I can’t think of a bigger contrast between 19th century naturalists than the differences between John James Audubon and Henry David Thoreau. First, a brief biography of each.
Audubon was born in Haiti during 1785 and was the illegitimate child of a retired French naval officer who owned a plantation there. His mother was a Spanish chambermaid, and he had many mulatto brothers and sisters because his father fornicated with his slaves as well. His father sent him to the U.S. to avoid Napoleon’s draft, and he overcame his prejudice against the English when he married an American woman of English descent. He owned a farm in Kentucky, but it failed. Birds always fascinated him, and he loved painting, so he decided to make a living painting American birds. His well-known books include The Birds of America, The Quadrupeds of America, and An Ornithological Biography. Journals of his travels down the Mississippi and Missouri Rivers are also enjoyable to read. Audubon died of a stroke in 1851.

Portrait of John James Audubon.
Henry David Thoreau was born in 1818 and lived in Concord, Massachusetts for almost his entire life. Unlike Audubon, he did very little traveling, instead focusing on the nature he found locally. His most famous book is Walden–a collection of essays about the more than 2 years he spent living in a cottage next to Walden Pond, a mile from any neighbors. He went there for solitude to write his first book, Travels Down the Concord and Merrimack Rivers. Walden is his most popular book, but I enjoyed Cape Cod and The Maine Woods more. Thoreau disdained the common labor most people performed then, but his books didn’t sell well, and he was forced to work as a surveyor. He never married, and he didn’t like to socialize with young ladies. He may have been gay during a time period when this was especially taboo. He died in 1862 of tuberculosis.

Portrait of a young Thoreau.
Audubon grew up speaking French. English was his second language, and it is noticeable in his earliest writings, but he did eventually master English, and his later writings reflect that improvement. His writing is straightforward with no pretention and is mostly factual with just a little embellishment. Thoreau’s writing is often deeply philosophical. Although he was a man of science, he would invite appreciation of nature’s beauty with long spiritual discussions. He had a much greater command of the English language than Audubon, but I prefer reading the latter’s writing. Thoreau could write 5000 words describing what different kinds of snow looked like, and I think that was tedious.
Audubon bought, owned, and sold slaves; and he was shockingly cruel to animals. Thoreau was adamantly opposed to slavery and was an occasional vegetarian opposed to the wonton slaughter of wild animals. (White tail deer were already extirpated from Massachusetts during his lifetime.) Thoreau was part of the Underground Railroad and helped escaped slaves get to Canada. He was the only person in his neighborhood who supported John Brown’s ill-fated attempt to spark a slave rebellion. Audubon strongly supported slavery, and his best friends were all slave-owners. He killed thousands of animals, just so he could get a better view to paint a realistic portrait. He once nailed a live bald eagle’s talons to the floor in order to take a lifelike portrait of it. He even experimented on his own hunting dog, feeding it parakeets to see if it would be poisoned by the toxic plants they ate. The worst Thoreau did was to temporarily hold some small animals captive, so he could observe them. He detained a screech owl, a flying squirrel, and turtles. He carefully returned the owl and the squirrel to the exact place he captured them. Clearly Thoreau was a much more humane person than Audubon.