How Birds Nest

In evolutionary terms, bird nests resemble human fabrications.

Posted May 01, 2019

It is spring in the northern hemisphere and birds initiate the remarkable architectural feat of building a home for the young. How is this accomplished?

Hormones and Getting in the Mood

Much of the dynamics of reproductive behavior of birds was revealed by research on pigeons (ring doves). The male of a pair spends several days in the elaborate courtship of the female. He struts about bowing in front of her while cooing. The function of male courtship is to prepare the female for nesting by altering her hormonal state. A female that is not vigorously courted will not move on to subsequent stages of reproduction, selection of a nest site, nest building, copulation, laying eggs, and incubation (1).

Once a nest site is selected, the male bird occupies it and emits a different call, known as a nest coo. When the female begins emitting her own nest coo, indicating that she is ready to build a nest, the male begins collecting nest material and they construct a nest. Once the nest is completed, the female is ready to begin laying eggs.

The presence of eggs in the nest brings on hatching behavior. Once the chicks arrive, the parent birds are very strongly motivated to feed them, responding to the gaping display inside the beak of the youngsters that inform parents of their need for food.

The entire sequence of reproduction involves a complex interaction of environmental cues with internal hormonal states. These proceed in a predictable sequence. If the female is not courted by the male, she does not reproduce because her hormonal; condition is not correct for nesting. If she does not build a nest, she will not copulate or lay eggs.

The Complexity of Avian Reproduction

All ring doves go through pretty much the same sequence of activities in raising their young. While incautious people, assumed that these activities were somehow built into the birds as “instincts”, the truth is a great deal more complex with profound implications for how species-typical behavior is transmitted across generations.

Nesting and reproducing are orchestrated by a complex interplay of internal biology and environmental cues. Brooding birds develop an inflamed area on their underside known as a brood patch, for instance. This is soothed by sitting on cooler eggs. As the eggs warm up, their soothing property declines so that the parent bird repeatedly turns the eggs to bring their cooler surface to the top. This is essential because eggs that were not turned would yield chicks having serious joint problems.

All of this illustrates the complex interdependence of internal hormonal states, environmental cues, and social influences in a pattern of reproductive activities that used to be thrown into the bin of “instinct” as though they were inherited in whole cloth. That is clearly not true.

Nest construction cannot be genetically transmitted across generations. Why do members of a species build nests that are so uniform?

One topic that has received a lot of attention is the way that birds master skill in building very elaborate nests, such as those of weaver birds.

The Weaver Bird's Nest

Bird nests vary greatly by design and complexity. Eagles construct simple nests that are made by dropping large numbers of sticks on top of each other at random. The resulting platform is surprisingly stable and quite good at protecting the eggs from ground-based predators.

Wrens make much more elaborate nests that are spherical with an entrance opening at the base. These are carefully woven together with small twigs and carefully lined with a variety of soft materials like moss and sheep's wool to keep chicks warm.

Weaver birds nest in colonies and their nests are even more elaborate than those of the wren because they have a central suspension system attached to the top of the nest instead of being lodged securely in the fork of tree branches. This design permits large numbers of weavers to nest together and use group defense against predators. It is also difficult for predators to penetrate individual nests as they sway like a pendulum.

How does the individual bird arrive at such a complex architectural feat? Apparently, they do so through practice. A young male builds faulty nests that are rejected by females and quickly abandoned. With continued practice, he eventually produces a nest that is usable and goes on to reproduce.

For weaver birds, nesting may have species-typical characteristics (despite different materials being used in different locations) but it requires not just considerable experience and skill development but is also a product of social learning

They imitate the nests in which they were raised and the final arbiter is female approval based presumably upon what the female has learned about acceptable nest construction based on what she knows about the design of other nests in the community. How birds build their nests is an entertaining excursion into animal behavior but does it have any significance for humans?

What Birds Reveal about Human Behavior

So far, this post was concerned mainly with how the different elements of bird behavior get organized into the sequence of successful reproduction in nests.

We know very little about how fine-grained elements are passed on. How does a bird know how to grasp a twig correctly and tuck each end into the nest being constructed? It seems unlikely that this behavior has any simple genetic explanation specific to nest building. Grasping the nesting material may use movements already mastered for foraging, such as pulling a worm out of the earth. Weaving actions are less easily explained in those terms.

Whatever about the finer components, it is clear that nest building is not some genetically-programmed “instinct” that is built in at conception.

An evolutionary analysis is relevant to items that are fabricated even if they have no obvious genetic correlates (2). Useful objects, like nests, are taken along for the ride with successful populations. Ancestral Inuit who began making clothes from the skins of caribou were likely much more successful than those who relied on other materials. In doing so, they were co-opting a biological adaptation by the caribou to cold conditions. How is that different from a weaver bird co-opting the adaptive structural properties of blades of grass to weave a nest?

References

1 Barber, N. (2002). The science of romance. Buffalo, NY: Prometheus.

2 Mesoudi, A. (2011). Cultural evolution: How Darwinian theory can explain human culture and synthesize the social sciences. Chicago: University of Chicago Press.

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