ago.

And with the ancient city under the Isis, the fate of its first century art and architecture, and its rare bald northern ibis hang on a thin thread.