Behaviour of nesting birds

by K. G. H. Munidasa

Ceylon Black-headed Oriole with nestlings	Purple-rumped Sunbird (female) feeding nestlings

As the nesting season approaches a great change takes place in the mind of a bird. This shows itself in a variety of ways. The members of one species which stay together in groups or flocks throughout the year begin to break up.

The cock birds may map out territories for themselves and defend them against intrusion by other birds of the same species. This is the practice among most birds and is particularly noticeable in more aggressive types like drongos and bulbuls.

It is also during this period of greater activity that the birds are more vocal with their singing reaching a peak. Broadly speaking, in the realm of bird behaviour, the song is its mode of proclaiming possession of a particular territory. In our own language this should sound as, "Here I am, and here I mean to stay!"

Practically, all birds in their nesting season live in more or less well-defined territories, never trespassing on those of others, until the young leave the nest.

All this change of activity has one aim - that of raising a family and thereby assuring the continuation of the species, hence the focal point of the birds' family life is the nest.

No bird needs be taught how and where to build its nest. This is solely an instinct present in all birds, and each species builds a typical nest in a typical site.

The approach to nest-building raises a whole set of interesting problems. With some birds like the bulbuls, tits and parakeets, the selection of a site for the nest may take place weeks or perhaps months before building commences. With others like the paradise flycatchers, orange minivets and cuckoo shrikes, the cock may choose a suitable nest-site even before it has chosen a mate, although it subsequently takes very little interest in the actual construction of the nest.

The thrushes and robins select sites in bushes and trees and will crouch there for sometime in a bare fork or among the branches, apparently getting the 'feel' of various locations for the nest. A cock finch lark would scrape out a number of depressions on the bare ground and the hen lark would finally select one of them for the nest.

In the case of small birds like the pipits, bush larks and some prinias, which nest on or near the ground beneath tufts of grass and herbage, the hen may be seen continually flying down to a particular place and crouching there for a few moments to draw the attention of its mate to the particular site.

Share of work

Ceylon Paradise Flycatcher (male) with young	Small Scaly-bellied Green Woodpecker (male) at nest hole.

The role taken by the two sexes in constructing the nest too varies widely with different species. With some birds, both the cock and hen take a full and equal share in building. In others, the hen does the building and alone collects all the material. Yet, in some, especially among sunbirds, the hen is the sole builder, but does not collect the material for the nest, the cock being the builder's labourer. Finally, there are birds which collect the material together, but only the hen does the construction, while the cock keeps guard from a vantage point.

The material used in nest-building and the complexity of the construction may also vary.

A common material, however, is well-dried leaves, which are generally reinforced with dried sticks, grass, fibre and even pellets of mud. Cobweb is a binding agent used by practically all birds in nest-building, while the swifts use their own saliva.

Some birds build highly complex nests, while other birds build nests not worthy of the name.

As a rule, though there may be exceptions, those birds whose young are helpless and bare of feathers at birth, construct nests which are of good workmanship, while in those species whose young are well developed and capable of running about soon after hatching from the egg, rudimentary nests are seen.

Loten’s Sunbird (female) feeding its young

Some birds ensure the best concealment for their young by placing the nests in cavities on trees or tunnels in the ground or dug on earth banks.

Coloration of eggs

The coloration of eggs is also a fascinating subject to study. It is interesting to observe that the eggs of many birds which nest on the bare ground have evolved protective coloration; birds, which usually place their nests in bushes and trees lay coloured eggs, while those laid in tree-cavities or inside tunnels in the ground are always pure white or whitish.

In ground-nesting birds, the protection afforded to their eggs by the coloration effect is further advanced by the way the clutch is arranged in the nest. The eggs of the ground nesters are said to be "pyriform" in shape, that is, broad and rounded at one end and more pointed at the other. When the clutch, usually four in number, is arranged in the form of a cross with the pointed ends facing inwards to a common centre, they take up the minimum of space.

This is a remarkable provision of nature that ensures that if an egg of the clutch accidentally rolls out, it would not roll away, but pivot round with the pointed end still pointing to the centre.