Bill tip organs in waterfowl
In geese and ducks, the outer surface at the tip of the upper bill and the outer and inner surface of the lower bill are covered by a horny plate. This is relatively flat and is shaped somewhat like a human fingernail. Hidden underneath the hard outer horn of the surface of the nail are many ‘touch papillae’ which protrude from the deeper part of the skin around the flat rim of the bill. The tips of the receptors reach the surface. Even the naked eye can see the small pits or surface irregularities which
Figure 4.1 Bill tip organs in waterfowl. Touch-sensitive receptors are clustered in pits arranged in rows with the tactile receptors protruding through the surface of the hard keratin inside the upper bill, around the rim of its tip. The number of touch receptors and their actual arrangements vary between species and possibly between individuals. In this diagram, the location of the bill tip organ in a Mallard Anasplatyrhynchos is highlighted in white on a photograph of the inside of the upper mandible. The enlarged photograph of the organ shows the pits arranged schematically in rows. Photograph of Mallard by Bryan William Jones; the base photographs of the bill are from Gottschaldt (1985).
occur in continuous rows around the rim of the inside of the bill. It is these pits which contain dense clusters of touch receptors (Figure 4.1).
The number, size, and shape of individual touch papillae vary between wildfowl species. These interspecific differences suggest a fine tuning of the structure of the bill tip organ to meet the sensory demands of detecting various types of food items using touch cues alone. Thus, there is highly likely to be a sensory ecology of bill tip organs in wildfowl that is yet to be understood. Neighbouring touch papillae are mechanically isolated one from another and it is argued that this should allow fine spatial tactile discrimination by the bill tip organ (Gottschaldt 1985). Wildfowl with such organs may well be able to discriminate between different objects and/ or the surface structures of objects using tactile information alone. Indeed, there is experimental evidence that Mallards Anas platyrhynchos can distinguish between real and model peas buried in soft substrates using only cues derived from the bill tip organ as sight of the buried peas was not possible (Zweers and Wouterlood 1973). However, taste may also be involved in this behaviour (Berkhoudt 1985); see 4.2.1.