Hi plum612!
I can understanding some of your confusion on this one. As you write about (D),
if a female is put in a cage next to the bird, we lose the need to attract females--one is already present (though unable to get through bars).
What the LSAT test writers are going for with (D) is that there's a cause-effect relationship: a female being present makes the male starlings use more aromatic plants in decorating their nests. The fact that it is caged is besides the point in the terms of "attraction" being used in the stimulus. That is, one might argue that the males couldn't "attract" a caged bird--but that's injecting too much unknown information, if the suggestion is that the caged bird couldn't move. For all we know, for example, it might be a large cage. In that case, (D) is reinforcing the mentioned cause-and-effect relationship.
Moreover, the sense of "attraction" that is used doesn't directly link it to attraction as "physically drawing closer." Rather, it seems to be broader, with attraction just referring to a mating pattern as a whole.
Finally, answer (A) doesn't strengthen the conclusion--the conclusion is "the function of the decorations is to attract females." If starling
adults could defend against the insects repelled by the aromatic plants, there would still be a question of whether the plants are there to protect
nestlings or instead to attract females. Answer (D) reinforces that cause-and-effect relationship.