Birding: Bird Nests and Nest Building
Nests and nest building
A nest may just be a cup-shaped depression scraped in the soil or a highly intricate structure woven together with a variety of different materials. An all-round naturalist that I know sometimes takes old birds’ nests from a wood he knows well and identifies the plants and other materials of which the nests are composed. He can then trace how far the birds have travelled to collect the material for the nest.
Those birds which nest on rocky surfaces or in burrows use hardly any nesting material. Some ground-nesting birds may, by swivelling themselves around on their breast, make a scrape in the ground, to which they bring material. Some waders, such as Lapwings and Avocets, if they are close to their nest scrapes may pick up pieces and lay or toss them on the nest. Avocets seem to be able to build quite substantial nests in this way. Some material probably helps to camouflage the nest and eggs, such as when Stone-curlews and Oystercatchers line their nests with a few chips of rock or rabbit pellets.
Most song birds make use of what material there is to hand. A Wheatear I observed on Skokholm, began by taking in dried bracken or heather stems. The cavities of the rabbit nurseries which they were using were rather too large to hold the nest steady. The stems of these plants were the foundation into which they built a loose cradle of grasses which held the tighter-textured nest cup, also made with grasses. Finally the nest was lined with Rabbit and Soay Sheep hairs and the occasional Manx Shearwater feather.
My observations of Goldfinches have shown that as well as using grasses they also use Forget-me-nots and Sweet Alyssum, which they placed corolla outwards on the nest. This had the effect of camouflaging the nest amongst the leaves of fruit trees, limes and cherries. The significance of flower gathering is not yet fully understood. Goldfinches are also one of the birds that makes use of spiders’ webs to bind the first pieces of nest material to the twigs of the tree.
Mud is a well-used building material. Blackbirds and Song Thrushes line their nests with it, while Nuthatches plaster the entrance of their nest holes with it to reduce the size. The Swallow and House Martin are well known for using mud pellets to make up the cup of their nests which are often found under the eaves of houses.
The actual construction of the nest is an instinctive action; birds of the same species build nests that conform closely to a given pattern without any instruction. However, there is some evidence that birds build better nests as they grow older and benefit from experience. The movements which they use to build the nest, therefore, tend to be rather stereotyped. There appears to be three movements which are common to most passerines. Professor R. A. Hinde calls them ‘pulling and weaving’, ‘scrabbling’ and ‘turning’. In ‘pulling and weaving’, loose strands of nest material are pulled towards the breast of the bird, which is sitting in the cup of the nest, and then pushed down into the cup. The female ‘scrabbles’ by pressing down into the cup and pushing back hard with each leg alternately. In ‘turning’ the female turns round while sitting in the cup and thus shapes the cup of the nest. The female normally builds the nest, sometimes helped by the male. In a few species the male does the major part of the building assisted by the female. The notable exceptions, perhaps, are the unlined nests built by male Wrens and some other species which are used by the males for roosting. The time taken to build a nest may range from virtually a day or so to perhaps several months, or even years, in the case of a bird of prey which may add an odd stick to an already existing nest throughout the breeding season.
Small passerines, which winter in Britain, may take as much as a month if bad weather intervenes and stops building. Summer visitors to this country, because they arrive later, build their nests rather more rapidly. Between two and three weeks elapsed between the first time that I saw a pair of Wheatears carrying nest material and the laying of the first egg. Generally speaking, the further north the species is nesting, the quicker the nest is built because the breeding season is shorter. In cases of necessity, when for instance a nest or clutch of eggs has been destroyed, repeat nesting and even second clutch nests can be built in a day or so.
In Wheatears, I found the first eggs of repeat clutches – that is clutches replacing those destroyed – two days to four days after the first clutch had been destroyed.