Much of this sentence is simply clutter. So many of the phrases add extra description, but are the kinds of things your mother would tell you to put away and “declutter” – namely, the prepositional phrases. So let’s get rid of the clutter with “to the zoo”; “often”; “in to the leafy aviary”; “on the branches”; and “whose tails trail like brightly colored splatters of paint on a green canvas”.
On the GMAT, description often serves as clutter, so if you can envision the sentence without the descriptive clutter (similar to how your mom wanted to envision your bedroom), you’d be left with;
Visitors have looked up and saw macaws resting.
Without all of the clutter, your ear should tell you that this is just wrong – the expression should be parallel in timeline: “Visitors have looked up and seen macaws.” And that only leaves D and E.
Now, to make this next decision you’ll need to bring back some of the description, as you can see that the only remaining decision is between “with tails trailing” and “whose tails have trailed”.
And here, yet again, is where your mother’s life lessons can help you. What did you often do to make sure your room passed your mom’s test? You took anything that *might* be considered clutter, buried it in a closet or under a bed, and then dug back in to pull out the things that you really wanted. And that’s the case on GMAT Sentence Correction – when you “eliminate” clutter you don’t get rid of it forever, you just ignore it temporarily. Here if you bring back the description in question, you have:
(D) seen macaws resting on the branches, with tails trailing
(E) seen macaws resting on the branches, whose tails have trailed
Here the description/modifiers are important, and astute test-takers should see that branches don’t have tails, but birds do (your mom probably took you to the zoo, too – one more lesson to thank her for). So E cannot be right, and the answer is D.