I recently had a new follower to my Instagram page, who was admiring some of my images of Flamingos taken in our local zoo, ask, "are these shot in natural light or did you have external lights help .. cos the dark backgrounds really make the flamingos pop." Interesting question, as I know there are many photographers who do use external lights -- often speedlights -- to add some fill and create that "pop" he was talking about to their wildlife photos. Thing is, I don't. Ever. When it comes to wildlife, it's 100% natural light, always. I take that stand because in US National Parks, where I love to shoot wildlife, using artificial light, be it speedlights or flashlights or even your vehicle's headlights to light wildlife, is illegal. Since I'd rather not have that discussion with a National Park Ranger, I simply adopted the attitude that I would forever refrain. Additionally, I like the idea of maintaining the same look in all of my wildlife photos, regardless of location, so it just became standard operating procedure to keep everything, including the light, as I find it.
This means finding other ways to make my images pop, and keep the critter I'm shooting from blending into their background. And for the most part, that last thing is the key -- the background.
I have heard it said that the best photographers are keenly aware of the backgrounds in their photos, and while I would never clump myself in the category of the "best," I certainly make every effort to make sure the backgrounds, especially in my wildlife photos, is designed to make the animal stand out, while calling as little attention to itself as possible. Of course, this applies to portraits of people, too!
The traditional way of doing this, and the one that most people think of first, is by blurring the background, creating a soft, diffuse bokeh. As most photographers know, that bokeh is usually obtained by shooting with a long lens and a wide aperture. With wildlife, that usually means anything from 200mm to 800mm, from f/2.8 to even f/5.6. Since buying a Nikon 500mm PF lens, which has a maximum aperture of f/5.6, I have found I can get a wonderful bokeh by simply keeping my subject closer to me, while maintaining some extra distance between my subject and their background. The shorter the former and the longer the latter the more and better the bokeh will be.
But then there also is the idea of shooting with a dark background, and there are two ways to gain that dark background. The first way is to try to shoot against a background that is itself very dark in tone. Dark foliage, dark trees or rocks, dark water, all work well. The second way is to shoot a subject that is well lit, but where the background is in deep shadow. In both cases, it is very important to expose for your subject. Traditional spot metering usually works quite well, as does what Nikon calls center-weighted average metering, which is accomplished by utilizing a narrower angle of view than their matrix (averaging) mode, but not as tight as true spot metering.
Either of these methods -- a naturally dark background, or a background in the shadows -- can be accomplished in both the great outdoors or in many of our modern zoos. All it takes is a keen eye, and the willingness to really examine those backgrounds.
In a nutshell, keeping your backgrounds as simple as possible, and quite often, as dark as possible, will make your subjects pop, and make your images standout.