Very good observations! The dark stone at the top was definitley a focus point in the configuration and is also propped up (small standing stone.) As a matter of fact you could analyze the shape of this top standing stone and say it is "receptive" like this:
http://3.bp.blogspot.com/-jGi6JUJnI2...600/Marks2.jpg
Different types of stones as well as shapes and sizes were all taken into consideration with the building of all these structures, including the first post I made here of the shrine, and no 2 are alike which makes it fascinating. I know in other cultures around the world such as the Roman Empire that at one time or another they started producing same-size bricks for instance to make everything uniform. Although there are bountiful production sites in the ancient America's including my region they never made things uniformly the same which I guess is another classification for saying its indigenous.
Also with the U-Shape structure it is along a hill-side. It is basically at the base of a small knoll that is the highest point in that immediate area. Places where earth, sky and water meet, especially water flowing in 2 or more directions (such as the Echo Lake site I posted- headwaters of the Charles, Blackstone & Sudbury rivers) was important. Like you were getting at spirits were important, the cosmos, ancestors, all those elements. Here is another distinct U-Shape stone ring/ prayer seat at another site also by a hill-side:
http://1.bp.blogspot.com/-BU8rIrhvRM...600/zain16.jpg
And a nearby cairn:
http://4.bp.blogspot.com/-9tmFwxtZp0...600/zain25.jpg
As for the birds- yeah, I wish I could get better pictures of those. It is more apparent when looking at the sites in real life, although one still needs some schooling the way I did to start seeing how these sites turn up everywhere/ the different features. Here are some more smaller birds propped onto boulder platforms only about 20 yards apart:
http://3.bp.blogspot.com/-YDRPltJkvr.../wonder28A.jpg
http://2.bp.blogspot.com/-JqXoMwE6Db...0/wonder16.jpg
Thanks for taking an interest! I will post more here and there- thank you for realizing that these structures are more than just piles of stones!