Sunday 10 May 2015

Gestalt - Beak Structure and Shape (Part 1)

Though beaks come in all shapes and sizes they have a consistent design, as discussed HERE.  Though the beak (bill, or rostrum) is a bony structure the upper and lower mandibles have a degree of flexibility.

The outer sheet, the rhamphotheca, contains colour pigments which give the beak colour as discussed HERE.  The rhamphotheca, while subdivided into two segments (rhinotheca of the upper mandible and gnathotheca of the lower) is seamless in most birds.  The albatrosses are among a small number of bird families with a more complex rhamphotheca (note topography below).  The colour pattern of various different segments in albatross rhamphotheca can be an essential part of the ageing and identification process.

The tomia (singular tomium) are the cutting edges of the bill.  Designs vary greatly, from serrated (eg. in mergansers) to ridged (eg. in granivorous birds) to hooked (eg. raptors, shrikes etc.).

The nares or nostrils are generally fairly prominent features, particularly for example in the tubenose seabirds (eg. Tristan Albatross above).  But, if your speciality is diving it may be a disadvantage to have these structures providing access for water into the lungs.  So on some species eg. cormorants and gannets, the nares are missing.  These birds breath entirely through the mouth.

The cere is a waxy structure at the base of the bill of raptors, owls, skuas, parrots, turkeys and currassows.  In all but the owls the nares are found within the cere.  In owls the nares are distal to the cere.  The cere can be an indication of age and/or sex.

Though sometimes confused with the cere, the operculum is a different structure.  It is cartilaginous, not waxy.  It is most recognizable as the swollen, bulbous mass at the base of the bill in pigeons and doves.  It has a functional purpose in many birds.  It can be used to keep water out of the nasal cavity in some diving birds, as a pollen guard in flower feeding birds or to keep dust out of the nares eg. in seedsnipes.

Bill Tip Organ
Birds which probe the soil and mud typically have an internal bill tip organ to help them locate prey.  Shorebirds show some of the greatest variation in bill design owing to the rich availability of specialist niches in the intertidal environmental.

Intra-specific Variation
In many species younger birds have shorter bills than adults.  Males may also have longer and/or broader bills than females.  In a few species there may be some sexual-dimorphism in bill design.
Confounding Factors
During feeding, birds can accumulate material on their beaks which can confuse an identification.  Whether it's food material, soil/mud or water, it can all blend in and mask the real colour and shape of the bill.

Here we can see how a single drop of water can drastically alter the appearance of the bill tip.  Depending on the angle of the subject and the light source the water droplet may mirror and take on the colouration of the bill.  Though Semipalmated Sandpiper (Calidris pusilla) generally has a slight bulbous tip, this is accentuated by the fact the bill of this species often carries this little water droplet.

Interestingly, this bird also seems to have some deformity along the dorsal ridge of it's upper mandible (it's culmen).  Birds can overcome some incredible bill deformities or damage, adapt and survive.

Nest-building Common House Martin Delichon urbicum.

For more on beaks see HERE.

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