I do find some animals are far easier to approach than others, but largely it's just been a matter of years of practice; learning about the animals' behavior and how not to seem too imposing to them. Crazy as this may sound, I do also talk to them reassuringly in my mind as I approach, as I do believe this affects the energy I emit, that I do feel they are to a greater degree sensitive to. I do as well always hike in sandals or moccasins which allow me to move almost silently over most terrain. That does helps greatly, as many of the animals don't seem to realize I'm there until I get quite close. And though that is definitely a key, I do find many animals seem quite comfortable with my presence.
I'm generally not fond of pictures of myself, but here I am, out for a typical hike. Birkenstock clogs and all!

Effective, if not the most fashionable. Possible fodder for the next photo contest!
Here is a friend from last season. A regular subject, I photographed throughout the summer. Sometimes seeing him a couple of times a week. Almost always perched on this tall stump, directly above his den. I nicknamed him "Divot", as he was very easily recognizable from the missing piece of his right ear.
This was one of my closest encounters from last year. A beaver I had spotted from the road on the way home. I climbed across a treacherous drainage ditch to get this photo, believing myself realtively safe. Despite their reputation of being extremely shy of humans, after slapping his tail in a territorial warning many times, he proceeded to get closer and closer. So close, in fact, I beat a hasty retreat shortly after this photo was taken, as they have been known to defensively nip animals as large as bear. So much for the wildlife guides!
This was perhaps one of my luckiest encounters last year, and one of the rarest things I have ever photographed. An endangered Blanding's turtle, complete with attached radio transmitter (upper-right).
And this is one the first photos taken with a new camera early in the previous year, of some Mallard ducklings. This is probably, undoubtedly the most popular photo I have ever taken.
