This article is from the Birds FAQ, by Lanny Chambers with numerous contributions by others.
In the early twentieth century, several governments realized that
the protection of migratory birds was not something one nation could
accomplish alone, because birds do not respect national boundaries.
The treaty was signed by the United States and Great Britain (on
behalf of Canada) in 1916 and was implemented in the United States
by the Migratory Bird Treaty Act. The United States has similar
treaties with Mexico and Japan, and it also signed one with the
The Act makes it illegal to "pursue, hunt, take, capture, kill,
attempt to take, capture, or kill, possess, offer for sale, sell,
offer to barter, barter, offer to purchase, purchase, deliver for
shipment, ship, export, import," etc., migratory birds, parts of
their bodies, or their eggs or nests. Governmental authorities
may make exceptions to allow, for example, hunting seasons or
research work; in these cases, licenses or permits are involved.
The "take" provision above makes it imperative that birders
refrain from harassing birds that are attempting to nest. See
"Birders and the U.S. Federal Laws" in the October 1992 _Birding_
for more information. Note also the "possess" provision above;
it explains why wildlife rehab centers do not give molted feathers
to persons who request them.
In the United States, the Act appears in law at 16 USC 703-711 and
is implemented by regulation at 50 CFR 21.11, 10.12, 10.13.
Web servers where you can find the text of those laws, species list, etc:
Click the line "Code of Federal Regulations" and then find the above
regulations (eg 50CFR10, ETC).