Plant Eaters

Plant Eaters can also be called Grazers and Foragers. I use the term Grazers for birds which eat plant materials in various forms either on land or in the water. Birds which have evolved to take other foods besides plant materials I call Foragers.

Plant materials contain cellulose which is highly desirable as a foodstuff because it is a pure carbohydrate made up of molecules of sugar joined together. Cellulose can provide energy when it is digested but the process requires special enzymes and is rather slow. For birds to provide enough energy to support their high metabolic rate they need a gut capable of holding quite a lot of cellulose. It appears that the Grouse is probably the smallest bird which has a large enough gut to live entirely on plant leaves and shoots. For an Ostrich, with a large body and a low metabolic rate, life as a vegetarian is easy.

Other plant eaters which live mainly on grass and water plants are the Waterfowl like Swans, Geese and Ducks. All are large bodied birds.

Terrestrial Habitats.

I have split the plant eaters into those found in terrestrial habitats and those which have evolved into aquatic habitats:-

Open grassland and scrubland

Ostriches prefer large open spaces where their good vision enables them to spot potential predators and run if necessary. They appear to be eating grass but are actually being quite selective taking nutritious shoots and roots. Emus can be considered to be counterparts of the Ostrich.

Bustards are mainly birds of Africa which have established niches in sub-habitats, which can be further divided according to the degree to which trees and bushes are present. Some like a wet habitat and others prefer it to be arid. Altitude may also play a part. Small Bustards are often called Korhaans.

Thick-knees, also known as Stone-curlews, rely on their plumage to provide camouflage and prefer to freeze or run fast rather than fly. They are nocturnal foragers and although they will eat plant materials and tubers, most species have become omnivorous. The Bush Stone-Curlew could even be classed as a predatory carnivore. Field guides often call the Thick-knees Shore Birds or Waders but I hardly ever see them in that habitat.

Their behaviour and the habitat in which I have seen them leads me to place them with the Plant Eaters. They tend to favour lightly bushed grassland and even semi-arid areas. In Norfolk, Breckland and Weeting Heath, areas of finely cropped grassland on sand and chalk, are favourite habitats for the Eurasian Stone-Curlew.

Open areas of grassland / wetland

Cranes are described as both generalists and opportunists feeding on a wide variety of plant and animal foods. They walk calmly and sedately as they forage for insects, plant materials, seeds, berries and small animals. They fly well so don’t need to run to escape predators. Field guides often refer to them as Waders but I rarely see them in water. They exhibit characteristics which have enabled them to evolve to occupy specific niches within sub-habitats as outlined below:-

They appear to require a large foraging area for survival. In relation to breeding I have seen it stated that nest sites need to be 3000 metres apart and that fragmentation of breeding areas, for instance by agriculture, poses problems for the survival of their young.

Birds with shorter beaks feed in dry upland areas taking insects, seeds, leaves, berries, fruits, worms, lizards, reptiles, and small mammals. Those with longer beaks favour wetlands where their food includes roots, bulbs and tubers as well as crustaceans, small fish and frogs.

They are able to shift their feeding strategies with the seasons and even on a daily basis if necessary. I am sure this is why the 15 species which we know today have found suitable habitats in four of the five main regions of the world. They are absent from South America.

Limpkin is found in the Americas, West Indies and Argentina. It looks like a Rail but skeletally is more like a Crane with which it shares the need for quite a large territory, in this case several hectares. It feeds on frogs, lizards and especially apple snails the availability of which is an important factor influencing the local distribution of this bird.

Storks are carnivorous, eating fish, amphibians, reptiles, rodents and small mammals. Their ability to eat almost anything has led to 19 species establishing niches in most regions of the world. Their distribution favours Africa and Asia with just 1 species in South America.

White Storks favour open meadows, wetlands and swamps where they eat crustaceans, frogs, worms and insects. Black Storks favour wooded country and even ravines where they eat small mammals, lizards and snakes. Marabou Storks are bare headed and appear to be closely related to the Vultures in that they both feed on carrion. Two species found in Africa and Asia eat water snails. Jabiru of Central and South America is a massive Stork found in freshwater areas. It has a varied diet including molluscs, dead fish and carrion.

Ibis are medium to large waders found on all continents except Antarctica. Their distribution favours Africa and the Americas with just 1 species in Europe. Like the Storks they seem to prosper in almost any habitat. They feed both on land where they take insects, frogs and snakes and in the water where they sweep their beaks from side to side searching for crabs and crayfish. They have long, thin down-curved beaks which they use to probe the mud for invertebrates and vegetation. Again the 27 species known today have evolved into some very different habitats.

As with the Storks a genetic variation has led to un-feathered heads being beneficial in avoiding muddy head plumage in a species rather obviously named the Bald Ibis. The Northern Bald Ibis can still be found in Morocco and the Southern Bald Ibis is present in the Drakensberg Mountains of Lesotho. Both nest and breed on cliff ledges and eat lizards, reptiles, small animals and birds.

Low ground cover

Some of these plant eaters are reluctant fliers and favour areas of long grass and shrubs in which they can hide to avoid predators:-

Turkey, Grouse, Partridge, Francolin, OW. Quail, Pheasants are found in Eurasia and Africa. New World Quail scratch and dig to find plant roots and tubers. They have found niches in North, Central and South America.

Wooded & forested areas

Various plant eating birds favour wooded and forested areas which provide protection against predators:-

  • Tinamous have evolved to fill a very wide range of sub-habitats in South America. They have established a variety of niches ranging from open grassland to dense forest and at elevations ranging from 500 to 4000 metres.
  • African Guineafowl favour woodland and forest which provides a source of food as well as protection from predators.
  • Megapodes such as Scrubfowl and Malleefowl are primitive mound nesting birds of Australia which can be readily seen in open grassland but are never far from woodland or forest cover.
  • Cracids such as Chachalcas, Curassows and Guans quite primitive birds which are found in the Americas.

Aquatic Habitats.

Collectively birds which I call Waterfowl are found on inland water bodies. Screamer and Magpie Goose illustrate the rather primitive appearance of the earliest of our modern birds. They are only found in South America and Australia respectively. Whistling Ducks have become established in the tropical parts of Australasia, Africa and the Americas where they favour marshy and swampy inland water bodies. Shelducks are equally at home feeding on land or in the water and appear to be intermediate between Geese and Ducks. They have established geographic niches in Eurasia, Australia, New Zealand and Africa. Steamer Ducks are only found in South American coastal waters. They appear to be related to the Shelducks.

Under-water feeders - Swans.

Grazers and grassroot feeders - Geese, Shelduck.

Divers and dabblers - Ducks of various common names which have evolved in a variety of niches. Mergansers and Smew, which are perhaps suggestive of the Grebes, are Ducks which pursue and catch fish. Although the Ducks in particular have been very successful these are the only species which show signs of evolution outside the typical body form of the Duck.

Dive for plant materials - aggressive - Coots favour open ponds where they dive to take water plants from below the surface just like some of the Ducks. They also take small aquatic animals and fish. These birds have established niches in Eurasia, Africa, North and Central America and particularly South America.

Marshland and Reedbeds- Swamphens, Gallinules and Moorhens like the cover provided by reed-beds. They feed on tender reed shoots. These birds are clumsy fliers but have considerable stamina and can be found in all the main regions of the world.

Trees in swamps - Hoatzin eats leaves from the trees.