In life things aren't always black and white. In fact, it's usually a great, big patch of gray area. Heaney is skilled at showing us the whole picture in a very small frame. Through realistic and detailed description, he shows us the complicated intricacies that surround us and help us frame our fluctuating opinions. In this particular poem, we see the beauty and ugliness intertwined in nature, and the impressions it makes on our young speaker.
If you're into what Heaney's throwing down here, you might like some of his other poems. Check out "Digging" and "Blackberry Picking", for example. They're from the same collection as "Death of a Naturalist." You'll recognize the trademark natural setting as a backdrop for Heaney to paint a picture of life and its complex relationships and shifting opinions.