Charles Elton, in his book, The Pattern of Animal Communities, having described the main habitat systems, goes on to point out that the terrestrial systems can be grouped into a series of habitats corresponding to an ecological succession from bare ground up to a climax woodland. He uses four commonly recognizable formation types. Open ground includes the soil and any plants up to 15 centimetres in height. The field layer comprises meadows, heather moors, low bramble, and any stage towards the climax vegetation which is less than 2 metres high. Scrub layer grows to 5 metres high and any vegetation above that is the woodland or tree layer type. Although one is dealing here with different types of communities, the same stratification is quite often detectable in a climax or well-developed woodland, as A. J. Willis points out in his useful book, Introduction to Plant Ecology. Obviously the divisions of the strata are not precise as the birds do not keep to the special layers and it is often more valuable to study the actual perches or heights above ground at which the birds are feeding, nesting and so on.
Many people have made studies of the use that birds make of different levels of their habitat. They are often particularly interested in the ‘niche’ which the birds fill and whether there is competition, which may occur when two different species occupy the same habitat and seek similar food supplies. It is unlikely that both species will be equally competent and the less efficient will be at a disadvantage, especially when food is in short supply.
Dr Perrins, in his book, Birds, summarizes the differences in the feeding habits of some closely related members of the tit family. Dr Perrins and members of the Edward Grey Institute at Oxford found that in general terms Great Tits tended to feed low on Hazels or on the ground; Marsh Tits hunted low down on the large branches of Oak and Beech trees; Blue Tits, on the other hand, tended to feed high on Oak leaves and smaller twigs, while Coal Tits fed high up in conifers.
This type of study is worth repeating in different kinds of woods and in different parts of the country. It is hard and sometimes tedious work. However, although you may not learn anything new, you will gain an insight into the more detailed feeding arrangements of the species.
I have, personally, looked at the ways birds forage on the sandy shores and rocky coasts of the island of Alderney. I wanted to establish how the different feeding techniques and hunting methods used by such waders as the Dunlin and Ringed Plover lessened the chances of the two species competing. Some of the animal life on which these waders feed is found on the surface, whilst others are found under rocks and seaweed or below the surface of the sand. In order to exploit this The Dipper is a thrush which obtains its food from the bottoms of mountain streams. By spreading its wings it is able to use the flow of the water to overcome its natural buoyancy.
Range of food the waders have different shapes and lengths of bill as well as different methods of feeding. Some waders stab at the surface of the mud, others like the Curlews and godwits wander about probing deeply. Plovers usually run for 50 to 100 centimetres and then search for food. Turnstones, as their name implies, turn over stones and seaweeds in their search for sandhoppers. Dunlins, in wet sand or mud, probe deeply but on hard and dry sand they can only peck at the surface.
Some extensive studies of all the factors which influence a bird in .its choice of habitat have been made, chiefly by American ornithologists, but even so the distribution of a species through its habitat is still a relatively unexplored field in avian ecology. If you visit, for example, a number of different types of oakwoods with different types of coppicing and count the birds on each visit you may eventually deduce that a certain species shows a preference for a wood with certain characteristics. But you are unlikely to understand the reasons unless you study the bird itself and discover what use it is making of each part of the habitat.
As an example, let us look at the use that the Wheatear makes of its habitat. As soon as the Wheatear arrives from the south it must feed, so that immediately it will discover whether food, which is neither too large nor too hard to eat, is available. The Wheatear hunts by hopping for a metre or two, or making a type of ‘running hop’ like so many of the chats and thrushes.
This gait requires that the ground must be fairly bare of vegetation or heavily grazed, otherwise the longer grasses and other plants would obstruct the bird’s passage. Indeed, when the Wheatear does occasionally hunt amongst the longer grasses it often hovers like a Whinchat or Stonechat, before it drops in to the grass after its prey. Species which feed in the longer grasses, such as the Skylark and Meadow Pipit, tend to walk. Furthermore, they have a long hind-claw which can presumably act like a snowshoe, trapping a mass of grass-blades beneath it.
Almost every part of the habitat can be used for nest-sites. No birds nest underwater, unless you claim that the Dipper qualifies as it can use a ledge behind a waterfall. Several species do build floating nests, such as the Black Tern and Great Crested Grebe. Waders with precocial young nest in damp wetland habitats on which the young can forage for food within a day or two of hatching. On drier soil in suitable coastal areas hole-nesting birds such as the Puffin will be found. There is a similar variety of nesting preferences in woodlands: Chiffchaffs and Willow Warblers build on or close to the ground, while Garden Warblers and Blackcaps nest in the field layer. Linnets choose the thick cover of bushes and hedges. Higher up, Carrion Crows nest among the thicker branches, while Goldfinches and Redpolls squeeze their nests into the ends of branches. The holes and crevices are used by Robins, tits and woodpeckers. One of the most comprehensive surveys of the nest sites of British birds is to be found in a Field Guide to Birds’ Nests by Bruce Campbell and James Ferguson-Lees.
While most species build their nests in cover, there are others which appear to prefer to see over long distances such as the Stone-curlew, and moorland species such as the Golden Plover. They nest in fairly open positions but their plumage disguises them or their eggs are cryptically coloured. Most birds of prey nest where they have good views over the surrounding countryside. It is interesting to ponder how small a field a Lapwing will nest in or how close to a hedge. Some species, particularly the small woodland birds, tend to use cover to hide from enemies. Cover can also be useful to the hunter. The Sparrowhawk’s method of catching his prey, springs to mind, as it flies low over or round bushes, thus surprising his victim.
Almost whenever there is a bird you can study its living conditions. One of the great advantages of the study of birds in relation to their environment is that you have to be aware of the other animals and plants that make up the ecosystem, thus widening your interest into the functions and structures of nature.