Does the large bill of the hornbills is a hindrance in their visual field?
Well, to a large extent, yes, but it also has its advantages related to precision-grasping and sunshades.
Interspecific comparisons of the topography of avian visual fields have indicated that the extent and position of the frontal binocular field is related to the degree to which vision is employed to control the position of the bill or feet when they are used to take food items.
A study on visual field topography in Tockus leucomelas (Bucerotidae), the Southern Yellow-billed Hornbill, has shown that bill indeed intrudes into the binocular field. This intrusion of the bill restricts the width of the binocular field but allows the birds to view their own bill tips. It is suggested that this is associated with the precision-grasping feeding technique of hornbills.
When feeding, hornbills employ ‘precision-grasping’. The bill is used as a pair of forceps, grasping an item between the tips and then tossing it back into the throat or further back into the mouth. Items are often manipulated in the bill tips.
Interspecific comparison shows that eye size and the width of the blind area above the head are significantly correlated. The limit of the upper visual field in hornbills is viewed through the long lash-like feathers of the upper lids and these appear to be used as a sunshade mechanism.