With a telephoto lens, netting and bars are easily overcome. For our protection and theirs, the animals at the zoo are kept in enclosures to keep from getting out. Unfortunately, this ofte places a barrier in the way that makes photography difficult. Chain link fences, netting, bars, and other devices obscure the view.
But this problem can be over come. With a long focal length lens set to a wide enough aperture. thin objects like chain link, disappear in the camera’s view. This caused by light diffraction. The light passes by the object and will recombine on the other side to allow the image to pass through. The are are slight signs that this effect is happening in this image. Look closely in the circular shapes of the out of focus background (referred to as Bokeh) you will see dark lines looking like a chain link fence. This is actually the foreground fence incompletely removed from the image. These line are visible due to the darkness of the image. In a brighter image they probably would not have been visible.
To shoot through a barrier such as this you need to get as close as safely possible to the barrier with a long focal length lens with a a wide aperture. I find at least 85mm with an f4.0 to be the minimum. and wider or smaller an aperture and the barrier elements begin to creep back into the image like an annoying ghost.
This technique is not limited to the zoo. By far the most popular venue to use this technique is at the ball field. With stray balls flying everywhere, home plate is often shield by nets and fences that pose a challenge to sports photography. With proper practice this factor can be mitigated. One caveat though, the ability to cover action midfield is hampered shooting through a fence as the auto focus on your camera will be confused by the fencing material and will fight you as you track the action from base to base.
to learn more about Bokeh:
http://en.wikipedia.org/wiki/Bokeh