Natural law can refer to observable rules that relate to things in nature, and, in this sense, I believe "The Eagle" does address natural law.  The eagle in this poem has supreme power: he can fly higher than other animals, he's incredibly strong, and he is a lethal predator.  Typically, the strongest animal in a group will maintain control of that group.  In a pride of lions, the strongest male rules; in some societies, the person with the biggest army behind him rules.  Here, the eagle rules because he is also the most powerful.  The "wrinkled sea" that "crawls" beneath him is representative of other organisms on which the eagle might prey, and they are described as crawling in order to point out their relative powerlessness compared to this bird's supreme power.  Therefore, the poem can be read as a representation of the natural law which dictates that whichever animal is mightiest rules the others.