UC Berkeley

The maxim “form ever follows function” reflects a philosophy of design in which the shape and composition of an object should primarily express its intended purpose. While this concept is most often associated with architects of the late nineteenth and early twentieth century, such as Frank Lloyd Wright and Louis Sullivan, one only needs to step outside and observe the natural world to see this principle reflected in all its living inhabitants. Whether it be the aerodynamic shape of a bird’s wing, the bright, warning coloration of a toxic newt, or the interlinking threads of a spider’s web, functional design is everywhere in nature. The nests constructed by the over 10,000 species of bird are perhaps the best demonstration of this phenomenon. The basic purpose of a nest is to act as a receptacle for the eggs and developing young; however, careful examination of a nest can reveal information about a species' life history, physiology, and ecology. Moreover, a nest provides a permanent, physical record of a bird’s behavior which can, in turn, be studied within the context of its evolutionary history.

The nests of weavers (family Ploceidae) are some of the most distinctive and complex among birds. Each of the more than 100 species of weaver constructs an enclosed nest, some of which are assembled using a series of intricate knots, resulting in especially strong and secure structures, such as those built by many Ploceus and Malimbus species. While some features of their nests are shared among weaver species, their nests can also vary tremendously in size, shape, material composition, and physical location in the environment. This variation in nest design is likely a reflection of ecological, behavioral, and morphological diversity exhibited by the family, whose members have diversified into every imaginable habitat type across the continent of Africa and, to a more limited extent, to parts of the Asian tropics and some islands in the Indian Ocean.

In this dissertation, I investigate evolutionary patterns of nest design across the Ploceidae and place my results in a phylogenetic framework in order to further explore phenotypic diversification and speciation within the Ploceidae more broadly. My doctoral research uses an integrative approach to study evolutionary processes by combining character data from nests and study skins housed in museum collections with molecular data from DNA sequences generated in the lab. To address my broader research objectives, I divide my dissertation into the following three chapters: (1) Evolutionary patterns of nest design in the weavers (Ploceidae); (2) Nature’s basket weavers: the evolution of bill shape and nest design in weaver birds; and (3) Phylogenomic analysis of the Ploceidae: unraveling the weaver phylogeny using ultraconserved elements and whole mitochondrial genomes.

In Chapter 1, I conducted a species-level phylogenetic study of weaver nests to elucidate patterns of nest character trait evolution across the Ploceidae. In addition, I sought to deduce the ancestral nest form of the family and, in doing so, explore the hypothesis that nests tend to increase in complexity through evolutionary time. I collected several discrete and continuous measures of nest design for each species, principally from museum specimens and supplemented with information gathered from online and primary literature sources. I then carried out phylogenetically-controlled analyses using a multi-locus phylogeny of the Ploceidae reconstructed using sequence data for five nuclear and five mitochondrial genes available on the National Institute of Health’s GenBank database, and I combined the sequence data with additional mitochondrial sequences I generated. These analyses indicate that the ancestral weaver nest was most likely a simple, globular structure that lacked an entrance tube and that was either bottom- or side-attached to its support structures, a finding consistent with the sister relationship between weavers and the waxbill finches of the family Estrildidae, whose members largely construct enclosed, bottom-attached globular nests. Furthermore, the general shape of weaver nests is largely conserved within each of the family’s major clades; however, other features of nest design, including nest size and attachment type, can vary considerably between even closely-related species. In addition, apparent gains and losses of complex structures, such as the short spouts and elongated tubes characteristic of derived nests, demonstrate a more varied and complex evolutionary history of nest design in the weavers.

Chapter 2 focuses on the relationship between bill morphology and nest design in weavers. Specifically, I use a geometric morphometrics approach to characterize bill shape and size, and investigate whether the bills of weavers and the design of their nests reflect patterns of co-evolution while considering the alternative hypothesis that variation in bill morphology is chiefly determined by differences in diet among species. Two-dimensional landmark data were collected from photographs of museum study skins and combined with the nest character data and phylogeny of the Ploceidae used in Chapter 1. In addition, information on weaver diets made publicly available by Song et al. (2022) was downloaded from the Dryad Data Repository. The results of this dissertation study show that bill morphology is predominantly linked with dietary differences among species, thereby supporting the conventional view that the primary role of the avian bill is that of a feeding apparatus. The analyses also reveal significant relationships between bill morphology and certain aspects of nest design (including nest volume and nesting material density), suggesting that weavers use their bills secondarily as tools. Moreover, while other studies of avian tool use have documented numerous instances in which birds will manipulate objects with their bills, to my knowledge, this is the first study that links bill morphology and nest design.

In Chapter 3, I re-explore the evolutionary relationships among weavers by using a phylogenomics approach to construct an updated phylogeny of the Ploceidae. I use thousands of ultraconserved elements and mitogenomes extracted from whole genome sequence data I generated to build new weaver phylogenetic hypotheses. In doing so, I compare and contrast the results of the nuclear and mitochondrial datasets and consider the efficacy of alternative phylogenetic tree-building methods. Lastly, I compare my results with other molecular-based Ploceidae phylogenetic hypotheses from recent studies.