It heavily depends on what you are searching for in the field:
When I'm just walking around, I try to have a tele-zoom lens with me, currently, I'm using a 100-400 Canon lens. Although it fails in bad weather and a too near distance (maybe a snake or something directly in front of your feet), I can easily switch from smaller reptiles that may easily abscond to big animals or birds.
If we are looking especially for Vipers, I mostly have my 100mm macro on the cam.
If I'm exploring a new habitate, I use a 18-55 lens, so that I can easily take some shots of the nature and good places.