PAUL DONOVAN investigates the diverse set of species that lead much of their life on the ground
ALTHOUGH the majority of bird species could be classified as being either arboreal or semi-arboreal, a good number have chosen to live life on the ground. Of these, most can take flight whenever they wish, but there are a few that have lost the ability to fly altogether.
Powerful legs and feet
Essential for adopting a terrestrial life has been the evolution of robust legs and feet for stability, walking, running, hopping and grasping. Most birds have four toes, with three facing forward, and one rearward. However, the number of toes can vary greatly depending on the species. For example, the ostrich has only two toes on each foot – the only bird so equipped. The toe muscles of ground birds are also strong to allow for gripping and manipulating food, or for perching on rocks.
Thick, sturdy bones
Unlike their arboreal cousins, ground birds that are flightless do not need a lightweight skeleton. Their bones have increased density, stronger tissue, a thicker layer of skin, and greater mass. All of these are required to support their body weight and withstand the stresses of running or walking over long distances in the search for food. Thick bones help to absorb the stress and impact of running, thus reducing the likelihood of fractures. Strong bones also offer protection to the internal organs against injury. Burrowing species also need strong legs with sturdy bones to dig out their cavities. Great force is often required to shift stones and delve into hard, impacted soil.
Camouflage and coloration
One of the challenges of life at ground-level is that you are exposed to a host of predators. Some developments of terrestrial birds to tackle this threat are effective camouflage to blend in with their surroundings; clear and comprehensible alarm calls; and the tactic of “freezing” motionless.
Burrowing behaviour
Terrestrial birds dig holes for a variety of reasons. Nesting and sheltering from the elements are obvious examples, but burrows can also be used for evading predators, rearing the young and storing food. When food is abundant, reserves can be stored for times when times are leaner. There are also adaptations in egg incubation, brooding and chick-rearing within these burrows. For example, to deter unwelcome predators from investigating their cavity, some owls, such as the burrowing owl, make a hissing sound that mimics that of a potentially dangerous snake.
Foraging aids
Specialised beaks have evolved for foraging, cracking seeds and probing into the earth. Birds such as chickens and turkeys have strong scratching legs and sharp claws to uncover food in the soil or leaf litter. Others probe and scan to pinpoint food hidden in the ground. A bird’s acute hearing can pick up the movement of insects beneath the surface. With remarkable accuracy, the beak is then pushed into the ground and the insect or earthworm pulled out.
Dust-bathing
This behaviour is one of the ways in which ground birds maintain good feather condition. Over a period, oil applied to the feathers will deteriorate and lose efficiency. Before new oil is applied, the old must be removed, and this is why a bird will bathe in dust. It shuffles itself into the soil, and kicks up a plume of dust around itself. This settles on to the bird and gradually works its way deep into the feathers, where it sticks to the oil. Once the bird has bathed, it will stand up, ruffle its plumage and shake itself clean, before preening and realigning the feathers.
Dust-bathing goes beyond simply keeping the feathers clean, for it also helps the bird to rid itself of unwanted parasites.
Strong tail feathers
Tail feathers evolved to give birds balance and support while walking or running. But a tail can also serve a means for communicating and courtship. In many terrestrial bird species, the tail may be less prominent than that seen in arboreal birds. In the ostrich it is short and stubby. Yet in other cases, such as the peacock, the “tail” – or the tract of elongated coverts above the true tail – is large, colourful and ornately patterned. This “train” may be carried in an elevated position, or drooping.
Setting boundaries
Where resources are limited, ground birds will often establish territories which they defend against others. Within these territories are their nesting sites. Although a male will tolerate many females within his territory, even when resources are scarce, and allow them to come and go as they please, it is a different story with other males. These will be chased off robustly, and clashes – violent or ritualised – often take place.
Migration and dispersal
Once food resources become scarce, many ground birds migrate or disperse to more fertile areas. This is often timed to tie in with emerging insects, or arable farming practices such as seasonal planting. Such movements may also take place to escape harsh weather, although, some species can weather out the most inclement conditions. Migrating to seasonal breeding grounds may also take place and, likewise, will coincide with the seasonal appearance of certain insect species, or plants fruiting.
Successful changes
Terrestrial birds have shown extraordinary resilience and flexibility in the face of human activities such as wholescale habitat destruction, farming or urbanisation. They have evolved specialised behaviours such as foraging, courtship displays, and predator avoidance. As a result, birds flourish at ground level in forests and grasslands, deserts and backyards. Many ground birds species exploit habitats created by humans.
At the core of this adaptability, for many species, is a robust skeleton, as we have seen. Additionally, many such species have reduced their wing size, even to the point where the wings do not support flight.
Biologist Paul Donovan, a resident of Zambia, has studied many species of birds during his work as a zookeeper, as well as in the wild.
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