“…now on the finches in the palo santo tree you see adaptive radiation’s results, a fluorescent spray of horn.”
I don’t understand the bold parts of this sentence. What does “a fluorescent spray of horn” mean?
Hi Cooliegirly
Here is my guess: 
I understand that the finches have mutated/adapted/become different. Horn may refer to their beaks, fluorescent would then be a description that characterizes the “new” beak color. Spray could be a description of the tree with many finches in it (i.e., the finches are scattered all over the tree, but what is particularly noticeable in the “spray of birds” is the color of the beaks).
Amy