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Neural basis and cognition of nest building in birds

Final Report Summary - BIRDNEST (Neural basis and cognition of nest building in birds)

How do birds know how to build a nest? Nearly all bird species build a nest in which they lay and incubate their eggs and also raise their young. Nest sites, structures and building materials vary enormously across species but we know very little as to how birds go about the physical task of building, which includes choosing and collecting nest material as well as manipulating and inserting it into the nest. Do birds have to learn how to build, and if so, what do they learn? How do they choose nest material? Which parts of the brain are involved in building, and do hormones or the behaviour of breeding partners affect how and what birds build? In this project I aimed to address these questions by combining behavioural experiments with hormone assays and analyses of neuronal activity. I used the zebra finch (Taeniopygia guttata) as my study species because these birds readily breed in captivity all year round, and they also readily build with artificial nest materials, which makes controlled laboratory experiments possible.

In the first part of the project I, together with colleagues, addressed whether zebra finch males, which are the principal builders in this species, learn about the structural properties of the materials with which they build. In an experiment where the birds built nests with different types of cotton string we found that zebra finch males learn to avoid unsuitable nest materials (based on length and/or rigidity) but their individual experience (whether they build with the best available material, or material that is too short and/or too flexible) affects which material properties they will subsequently avoid. These results show that rather than being ‘instinctive’, nest building has individually learned components, and birds can attend to specific physical properties instead of following simple decision rules (e.g. preferring familiar material if it is suitable, or switching to any other material if it is not) when choosing nest material.

We then studied whether zebra finch females affect the building of their male partners so that the behaviour of the pair appears coordinated. In monogamous birds (like the zebra finch), partners can coordinate other parts of their reproduction including incubation and feeding the young, and it has been suggested that familiar partners may be better coordinated than new pairs. Through analysing video recordings of pairs building nests in a laboratory experiment, we found that zebra finch males bring material into the nest faster when their female sits in the nest, but the individuals of a pair do not seem to coordinate other building behaviours (the amount of time spent in the nest and how much the birds arrange the material in the nest) either through sex-typical roles or more individual interactions. It seems, therefore, that zebra finch males build mainly to the beat of their own drum, although females have a way of hurrying them along.

Sex steroid hormones affect and control many breeding behaviours but whether they have a role in regulating building is not known. As a first step to find out, we set out to measure whether concentrations of circulating sex steroids (plasma testosterone in males and estradiol in females) change when zebra finches start and stop building. Preliminary analysis indicates that they do not. Therefore, at the moment it seems that sex steroids do not regulate building even though they play roles in other reproductive behaviours such as courtship and aggression. However, these data have to be analysed in more detail.

The cerebellum is a brain structure involved in motor control, and there is evidence that the cerebellum is bigger or its surface is more folded (indicating increased volume and ‘processing power’) in birds that build more complicated structures (bowers or nests). To establish whether the cerebellum has a direct function in building, we conducted two experiments in which zebra finch males had an opportunity to build in a laboratory. In the first experiment we found that at least three folia (anatomical folds that also have functional differences) of the birds’ cerebellum are involved in nest building: neural activation in these areas (measured as immediate early gene expression) increases when male zebra finches deposit and tuck material into the nest more. We are currently studying whether this activation is specific to building, or whether it is similar when the males can only manipulate nest material without an opportunity to build.

The results from this project are mainly aimed at the scientific community, and will be published in journals specialising in animal behaviour, neuroethology and/or behavioural endocrinology. They are of wide interest because nest building is common and crucial for the reproduction of many birds, but it is also seemingly sophisticated behaviour comparable to tool use because it includes selecting, shaping, and manipulating material for a specific purpose. In this project I have accumulated data on the actual building behaviour of birds (rather than simply measurements of nest morphology), hormone profiles and brain activity, all of which are difficult to collect in the wild or from seasonally breeding birds. In this project we have used multiple approaches, such as manipulating the experience of individual builders to study learning, and mapping the hormonal and neural underpinnings of building. Combined with other work carried out by our research team e.g. on the role of social learning and early life experience on nest building, we are making inroads into understanding how birds ‘know’ how to build a nest.