That is a really interesting issue. The Migratory Bird Treaty was the first treaty used in combination with the Interstate Commerce clause to give the federal government the power to regulate private natural resource use. This expanded into the federal Endangered Species Act.
There is an excellent Australian synopsis about the issues of treaty making and federalism: http://www.aph.gov.au/Senate/committee/legcon_ctte/completed_inquiries/pre1996/treaty/report/ (Chapters 3 and 10) It explains how the International Community dislikes the problems that our federal form poses in approving and implementing treaties. The Vienna Convention on the Law of Treaties assumes that there is a sovereign at the top that has the capacity to enter into a treaty. In our system, the People are sovereign and power is fractionalized among layers and branches of government. With treaties that reach into domestic matters, that poses a real problem.
It also explains that International Law can apply to a country that has not even signed a treaty. You can see where this would be a problem when Supreme Court Judges believe that International Law should be taken into consideration.
Some older treaties include a federal clause, but other governments don’t like this as they are bound and we are not. Sometimes we use a reservation. An example is the following federal clause from article 34 of the UNESCO Convention Concerning the Protection of the World Cultural and Natural Heritage:
“The following provisions shall apply to those State Parties to this Convention which have a federal or non-unitary constitutional system:
“(a) with regard to the provisions of this Convention, the implementation of which comes under the legal jurisdiction of the federal or central legislative power, the obligations of the federal or central government shall be the same as for those States Parties which are not federal States;”
“(b) with regard to the provisions of this Convention, the implementation of which comes under the legal jurisdiction of individual constituent States, countries, provinces or cantons that are not obliged by the constitutional system of the federation to take legislative measures, the federal government shall inform the competent authorities of such States, countries, provinces, or cantons of the said provisions, with its recommendation for their adoption.”
Generally, treaties made pursuant to the Constitution are the supreme law of the land on an equal footing with federal legislation. However, both implementing legislation and treaty provisons may be superceded by subsequent modifying or annulling federal legislation. The duty of the Courts “is to construe and give effect to the latest expression of the sovereign will.”
Supposedly, treaties should not give the federal government authority that has not been delegated to it in the Constitution. In implementing a treaty, Congress cannot abrogate the structural separation of authority under constitutional principles of dual sovereignty and assume either reserved State powers or powers reserved to the people. Under the principles of dual sovereignty, Congress may not impose legislation upon the States, although it may recommend implementing legislation to the States. Federal legislation must act directly upon individuals. The Courts will nullify State legislation that is contrary to “self-executing” treaty provisions or construe such legislation in harmony with a treaty.
In my (non-exhaustive) research, I came upon the following relevant cases that examined the treaty powers:
Justice Field’s opinion in De Geofroy v. Riggs, 133 U.S. 258 (1890):
“...That the treaty power of the United States extends to all proper subjects of negotiation between our government and the governments of other nations is clear. It is also clear that the protection which should be afforded to the citizens of one country owning property in another, and the manner in which that property may be transferred, devised, or inherited, are fitting subjects for such negotiation, and of regulation by mutual stipulations between the two countries. As commercial intercourse increases between different countries, the residence of citizens of one country within the territory of the other naturally follows; and the removal of their disability from alienage to hold, transfer, and inherit property, in such cases, tends to promote amicable relations. Such removal has been, within the present century, the frequent subject of treaty arrangement. The treaty power, as expressed in the constitution, is in terms unlimited, except by those restraints which are found in that instrument against the action of the government, or of its departments, and those arising from the nature of the government itself, and of that of the states. It would not be contended that it extends so far as to authorize what the constitution forbids, or a change in the character of the government, or in that of one of the states, or a cession of any portion of the territory of the latter, without its consent. Railroad Co. v. Lowe, 114 U.S. 525, 541, 5 S. Sup. Ct. Rep. 995. But, with these exceptions, it is not perceived that there is any limit to the questions which can be adjusted touching any matter which is properly the subject of negotiation with a foreign country. Ware v. Hylton, 3 Dall. 199; Chirac v. Chirac, 2 Wheat. 259; Hauenstein v. Lynham, 100 J.S. 483; 8 Ops. Atty. Gen. 417; People v. Gerke, 5 Cal. 381.
Justice Sutherland in United States v. Curtiss-Wright Export Corp., 299 U.S. 304 (1936) (This is a good case to read):
“It will contribute to the elucidation of the question if we first consider the differences between the powers of the federal government in respect of foreign or external affairs and those in respect of domestic or internal affairs. That there are differences between them, and that these differences are fundamental, may not be doubted.”
In 1956, the U.S. Supreme Court in Reid v. Covert observed that the Court has “regularly and uniformly recognized the supremacy of the Constitution [U.S.] over a treaty.”
As I recall, Larry Beecraft did some good research on treaties and municipal powers: http://home.hiwaay.net/~becraft/TREATIES.html
It explains how the International Community dislikes the problems that our federal form poses in approving and implementing treaties.
Which helps to explain(as if it were needed) our leaders' propensity to consolidate power in DC to better comform to the majority of the world's governing styles. We The Sovereign People of America are standing in the way.