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Unravelling an extended phenotype: sexual selection and the evolution of nest architecture in weaverbird defence against brood parasitism

Periodic Reporting for period 1 - WEAVERBIRD_DEFENCE (Unravelling an extended phenotype: sexual selection and the evolution of nest architecture in weaverbird defence against brood parasitism)

Reporting period: 2019-07-08 to 2021-07-07

Why is nature so diverse? In the natural world, the same problems recur but nature’s solutions manifest in strikingly different forms. As evolutionary biologists, we are interested in understanding the processes and mechanisms that lead to this variation. And increasingly, as scientists and as a society, we are turning to nature for inspiration to solve human problems and to counteract the consequences of human activity.

One dominant force in nature is coevolution – reciprocal selection pressures between closely associated species – which often leads to escalating arms races of adaptations and counteradaptations. Cuckoos and their hosts provide a textbook example of coevolution, by reciprocally selecting for better parasitic tricks (to fool hosts into rearing cuckoo offspring) or better host defences (to avoid rearing a cuckoo chick). However, we know much less about how these arms races between species interact with the social selection pressures within species.

The weavers (Ploceidae) provide a unique opportunity to investigate such questions. These small birds produce conspicuous, enclosed nests that vary both across and within species. The males produce the nests and the females choose their preferred nests in which to lay eggs. Weavers are also host species of the brood parasitic Diederik cuckoo (Chrysococcyx caprius) and the architectural design of weaver nests is thought to be an important barrier in keeping the cuckoo out of the nest, but we have a surprisingly limited understanding of this fascinating evolutionary interaction.

This project has two main objectives:
(1) Uncover evolutionary mechanisms that explain weaver nest structure: (a) is weaver nest architecture a defence against brood parasitism and if so, how do nests repel cuckoos? (b) to characterise the extent to which female preference for nest characteristics interacts with nest defences against cuckoos; (c) to document and examine the effects of variation in nest defences on host egg patterning (information used by hosts when making decisions about egg rejection), since nest structure influences the light environment in which host and cuckoo eggs are observed by the host.
(2) To draw inspiration from natural structures using interdisciplinary approaches that could provide bio-inspired solutions to problems faced by humans.
The Researcher has established field sites, logistics and local networks for examining the nest architecture of weavers and their interactions with Diederik cuckoo brood parasites. Initial field data has been collected and analysed, including physical nest morphometrics, and data extracted from calibrated and reliable imaging approaches. Early results are now emerging from the project, and subsequent analyses will be carried out during the final phase of the ongoing project. These initial results are suggestive of relationships between different nest characteristics measured from an individual nest, and between nest characteristics and the social environment. These results will be examined further and confirmed with additional analyses, to complete the projects.
A major technical aim of this project was to develop non-destructive methods for monitoring active nests in the field. The benefits of these approaches are threefold: (1) generating highly quantitative data on nest characteristics that are (2) repeatable, reproducible and sustainable, (3) reduce disturbance to active nests in the wild, by avoiding the need to remove nests from their environment. Work so far has led to the generation and application of non-destructive calibrated 2D and 3D imaging approaches of nests. These techniques have been applied successfully, and are now being developed further.

The research project has had several direct and indirect societal benefits. First, the Researcher is developing new tools and undertaking novel research projects that will contribute to our scientific understanding of the natural world. Second, the project is providing opportunities for education and training via research for undergraduate and graduate students, with direct benefits for the students in terms of skill development, and for society in terms of disseminating the scientific approach through research-led teaching. Third, the Researcher has participated in and led a number of outreach initiatives for local Bird Clubs, Ornithological Societies, and community-facing groups, including a crafting outreach event making decorative bird nests in collaboration with the Women’s Institute and Cambridge Museum of Zoology.
southern masked weaver Ploceus velatus with nest
lesser masked weaver Ploceus intermedius with nest
weaver nest non-destructive imaging methodology
southern red bishop Euplectes orix nest