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Parrots of the Wild

A Natural History of the World's Most Captivating Birds

UNIVERSITY OF CALIFORNIA PRESS

What are the Parrots and Where Did They Come From?

The Evolutionary History of the Parrots

PHYLOGENY

CONTENTS

The Marvelous Diversity of Parrots 3

Reconstructing Evolutionary History 5
Fossils, Bones, and Genes 5

The Evolution of Parrots 8
Parrots' Ancestors and Closest
Relatives 8
The Most Primitive Parrot 13
The Most Basal Clade of Parrots 15
Other Major Groups of Parrots 16
Box 1. Ancient DNA Reveals the
Evolutionary Relationships of the
Carolina Parakeet 19
How and When the Parrots Diversified 25

Some Parrot Enigmas 29
What Is a Budgerigar? 29
How Have Different Body Shapes Evolved in
the Parrots? 32

THE MARVELOUS DIVERSITY OF PARROTS

The parrots are one of the most marvelously diverse groups of birds in the world. They dazzle the beholder with every color in the rainbow (figure 3). They range in size from tiny pygmy parrots weighing just over 10 grams to giant macaws weighing over a kilogram. They consume a wide variety of foods, including fruit, seeds, nectar, insects, and in a few cases, flesh. They produce large repertoires of sounds, ranging from grating squawks to cheery whistles to, more rarely, long melodious songs. They inhabit a broad array of habitats, from lowland tropical rainforest to high-altitude tundra to desert scrubland to urban jungle. They range over every continent but Antarctica, and inhabit some of the most far-flung islands on the planet. They include some of the most endangered species on earth and some of the most rapidly expanding and aggressive invaders of human-altered landscapes. Increasingly, research into the lives of wild parrots is revealing that they exhibit a corresponding variety of mating systems, communication signals, social organizations, mental capacities, and life spans. In a great many respects the 360 or so species of parrots represent a textbook illustration of how the process of evolution can, over much time, lead to the diversification of many species from a single ancestral population.

At the same time, parrots are one of the most physically homogeneous groups of birds. Anyone with a passing familiarity with birds can instantly recognize a parrot by its sharply curved upper beak topped by a fleshy cere, muscular prehensile tongue, relatively big head and stout body, and distinctive zygodactyl feet with two toes pointing forward and two pointing back (figure 4). This combination of anatomical features clearly sets parrots apart from other birds. There are other, less obvious, commonalities in physiology, behavior, and ecology that tend to distinguish parrots from most other birds. These shared features illustrate another principal feature of evolution: that it tinkers with the materials at hand rather than starting anew with each species. In other words, major innovations are rare. What more typically happens is that features already present in an ancestor are slowly modified through natural selection over many generations to produce a constrained range of variations on the basic template as different lineages adapt to changing and localized environments.

In the following chapters we will delve deep into what recent scientific investigations have revealed about the lives of wild parrots. We will discuss how parrots perceive the world around them, how individuals go about their daily lives and interact with others, and how populations are adapting to a world that is rapidly changing. Our focus will be both on what these investigations tell us about parrots in general, and on what can be learned from the interesting exceptions to these generalities. But before we start this exploration, we want to set the stage by summarizing the current state of knowledge of the evolutionary history of parrots: Where did they come from, how did they diversify, who among them is most closely related to whom, and what does this evolutionary history reveal about the process of evolution itself? To understand these topics, we must first understand how scientists explore what happened in the long-distant past.

RECONSTRUCTING EVOLUTIONARY HISTORY

Fossils, Bones, and Genes

Reconstructing the past history of life is both a historical exercise and a scientific one. Scientists typically illustrate evolutionary patterns as trees, with the common ancestor of a group of species placed at the root, and existing species at the tips of the branches. The branching points between the root and the tips represent points where a single lineage split to produce two new lineages, while the length of each branch represents the amount of time or evolutionary change between branching points. As an aside, this representation of evolutionary history in tree form was an innovation of Charles Darwin himself, appearing first in his scientific notebook and then popularized in his seminal work, On the Origin of Species. These trees, or phylogenies, as they are termed by evolutionary biologists, are best viewed as hypotheses of how evolution occurred in a particular group of species. As such, they represent a well-informed supposition as to who is more closely related to whom, and when and how current species diversified from a common ancestor. As we will see below, such phylogenies also furnish predictions as to what traits or attributes might be shared among which species. Like all scientific hypotheses, they are subject to a rigorous process involving the collection and analysis of data and a careful evaluation of whether these results support or contradict the particular hypothesis in question. If the data are consistent with the hypothesis, then it remains standing as our best estimate of how evolution proceeded, for now. But, like all hypotheses, it is always subject to further testing and investigation with new data, and such investigations may well lead to modifications of the hypothesis and a new understanding of the past.

What sort of data do evolutionary biologists use to reconstruct evolutionary history? There are three primary sources: fossils of ancient taxa, physical traits measured from the anatomy of current specimens, and genetic data sampled from living or preserved animals. Fossils have the great virtue of concretely demonstrating how specific lineages appeared in the past, including lineages that have become extinct. Importantly, the geologic layer in which fossils are found provides context and can pinpoint when and where the lineage with this trait existed. Such data can be invaluable for calibrating the timing of branching points in a tree and grounding the hypotheses of how evolution proceeded in a group. The downside to fossils is that they can be hard to find and are typically fragmentary in nature, and thus provide only a partial view of the evolutionary past of an entire group of species. As we will see below, such is the case with the parrots.

In addition to fossils, scientists can use data from species still in existence and look for patterns of shared similarities and differences. These data can then be used to reconstruct a phylogeny that best explains the patterns of shared similarities. In the past these trees were often based on the straightforward principle of parsimony, which assumes that trees that require the fewest evolutionary changes are more likely than those that require more changes; now more mathematically sophisticated approaches often are employed.

Scientists prefer to build such trees using traits that are easily and reliably measured. The reason for this is simple: Even a few species can be arranged into enormous number of alternative trees with different branching patterns, each one representing a different hypothetical evolutionary history. Distinguishing between these alternate branching patterns is best done with measurements of lots and lots of traits (also called characters). More characters generally leads to better discrimination of the small set of trees that fit the data well from among the enormous forest of possible trees that could be constructed for a given set of species. Making these distinctions is a job best left to powerful computers applying carefully developed algorithms; with large numbers of species it can still take these computers weeks to sort through all the billions of possible alternative trees. It is still up to the scientists, however, to choose and measure their characters carefully so that the trees generated are most likely to represent sound hypotheses of evolutionary history.

Historically, the most abundant characters available to scientists were those provided by gross anatomy and morphology. Museum collections have thousands of specimens that are used for just this purpose, and they are carefully curated in impressive collections of study skins, skeletons, whole bodies in alcohol, and even nests and eggs. These specimens can then be used to painstakingly measure obscure details of the size and arrangement of bones and organs and compare these characters within and among different species. Such careful work exemplifies the classical approach to systematics, the branch of science that aims to reconstruct the evolutionary history of all organisms or, as it is colorfully known, the Tree of Life. Such knowledge was hard-won, however, as even the most creative and careful scientist eventually ran into limits as to how many morphological characters they could reliably measure across an entire set of specimens. This problem was especially acute when trying to compare across very distant branches of the Tree of Life separated by long periods of time from their common ancestor. (Imagine how few characters could be reliably measured across jellyfish, honeybees, and sharks, three distantly related members of the kingdom Animalia.) At the other end of the spectrum, early systematists also had difficulty with homogeneous groups in which many members shared similar values for most morphological traits, leaving few characters that actually helped distinguish among different groups. Such was the problem with the parrots, as their conserved morphology provided few external or even internal characters that varied enough to be useful in building well-resolved evolutionary trees. It took a landmark scientific discovery to break this impasse and eventually provide new insights into the evolutionary history of parrots and the entire Tree of Life.

This breakthrough was the discovery of DNA and the rapid rise of modern molecular genetics it permitted. In 1953, James Watson and Francis Crick, along with Rosalind Franklin and others, described the double-stranded helical structure of a molecule called deoxyribonucleic acid (DNA for short) and proposed that it encoded the genetic information necessary for life. This landmark discovery led to an explosion of studies into how these encoded instructions were used to build organisms, and how these instructions changed as they were passed from one generation to the next. This understanding of the basic molecular mechanisms of inheritance has benefited virtually every field of biology and opened vast new fields of study. The beneficiaries have included systematists, who were quick to realize the insights that direct study of genes themselves could contribute to reconstructing the evolutionary past.

Among the first pioneers of this new field of molecular systematics were Charles Sibley and Jon Ahlquist, who worked together through the late 1970s and 1980s to apply genetic approaches to understanding the evolutionary history of birds (class Aves). Their work culminated in 1990 with the publication of their monumental Phylogeny and Classification of Birds, the first large-scale study to apply DNA evidence to avian relationships. There was, however, considerable debate among ornithologists regarding their general approach, which relied on large-scale comparisons of overall DNA similarity across the entire genomes of pairs of species, and about many of their specific findings that resulted from this DNA–DNA hybridization technique.

Nonetheless, Sibley and Ahlquist's groundbreaking study did spur others to follow in their footsteps, and it revitalized interest in the relationships among major groups of birds. This interest was facilitated by rapid advances in biotechnology that started in the 1980s such as the invention of the polymerase chain reaction (PCR) and the mechanization of DNA sequencing. These technologies allowed researchers to isolate a single stretch of DNA from a sample, amplify many thousands of copies of it, and then read out the sequence of nucleotide base pairs. This DNA sequence could then be compared between species to look for patterns of similarities and differences. With the help of ever-improving computers, these patterns of sharing could then be transformed into trees of evolutionary relationships using many of the same approaches developed for morphological traits. The main benefit for molecular systematists was that they could now compile information from hundreds or thousands of DNA characters, whereas they used to struggle to find a few dozen characters from painstaking examination of morphology. These new biotechnological approaches have led systematists into a golden age of studies aimed at uncovering the evolutionary past of birds and other organisms. It is a golden age that continues today and will no doubt stretch on until such time as a comprehensive and well-supported hypothesis for the entire Tree of Life is produced. And, importantly for us, it has cast new light into the previously obscure history of the parrots.

THE EVOLUTION OF PARROTS

Parrots' Ancestors and Closest Relatives

The origin of parrots themselves is an evolutionary enigma. The unique set of morphological features shared by all parrots sets them well apart from other groups of birds and has made determining the identity of their closest relatives a challenge. In the absence of series of well-defined characteristics shared with another group, avian systematists resorted to proposing a long list of possible candidates as relatives, usually on the basis of a single feature that each shared with the parrots. Various proposed relatives included the pigeons, based on similarities of the humerus bone in the wings; the owls, based on the shared presence of a fleshy cere over a curved bill and features of the skull; the woodpeckers and their relatives, based on the shared presence of zygodactyl feet; the cuckoos and relatives for the same reason; the falcons or the owls, based on the hooked bill; and the toucans, based on the sharing of powder down. Others have noted morphological similarities with the mousebirds, an obscure group of small African birds composed of only six extant species that are able to switch their toes between the zygodactyl formation and the anisodactyl formation, in which three toes point forward and one backward. Most dismissed the shared presence of curved bills in the falcons and the parrots as a sign of a close relationship, instead explaining it as an example of convergent evolution, in which similar selection pressures lead to the evolution of similar features in distantly related groups. Others pointed out that the same argument could be applied to any of the similarities noted between parrots and other groups of birds. Clearly, morphology was providing little resolution to this thorny question.

The first attempts to answer this question using modern molecular genetics were only somewhat more successful. The comprehensive phylogeny produced by Sibley and Ahlquist using DNA–DNA hybridization suggested that parrots were most closely related to the cuckoos and to a group composed of the swifts and hummingbirds. The actual number of DNA comparisons on which this conclusion was based was limited, however, and the relationships were generally considered provisional until such time as better data were available.

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Excerpted from Parrots of the Wild by Catherine A. Toft, Timothy F. Wright. Copyright © 2015 Catherine A. Toft and James D. Gilardi. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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