Birds have depth, and when using a long lens to fill a significant part of the frame with a bird, I try and avoid large lens apertures if possible. I’m tired of seeing a bird’s eye sharp but the rest of the body -- all those beautifully colored feathers -- out of focus due to shallow depth of field. The whole point of photographing nature is to capture the exquisite detail in the subjects. Even if the ambient light is low, I sacrifice a low ISO for more depth of field. This is easier to do with perched birds, of course, because they don’t move quickly. I used a 100-400mm lens to capture this red-breasted toucan in the Pantanal, and my settings were 1/200, f/10, and 1250 ISO.