The canary girls were the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

's trinitrotoluene (TNT) shell makers of World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

 (1914–1918). The nickname arose because exposure to TNT can turn the skin orange-yellow (and cause other, more serious health problems), similar to the plumage of the bird, the canary

Domestic Canary

The Domestic Canary, often simply known as the canary, is a domesticated form of the wild Canary, a small songbird in the finch family originating from the Macaronesian Islands ....

.

Since most men were joining the military, women were required to fill factory jobs formerly occupied by men. The United Kingdom used slogans like 'National Service

National service

National service is a common name for mandatory government service programmes . The term became common British usage during and for some years following the Second World War. Many young people spent one or more years in such programmes...

' or 'Women's Land Army

Women's Land Army

The Women's Land Army was a British civilian organisation created during the First and Second World Wars to work in agriculture replacing men called up to the military. Women who worked for the WLA were commonly known as Land Girls...

' to encourage young women to join the work force.