The scene where the falcon starts freaking out was caused by somebody upsetting it from behind. If you look closely you can see the arm of one of the crew behind the bird causing it to be upset. I believe the arm might belong to Dan Haggerty, who is also uncredited for this film, but who worked on the film as the animal handler. He later went on to become "Grizzly Adams" in the TV series The Life and Times of Grizzly Adams (1977).
The winter scenes where they show the lake frozen, were not shot in the winter. They had to construct a winter scene by sinking cinder blocks into the pond and then covering it with plywood, before adding the "snow" topping.
Henry David Thoreau (July 12, 1817 - May 6, 1862) was an American author, poet, philosopher, abolitionist, naturalist, tax resister, development critic, surveyor, and historian. A leading transcendentalist, and is best known for his book Walden, a reflection upon simple living in natural surroundings, and his essay called Resistance to Civil Government (also known as Civil Disobedience), an argument for disobedience to an unjust state.
When Sam discovers the peregrine falcon nest, it is occupied by female game birds, not peregrine falcon chicks as is implied. Peregrine falcon chicks are white and downy with a slightly hooked beak. The birds in the nest are most likely female Japanese quail or some similar type of game bird.