The decisions were announced at the end of a week-long conference of the 93 parties to the Convention on Migratory Species at the Nairobi headquarters of the U.N. Environment Program.
Of the 21 additions, 11 were added to Appendix I of the Convention on Migratory Species, which mandates that parties to the convention must ensure the protection of these species and conserve their natural habitat.
The 10 others were added to Appendix II of the convention, which lists "migratory species that need or would significantly benefit from international cooperation."
These "threatened species throughout the world all gain from the scientific and practical support we can now offer, especially in developing countries," said Robert Hepworth, executive secretary of the Convention on Migratory Species.
The Basra reed warbler was among 11 species added to Appendix I of the convention as its natural habitat — the Mesopotamian marshlands in Iraq — was seriously damaged during the regime of deposed leader Saddam Hussein.
Found only on the Pitcairn Islands, the Henderson's petrel was also listed on Appendix I because of constant predation by rats and crabs. The bird has an extremely low fertility rate, at 20 percent, making conservation attempts even more difficult.