Laws of Muktzeh on Shabbos
9. Just as it is forbidden on Shabbos to handle articles that are considered "muktzeh" or "nolad" (1), so too, it is forbidden to place a container underneath them so that they can fall or drop into it; this is because, [once the muktzeh item is in the container,] one has, [in a sense], nullified the functionality of the container ("Bitul K'li Mei'hacheinoh"), because it may no longer be moved; [this is prohibited] because it is as if one has cemented the container to its place (2).
One may, however, turn over a basket in front of chicks (3) for them to climb on, since when there are no chicks upon it, it is permissible to take it away. If, however, chicks were positioned on the basket "beyn hash'moshos" (between sunset and the appearance of three stars) on Friday afternoon (4), it is forbidden to handle it the entire Shabbos (5).
(1) The halachic concept called "nolad" (lit: "came into existence") refers to things that come into existence on Shabbos (See HY 88:4), For example, it would be prohibited to move water which has dripped from an air conditioner on Shabbos, because it is considered "nolad."
(2) Therefore it is similar to an act of building, which is prohibited on Shabbos. Others say that nullifying the functionality of the container is tantamount to an act of demolishing, which is also prohibited (See Mishna Berura 510:20; see also HY 75:11).
(3) As we saw earlier, all animals are muktzeh.
(4) This is true only if the basket was made for that purpose, or the chicks were placed upon it intentionally. In either of these two cases, the basket is classified as a base to a muktzah object ("bosis le'davar Ha'asur"), and may not be moved. The next chapter is devoted to this concept, so I won't go into any more detail here.
(5) When something is muktzeh for the whole duration of "bein hash'moshos," in many cases it remains muktzeh for the entire Shabbos, This concept is called "Migo De'iskatzai Le'Bein Hashmashos, Iskatzai Le'kulei Yuma."