Birds can adopt different body postures depending on the type of activity. When birds are on the ground and they seek food, there can be head-up or head-down. When head-up, they are usually scanning the environment for potential predators or food (vigilance behavior). When head-down, they are usually in the processes of grabbing food with their beaks or handling food (foraging behavior).

One of the interesting things that occurs when birds move from head-up to head-down positions is that their blind spots at the rear of their heads change location. Thus, a bird head-up can easily see part of the sky above him (scanning for predators); whereas, when head-down, the sky is no longer in sight because of the blocking effect of the blind spot. Obviously, this could reduce the ability of a bird to detect a predator approaching. The extent to which this blocking effect is meaningful depends on the size of the blind spot. The size of this blind spot differs enormously among different bird species, which raises the question as to how much visual coverage different bird species loose when they are head-down foraging.  To answer this question, we need to compare the configuration of visual fields among different types of species.

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