What unites the different things the speaker sees ? Is there anything more than the fact that he can see them from where he lies ?

What unifies the sights the speaker sees is the rhythms of nature.  From his vantage point on a farm,  he observes the butterfly that sleeps on a tree trunk, the cows that graze in a nearby pasture, the manure that dries in the sun, and a hawk that heads home.  The speaker is only able to notice these things because he is at rest, outdoors, under a tree.  One way of interpreting the poem's last line, "I have wasted my life" is that perhaps the speaker has spent his life away from the natural world and missed out on these simple beauties.  In this way, the line could be read as a lament.  It could also be read as an expression of wry humor. Perhaps the speaker wishes for a more simple life and thinks that the animals have an ideal existence. Or, it could be that he finds the whole scene too rural and boring and is being a bit sarcastic about what he witnesses.