Bill Tip Organs

The most interesting aspect of somatic sensitivity in birds is the concentration of groups of receptors in the bills which have become known as ‘bill tip organs’. Bill tip organs occur only in certain bird orders and families in which they are readily interpreted as having a specific role in the foraging or bill exploratory behaviour of particular species (Cunningham et al. 2010; Demery et al. 2011). Bill tip organs turn the bill from being just a mechanical structure primarily concerned with seizing and manipulating objects into a ‘tactile exploratory organ’. They have been described by Gottschaldt (1985) as an avian equivalent of the mammalian sinus hair system in moles (Eimer’s organ, Eimer 1873) whose properties have been most recently studied in star-nosed moles Condylura cristata (Catania and Remple 2004) and found to be crucial for guiding the underground behaviour of these animals. Like Eimer’s organs, bill tip organs allow the bird to detect, and possibly identify, objects which are buried or at least are out of sight. Bill tip organs seem to allow these probing birds to distinguish between stones, molluscs, and crustaceans, without the need to bring them to the surface.

Bill tip organs do not seem to have any special receptor types but have high concentrations of known somatosensory receptors, mainly Herbst corpuscles positioned at or close to the bill tip. The link between bill tip organs and foraging seems to be clear. If the bill is used to search for, catch, and select food items, as in waterfowl and shorebirds, a well-developed bill tip organ is present. The structure of bill tip organs of geese and ducks are known in most detail (Gottschaldt 1985), although recent research has revealed details of the structure and mode of operation of bill tip organs in shorebirds (Piersma et al. 1998), kiwi (Cunningham et al. 2007), ibises (Cunningham et al. 2010), and parrots (Demery et al. 2011).

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