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Birds are of high public interest and of great value as indicators of the state of the environment. Some 11,000 species are a number relatively well to handle. From a scientific point of view, it is not easily answerable what a species is, since speciation and extinction are ongoing evolutionary processes and differentiation among species works on various traits. Contemporary systematics attempts to take into account as many criteria as possible to delimit species. The currently most influential approach is the use of genomic sequences, be it as a neutral marker or to discover the underpinnings of functional traits. The study of the outer appearance of birds nevertheless remains fundamental, since that is the interface between a bird and its biotic and abiotic environment. For the majority of bird species, acquired traits of vocal communication add to this complex. Birds can also vary the timing of important behavior such as breeding or molting. Most fascinating among circannual behavior are the long-distance movements that can quite fast evolve and have genetic bases. Despite such dispersal ability for many bird species, geographic barriers play a large role for distribution and speciation in birds. Extant, former, and potential future ranges of a species can be modeled based on the abiotic niches individuals of this species have. Within a species’ range, genetic and phenotypic traits vary and promote to process toward species splits. Beside geographic frameworks, ecological circumstances play a major role and contribute to natural selection but also trigger individual responses such as phenotypic plasticity, modification of the environment, and habitat selection. Anthropogenic global impacts such as climate and land-use changes (e.g., urbanization) force extant species to accelerated modifications or population splits or let them vanish forever. Only if humans leave more room and time to birds and other organisms can we expect to maintain such a number of diverse bird species, although they will keep modifying, splitting, and becoming extinct—but for natural reasons.

Species are the basic currency in biodiversity studies, but what constitutes a species has long been controversial. Since the late 1990s, debates over species have shifted from theoretical questions (e.g., What is a species? Which species concept is best?) to empirical questions (How can we document species both efficiently and accurately?). A growing number of taxonomists agree that species can be discovered and documented in many different ways, preferably by employing and combining multiple types of evidence (“integrative taxonomy”). This chapter examines how and why avian taxonomy has become integrative, how species hypotheses are documented and falsified, and how the growth of taxonomic knowledge provides new and valuable insights into the speciation process, biogeography, and conservation biology.

A genome comprises the entire genetic material of an organism and consists of DNA, which is in turn constructed of hundreds to billions of nucleotides. Nucleotides are organic molecules composed of three subunits: nitrogenous base, sugar (deoxyribose), and phosphate group. The DNA is differentiated into coding (genes) and noncoding regions. A gene is a specific region of DNA that encodes a function. All genes present within an organism represent its genotype. The genotype determines the phenotype, which is, however, additionally affected by the environment and the individual development (ontogenesis). A gene may affect a single or several phenotypic features (pleiotropy). Likewise, a phenotypic feature may be affected by one or several genes, with the latter comprising polygenic traits. In the process of gene expression, the information encoded by a gene is used to generate a product. The expression of genes is regulated by the external (temperature, stress, resource availability) and internal environment (metabolism, cell division cycle), and the gene-specific role in the respective tissue organism. Several processes underlying evolutionary change, e.g., mutation, genetic drift, gene duplication, selection, and migration, may change the genome at the level of single bases through genes to the organism. Such changes may result in population differentiation and eventually speciation. Molecular genetic studies on microevolution and speciation started with single genetic markers, e.g., the COI marker gene. Today, mainly genomic and transcriptomic approaches, making use of a large number of markers such as single nucleotide polymorphisms or microsatellites, are used to compare species, populations, and individuals.

The huge diversity of phenotypes and associated geographic patterns has made birds prime examples for studies in speciation. For this purpose, morphological approaches were first choice to assess the degree of relatedness between species and their intraspecific variation for centuries, until molecular genetic studies seriously challenged traditional morphology-based conclusions. However, the current development of multivariate statistics and the ease to blend morphological, phylogenetic, and ecological insight has gradually led to a reconsideration of morphology as a valuable tool for ornithological research. This chapter reviews the most important aspects of morphological variation in birds, how its plasticity can be assessed and to which extent phenotypic variation can be incorporated into a broader evolutionary framework that explains modifications of the avian body in the light of speciation processes.

Vocal learning has evolved several times independently in mammals and three major orders of birds. Of these only hummingbirds and passerine birds have complex songs, whereas the large vocal repertoires of parrots comprise various call types associated with different behavioral contexts. Generally, bird song has two major functions: territorial defense and mate attraction. In the latter context, particularly in songbirds (Oscines), the evolution of male song repertoires has strongly been driven by sexual selection: Song complexity and repertoire size have been shown to be indicators of male quality and are thus crucial traits for female choice. Today, the age of phylogenetics provides new methods for the study of the mode and tempo of organismic diversification and of trait evolution, e.g., of vocal learning. As a striking result, song learning seems to be associated with species richness across the avian tree of life. This provided recent evidence for the theory that song learning can act as a pacemaker of evolution. The spatial variation of song dialects is often correlated with genetic diversification. Extreme cases are small isolated populations, for example, on islands. In the field, the effect of song diversification as a barrier for gene flow can best be observed in zones of secondary contact between close relatives. Even in cases of hybridization, differences of song might affect female choice and thus lead to assortative mating and prevent gene flow in one or even in both directions. These are probably the most exciting case studies, where speciation in action (or in reverse) can be studied in the wild.

Globally, most birds reproduce to some extent seasonally, and the timing of their annual breeding events involves regulation by biological clocks. Biological clocks also regulate diel activities, including song and other courtship behaviors which occur at certain times of day. Differences between individuals in the timing of display and breeding (i.e., allochrony) can drive genetic divergence, contribute to isolation between populations, and ultimately lead to speciation. Isolation by breeding time is thought to be common in areas where reproductive seasons differ greatly over short distances, for example in tropical regions. Here we introduce the ways biological clocks of birds function, and review evidence for variation between individuals and between populations. The potential importance of allochrony for the speciation of birds is underpinned by the periodic growth and regression of their reproductive organs, by their rigid migration programs, and possibly also by their learnt, well-timed mating behaviors (in particular song). We exemplify isolation by time in tropical songbirds and in migratory species. Potentially, further contexts that could promote allochronic isolation in birds include the differentiation between urban and rural populations, as urbanization is commonly associated with modified timing of breeding and song.

Seasonal migration is the yearly long-distance movement of individuals between their breeding and wintering grounds. Individuals from nearly every animal group exhibit this behavior, but probably the most iconic migration is carried out by birds, from the classic V-shape formation of geese on migration to the amazing nonstop long-distance flights undertaken by Arctic Terns . In this chapter, we discuss how seasonal migration has shaped the field of evolution. First, this behavior is known to turn on and off quite rapidly, but controversy remains concerning where this behavior first evolved geographically and whether the ancestral state was sedentary or migratory (Fig. d, e). We review recent work using new analytical techniques to provide insight into this topic. Second, it is widely accepted that there is a large genetic basis to this trait, especially in groups like songbirds that migrate alone and at night precluding any opportunity for learning. Key hypotheses on this topic include shared genetic variation used by different populations to migrate and only few genes being involved in its control. We summarize recent work using new techniques for both phenotype and genotype characterization to evaluate and challenge these hypotheses. Finally, one topic that has received less attention is the role these differences in migratory phenotype could play in the process of speciation. Specifically, many populations breed next to one another but take drastically different routes on migration (Fig. ). This difference could play an important role in reducing gene flow between populations, but our inability to track most birds on migration has so far precluded evaluations of this hypothesis. The advent of new tracking techniques means we can track many more birds with increasing accuracy on migration, and this work has provided important insight into migration’s role in speciation that we will review here.

Most modern orders of birds evolved and diversified during the last 65 million years following the demise of the nonavian dinosaurs and pterosaurs at the Cretaceous-Paleogene boundary. Diversification rates in birds increased from c. 50 million years ago onward driven by significant rate increases in different clades scattered throughout the entire phylogeny. No slowdown in the overall diversification rate has been identified, and equilibrium diversity might not have been reached. Birds breed on all of the continents on Earth and have adapted to almost every habitat. Substantial variation in distribution patterns occurs among the different species, ranging from narrow-range endemics restricted to a single oceanic island or to a particular habitat within a small geographic area to species with a near-cosmopolitan distribution, breeding on almost all continents. As in most groups, diversity of bird species is greatest in tropical regions near the equator and decreases toward the poles. This pattern, termed the latitudinal diversity gradient, cannot be causally linked to a single mechanism and might be influenced by both evolutionary and ecological processes. Species richness within a given area is basically the result of speciation, extinction, and dispersal. Speciation commences with the accumulation of genetically based divergence between populations and is completed by the development of reproductive isolation among them. This usually involves a phase of geographic separation of populations without contact, a process termed allopatric speciation. Speciation with ongoing gene flow between populations, i.e., parapatric speciation, and especially the evolution of reproductive isolation without geographic separation, i.e., sympatric speciation, appear rare in birds. Distribution patterns of different bird groups particularly across the Southern Hemisphere have for a long time been interpreted as being the result of vicariance evolution. Vicariance is considered to be the split of a geographical range of a species via a barrier caused by a historical event like montane uplift or the formation of oceans through tectonic rifting. The formation of such barriers should promote episodes of allopatric speciation in multiple clades, generating congruent biogeographic patterns among them. Using dated phylogenetic hypotheses, however, several studies have recently revealed discordance between sequences of geological events and phylogenetic patterns. Consequently, only past dispersal events, often over long distances and across oceans, can explain the current distribution patterns of several avian groups. In general, landscape changes might not result in congruent temporal diversification patterns among different bird groups. It can be assumed that the older an avian lineage is, the more time it has to colonize an area across a barrier. This increases the likelihood of dispersal across the barrier and subsequent diversification on either side of the divide. In addition, bird groups with lower dispersal abilities are expected to accumulate genetic differences among populations at a higher rate than lineages with higher dispersal capability. Consequently, diversification patterns are the result of interactions between ecological properties of different avian lineages and their environment as well as the age of a given lineage. Geographic ranges of birds are generally limited by a suite of biotic and abiotic factors. Range expansion is not only an important first step in speciation but also influences the number of coexisting species and thereby shapes the turnover of biodiversity in space and time.

Avian evolutionary studies have recently benefited from a plethora of new techniques as well as conceptual progresses on the evolution of ecological niches. The so-called species distribution models (SDMs) allow for niche quantifications in a way that permits comparisons among species and populations. This review will introduce the theoretical background of niche concepts and niche conservatism, followed by an outline of popular methods for modeling and analyzing environmental niches. A comparison of ecological niches among native and non-native populations of invasive species can reveal niche shifts. They can point to evolutionary changes that evolved over comparatively short time scales of decades to a few centuries. On the other hand, ecological niches can also remain conserved over the invasion process. In a similar way, comparisons of ecological niches are also applicable among closely related taxa. Thereby, it is possible to infer changes of ecological niches over longer time scales and reveal otherwise hidden patterns and processes in the evolutionary history of avian clades. Finally, SDMs offer the potential to contribute to integrative taxonomic studies.

Human beings have a strong, innate desire to classify and name things. We like things to be clear-cut. The way we approach classification of birds is as good an example as any of this. So it always comes as something of a surprise to non-ornithologists to learn that how we classify birds at the level of the species around us is still subject of so much at times fiery debate. Various chapters in this book approach this from different perspectives. In this chapter, the focus is on reminding us that evolution is an ongoing, dynamic process and that appreciating this evolution can help us make sense of why it is sometimes so complicated to pin names on birds and indeed many other organisms. This will take us into a few particular aspects of bird evolution. One will be the process of hybridization between populations that may or may not be of the same species or between species that may or may not be each other’s closest relatives. Another will concern the study of genetic diversity that exists within a species. In particular, we will examine what we have learned from the way that that diversity has come to be apportioned and distributed across the geographical range and landscapes inhabited by a species. These two areas have opened windows into the dynamics of evolution that give us new understanding of bird species. Genetic boundaries between species and subspecies are frequently very “leaky.” Only certain parts of the genome, the entire complement of genetic material in a species, may be contributing to the differences that we can see between bird species. If the chapter can convey to the reader that we must learn to think of birds as continually evolving evolutionary lineages, then it will have had some success.