SPECIAL FEATURES OF BIRDS (6): Food and foraging
Birds occupy both a vertical plane and a horizontal plane above ground level. Not all birds forage for food at the same level above ground. Shorebirds, waders, ducks and other waterbirds may feed well below the surface of the water or even at the bottom of a shallow lake, swamp, lagoon or river. Other species, such as kingfishers, skim just below the surface of the water, and swifts and martins skim just above its surface. Some species feed well above ground or sea level. In tropical rainforests we have at least a three-tiered, if not a four-tiered, vertical environment which, at each level, extends horizontally as well. First, there is the upper canopy, occurring only in ancient and intact rainforests, made up of very tall trees that may be hundreds of years old. In this layer we often find the really large birds such as toucans or hornbills. At the top of the canopy, they have access to the airspace for taking off and landing and they are able to locate large branches that provide easily accessible roosting and landing spots.
Fruit-eaters living in the upper canopy, like hornbills, have the task of dispersing seeds; they enable seeds in the very top of the canopy to be carried some way off before being dropped for germination. The next layer of the forest, the main canopy, is often the top layer in regenerated forest. It houses a variety of birds that require shelter and also feed on fruit. Examples include fruit doves and the topknot pigeon. The next layer down is a sub-canopy, housing many smaller songbirds and sunbirds. Finally, there is the understorey. This is subdivided into a region used by the birds that occupy a range above ground (up to 10 metres) and an area for the birds that live and forage on the forest floor. Many of these species living at different levels of the forest never meet. Their particular niche in the forest may remain unique to them and provide all the resources they require. H.L. Bell showed some years ago, in the lowland rainforest of New Guinea, that even very similar sized birds of similar weight range (8–35 grams) and with a varied diet of insects can be ecologically segregated in the rainforest. Such birds include fantails and gerygones. Some fantails are found on the ground floor, grey-breasted rufus fantails in the understorey, the yellow-bellied gerygone in the sub-canopy and another gerygone (the fairy gerygone) in the main or even the upper canopy.
Sharing out the resources among the species is an important aspect of survival. Some of this partitioning is the result of adaptation that has occurred over evolutionary time. In other cases there is evidence of competition. If a species dares to leave its layer of the forest, it may be attacked and shown its place by others. Rainforest birds have a well developed community structure that helps to minimise competition and enhance full use of resources. Even mangrove forests, which do not reach great heights, are ‘zoned’ for different species according to foraging requirements.
Where birds forage may also be determined by sex. It has been known for some time that bark-foraging species such as woodpeckers, treecreepers and sittellas divide up their feeding range according to gender. R.A. Noske showed, for instance, that sittella males (which have larger beaks) feed much more often on trunks and main branches of trees than do females. Within a species, and in individual birds, there may also be a variety of foraging strategies according to their specific needs in the life cycle or in a certain season, which lead to the bird being in different locations and
using different methods of food acquisition. The north-western crow on Diana Island in British Columbia, Canada, for example, exhibits two types of foraging behaviour depending on the availability of food and the amount of food required. Only during nesting do parent crows forage at low tide as the water recedes. Apparently, it is easier to find food at low tide and so less energy is expended on basic requirements and the surplus energy can be spent on obtaining extra food for their young.
Foraging space is not only divided up according to vertical, horizontal and other locational cues but also according to time of day. There are exclusive daytime feeders (diurnal species), there are dawn and dusk feeders (twilight or crepuscular species) and there are those that are classified as night-time feeders (nocturnal species). So airspace and ground space are utilised over a 24-hour period and some birds get exclusive use of some hours over others. The species that occupy broad daylight live in an entirely different world from those that wake and feed at night. As Graham Martin has shown so well, the number of species capable of switching between day and night is relatively small as each requires specific adaptations of sight, hearing and other senses to be able to use the different conditions of illumination effectively. Birds have developed very sophisticated strategies to make sure they get the food they need. They must know where food can be found as well as the appropriate techniques for extracting it. Being able to occupy a niche in nature, and finding a niche that is not wanted by every other bird as well has resulted in very specific requirements for breeding, feeding and even roosting. As later chapters show, the social organisation of birds, the types and location of nests and different ways of rearing their young ensure a diversity of habits that is sufficient to allow the coexistence of many species. Nests may occupy the ground, the scrub, branches on trees, holes in trees, edges of cliffs, sand dunes, or the rocky shores of many coastlines. In an average inland property in Australia there may be as many as 50 to 80 bird species occupying the same area. In national parks, the species occupation rate may increase to 120 or even more.
While birds occupy large parts of the globe, many species have become so specialised that any reduction in their essential resources can immediately threaten their survival. Birds of the sea (pelagic birds) and those of the shore require undisturbed beaches for nesting but, with increasing beach cultures around the world, they find their terrain being contested by humans. Wetland birds require the continuous existence of shallow waters, but many wetlands are at risk because the water is being diverted to irrigate farms and support industry. Most bird species need shelter and roosting sites, whether in grasslands, open woodlands, rainforests or near the sea. As a result of the destruction of their habitat much of the diversity of bird species has become confined to small areas of the world, the areas with the richest variety of species occurring mainly in tropical regions. Here birds coexist with many other animals and plants because, as yet, resources for feeding and shelter are not in short supply. (To be continue)
A Part of Introduction The Amazing Book of Gisela Kaplan & Lesley J. Rogers “BIRDS, THEIR HABITS AND SKILLS.
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