Monday, April 23, 2012

The Bird-Bill….a unique adaptation for survival

The bill or beak for a bird is the most versatile anatomical component adapted for its survival. Apart from the common physical features of a feather covered body and egg-laying for breeding; it is the bill and the feet of a bird that vary according to the food and the habitat they dwell upon.

The stout  and strong bill of the crow
Extremes in bill shape are many. The bill of a crow may be considered a good example of an all-round tool. It is stout and fare in size being strong enough to kill small mammals and opening nuts. Fine enough at the tip for picking small insects etc. Woodpeckers which search for wood boring insects have chisel like bills for cutting holes in trees while the hooked bill of the birds of prey are for tearing flesh. Sand pipers, Woodcocks and Snipe have long probe-like bills for probing worms and invertebrates out of mud. Another unique adaptation is shown by the broad bill of the duck and the scoop like bill of the flamingo which has a sieve-like fringe along the sides. A mouthful of water taken and forced out through the fringes trapping microscopic larvae, algae, snails, insects and seeds to be consumed as food. Yet another extreme bill design is that of the Nightjars, Swifts and Swallows which catch insects on the wing. The reduction of the horny bill is related to the extra-large increase in the gape size. 

Curtsy - Sarath Kotagam / Gamimi Ratnaweera                
Birds grouped according to bill types  
Competition for food on a common feeding ground is managed by the bill shapes controlling exploitation of natural resources. The nectar in a flower is shared based on the reachable depth of the Sunbird's bill into the corolla tube of a flower. A bird with a shorter bill feeding first at the flower leaves a share for the longer billed. 

The most unique form of habitat sharing when feeding is among the waders and shorebirds. Waders and Shorebirds accounts for the largest species diversity in the avian world and much of them migrate from the temperate breeding grounds in the northern hemisphere to tropical feeding grounds during winter. Classified as ‘waders and shorebirds’ it implies that their survival is highly dependent on the wetlands. 

The estuaries and wetlands where the waders feed in winter are the most productive ecosystems in the world. The rivers bring in fresh water which is high in nutrients to mix in the estuaries with the saline water. The resulting intertidal mud flats support and raise an abundance of invertebrates of a limited range of species. 

Waders find their prey by sight and touch and at times by a combination of both these and by hearing as well. How they feed depends on their prey and where they feed. 

Sight feeding waders are characterized by their short bills and large eyes for acute sight. Sight feeders generally feed with ample space between one another. Being too close they would dash for the same prey wasting precious energy and time. Distraction of other birds will make each one a less efficient hunter while the chance of the prey to escape is also high. Plovers and Lapwings are sight feeders and use same ‘run & pause’ technique. Their long legs aid them to dash forward, pause and bend their heads rapidly to catch sight or sound of the prey. Plovers and Lapwings also use the ‘foot – trembling’; tapping its foot rapidly on the ground to disturb the prey and trick it to move disclosing its whereabouts. Plovers, Lapwings, Stints, Thick- knees all fall under the category of sight feeders. They are most active during the day time and the food is smaller prey which they have to consume in large quantities. 

The Redshank is a sight-feeder by day on mudflats feeding on shrimp which they find by sight. The flocks of day feeding Redshanks are much looser than when at night feeding on snails by touch, which brings them nearer to work in a close flock. 

Sandpipers are rapid feeders, also locating their prey by sight. Turnstones use their short bills to hammer open barnacles. 

Several species of waders probe beneath the surface to locate prey by touch. However none seem to feed exclusively by touch only. Some of them also pick food from the surface through sight, while probing is not entirely unintentional and sight is used to identify where to probe. Snipe with their long sensitive bills are highly efficient probers and are mainly active feeders in the night. 

The Curlew is a skillful prober fully inserting their bill in the mud but will also take the small crabs on the surface by pecking and grabbing them from beneath the surface of water by sight. When probing for larger prey beneath the surface to be able to grip the prey at the tip of the bill most probers can flex the tip of their upper mandible. Thus a wader can open the tip of the bill without opening the base of the bill. The Whimbrel probes from half depth to full bill depth and will move its head to change the angle and is able to extract and swallow its prey without completely withdrawing the bill.

Curtsy - Muthurajawela wetland report 

Structural adaptation of bills of wading birds for feeding at different depths in  a sand shoal

The heaviest bills among the waders are of the Oystercatchers. Their hefty bills are subjected to enormous wear and tear. Used in a number of ways, to prise open cockerel, knock mussels from anchorages, to stab into bivalves, smash shells etc. The shape of the bill of an Oystercatcher will give a clue to its feeding specialization. A square ended bill like that of a chisel end for those that hammer open mussels through weak points and pointed end for those that open mussels at the point where the shell closes. The horn like material covering their bills ware-off and the pointed ends would become square with usages. This wearing is such that the bills continue to grow very rapidly for the survival of the bird. This is unique among waders and it is said to grow at a rapid rate of 0.4 mm per day, which is three times faster than the growth of the human fingernail.

Even with such adaptability it is not that easy to locate prey with the changing temperatures and climates. In wintery atmospheres the ground temperatures drop and relatively the prey burrow deep to find warmth. The birds need to wait till the temperatures are right for them to surface. Likewise over here the most productive wetlands situated in the South East and North West arid zones get very warm during the day and the prey tend burrow deep to avoid the excess heat. Changing winds form ripples on the water surface barring sight and focus. Continuous deep probing for larger prey has the disadvantage of heat loss into the cold surrounding ground resulting in energy loss. This brings the bird to a resting position until the lost energy is gained through digestion. 

We bird lovers on the other hand frequent these wetlands as they are the ideal lookouts to identify and spot the rear migrant in their intricate winter plumages. Global warming and human intervention by encroaching and dumping has gradually threatened the healthy well-being of these wetlands. Deterioration of the wetlands will affect the survival of the world wader population that have evolved and migrated for thousands of years. So let’s take to our hearts the need to preserve, protect, and present them… wetlands for the future birders and waders………..

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