Photo: Gerhard Gellinger. Paul discusses how birds have evolved to build nests

 

In the first of three articles on nesting, Paul Donovan investigates how the technique of incubation has evolved

 

SOMETHING we take for granted in bird behaviour is nest building. But why and when did birds evolve the need to build nests, and why do not all bird species build a nest?

It is likely that the early birds laid their eggs in decaying leaf-litter or simply on the ground in a sunny area, as the heat would help to incubate them. The latter, of course, would predispose them to predators. There is also some evidence to suggest that the evolution of nest-building ties in with the birds’ need to have an environment that stabilises the heat generated to incubate the eggs. During the night time, when the ambient temperature falls, the birds may have had difficulty in generating sufficient heat to maintain the eggs at a constant temperature. The bird may be induced into a state of torpidity, which gives credence to the evolution of reliance upon partly, or wholly, some heat source other than that generated by the parent bird’s body.

Open nests

During the Mesozoic era, a shift in climate took place from a humid tropical/subtropical environment to one of drier extremes. Birds adapted to this by evolving several solutions to the problem. First, some may have placed more reliance upon direct body heat to incubate their eggs. Second, others evolved various means of creating an environment in which to incubate their eggs, such as the construction of egg-laying mounds as some reptiles do (considering there is a shared ancestry), excavating shallow pits that the eggs were laid in and then sat on, or lining the nest with an insulating material. Almost certainly, the precursors to modern-day birds had species that sat on and incubated their eggs. Such was the efficiency of this method of incubation that it was adopted by almost all bird species.

The evolution of open nest incubation (sitting on the eggs) probably arose from early birds laying their eggs on the ground and then covering them to conceal them from predators. Over time, they would conceal them less, preferring to sit on them. Of course, the parent bird could not sit on the eggs all the time through the course of the incubation period, as there would be the need for them to eat and drink. This then saw a shift in egg coloration, where the shell took on a hue similar to the surface on which it was laid; a form of camouflage. This meant that the parent bird could leave the eggs for a period of time, limiting its exposure to predators.

As early birds began to maintain a higher body temperature (particularly during the night) that was independent of environmental temperature, there was a big shift towards parental egg incubation temperature. As the parent bird could now remain with the eggs and defend them against predators, this meant that predation would diminish.

Furthermore, as birds spread their wings, so to speak, and migrated to cooler climates, the ability to incubate the eggs due to an enhanced internal body temperature saw them quickly colonise even the coldest of regions.

Cavity nests

Nesting in a cavity, such as that found in a tree, appears to have moved forward repeatedly in birds at every stage of their evolution and is seen in species from at least half of all the known recognised bird families. Typical examples are parrots, hornbills and woodpeckers.

Nesting in a cavity has a number of advantages; namely, a much more stable environment from fluctuating temperatures, warmth and protection from predators. In species such as the hornbill, the male will seal the female in the cavity and feed her through a small hole. This means she can devote 100 per cent of her time and energy to incubating the eggs and tending the chicks. Only when the chicks are ready to fledge, will the male release them.

There can be great competition among individuals, and even species, for prime nest cavities. This competition can get to the point where some smaller birds will even partially seal up a large cavity hole with mud to prevent larger cavity-preferring birds from commandeering it.

Cavity nesting can be seen in various guises. There are those birds that make use of pre-existing cavities, such as modifying a cavity, excavating one in a decaying tree or chiselling them out of hard living trees. Of course, cavity nesting is not necessarily restricted to tree cavities. Many birds will nest in holes in the ground or along the banks of rivers. As with tree cavities, such burrows offer the same survival features. The Southern African little bee-eater (Merops pusillus), for example, will excavate a burrow in a disused aardvark burrow or earth banks.

How these evolved probably came from birds that initially formed shallow scrapes in which they would lay their eggs. This then progressed to digging a short burrow and eventually a longer burrow. Some deep-digging birds can excavate a burrow reaching up to 2m (6ft 6in) in depth. These burrows are not excavated on a level plane, but at a slight upwards angle, which prevents water ingress during the rains.

The evolution of nesting has come a long way and is one of the reasons why birds have become so successful. The adaptation to nesting in a cavity/burrow ensures several great survival perquisites, namely safety, and a stable warm environment.

  • In his next article on July 18, Paul will look at the evolution of nests in trees.

Paul Donovan is a biologist with 30 years’ experience working in zoological collections where he looked after birds and reptiles.

 

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