The fascinating diversity of their phenotypes and associated geographic patterns has made birds prime examples for studies in speciation. However, like in other groups of animals, there is not always an immediate connection between external appearance and species identity. In order to correctly distinguish between interspecific differences and intraspecific variation, it is inevitable to scrutinize the extent of geographic and individual variability within and between potential species (Mayr 1969). As the limits of such variation may shift temporarily as a result of adaptive processes (e.g., Grant and Grant 2014), character plasticity has to be considered likewise. Of course, this does not only concern morphological traits. A plethora of molecular studies has shown that the variation of genetic characters (i.e., DNA sequence polymorphisms) is a powerful tool to define groups of organisms even on a very fine scale. This is especially valuable when combining morphological and molecular evidence. In the following, however, only the variation of external characters will be in focus.
Geographic variation refers to the differentiation between different populations (Mayr 1969). Obviously, geographic variation covers all aspects of phenotypic differentiation on various geographic scales (Cicero and Remsen 2007). There is no rule as to how large geographic distances have to be until morphological differentiation sets in, but evidently at least some (historical) amount of geographic isolation (vicariance) appears necessary (Price 2008). Consequently, geographic variation often is not only pronounced in birds with a wide distribution but also in those that inhabit ecologically fragmented ranges on smaller geographic scales. Traditionally, populations of these polytypic species are grouped as subspecies based on the particular composition of morphological traits at a given locality (Winker and Haig 2010).
Geographic differentiation can either be continuous (clinal) or discontinuous. While the latter is commonly found in patterns of insular distributions or wide geographic range separations (disjunct distribution), the former frequently occurs in more or less continuous continental ranges. Clinal variation, however, can well be incongruent between different phenotypic traits indeed. As a result of such incongruently directed character changes, each bird population at any intersection of different clines represents a certain morphological entity with a particular trait combination.
As with other forms of morphological variability, geographic variation is not restricted to particular traits and can basically affect any aspect of a bird’s physique. What makes it even more interesting in evolutionary respect is the often-found correlation between geographic variation and ecogeographic rules (e.g., Sun et al. 2017), indicating the selective effect of environmental conditions on the avian body. Moreover, eco-morphological insight, such as the connection between wing structure and flight performance or migratory distance, may be likewise reflected in inter- and intraspecific geographic variation (Winkler and Leisler 2005).
By definition, individual variation comprises all phenotypic manifestations at the same locality (Mayr 1969). Such local variation, however, does not necessarily need to be less pronounced than geographic variation. There are different forms of individual variation and all of these forms may be represented by interindividually variable characters again. Moreover, the forms of individual variation are not exclusive as they might occur even in the very same individual at different ages or seasons. The most frequent forms of individual variation in birds are sexual, age-dependent, and seasonal variation, complemented by local eco-morphological differentiation within populations.
Sexual variation includes all aspects of phenotypic differentiation between the two sexes. This is of course most obvious in those birds that exhibit a pronounced sexual dimorphism, be it in size, coloration (patterns), or in both. Sexual dimorphism does not need to be constant throughout the year, as more conspicuous breeding plumages might differ more strongly between the sexes than non-breeding plumages, indicating the overlap with seasonal variation. In addition, sexual dimorphism may not only be due to sexual selection alone but might also have an ecological component. In many raptors (Falconidae, Accipiter), there is a substantial size difference between the two sexes with the larger females apparently exploiting a different food spectrum than the smaller males. Within females and males, again, there is variation in the respective characters that defines the extent of sex-specific characteristics. Other special cases are those birds in which the external appearance of one sex is strongly correlated with specific aspects of the mate choice process and where strong sexual selection leads to extreme phenotypic diversity within one sex. The prime example is the Ruff Philomachus pugnax, whose males are individually distinct in plumage patterns and coloration. But there is not only the males’ extreme plumage variability (that can be subdivided into main character groups) but also the existence of female-mimicking males that also form an intermediate size group in wing length between “proper” males and females (Jukema and Piersma 2006).
Regarding age-dependent variation, a broad range of character differences have to be considered. As far as the feather coat is concerned, consecutive plumage generations of the same individual are involved. A precise knowledge of molt sequence is thus necessary to properly assess this kind of phenotypic variation (e.g., Jenni and Winkler 1994). It is the nature of things that during molt transitional plumage stages occur and that particularly in long-lived birds (e.g., eagles and vultures) persistent composite plumages exist for years that comprise feathers of several generations. As a rule of thumb, a distinction can be made between plumages worn before becoming reproductively active (juvenile and so-called immature plumages) and those worn by reproductively active birds (adult plumages). Nonreproductive plumages might differ in terms of dimensions, structure, and coloration and patterns in being shorter, differently shaped (particularly in the large flight feathers), appearing more fluffy and being less strikingly colored and patterned. In addition, it is commonly the case that nonreproductive plumages are very similar between sexes even if the adult plumage is clearly sexually dimorphic. Moreover, there is growing evidence that bird plumages may also be subject to senescence (“progressive greying,” van Grouw 2013). As birds apparently remain reproductively active until their death, however, this stage has to be considered a representation of the adult plumage as well. But not only plumage traits are subject to age-dependent variation. Other physical features like iris, leg, or bill color as well as structural differences because of not yet full-grown characters (e.g., bill size and shape) might be concerned likewise.
Another often-found form of variation is seasonal: In the course of the year, many birds acquire two different plumages of which one usually is more conspicuous. Since in most cases such conspicuousness is explained by the demands of mate choice and breeding, the breeding plumage tends to be more prominent – at least in one sex (cf. sexual variation). In the more unobtrusively colored or patterned non-breeding plumages, possible phenotypic differences between males and females might become even less pronounced. In a number of birds, particularly among nonmigratory species of the temperate regions, seasonal variation in external characteristics is an adaptive way to cope with substantially changing environmental conditions. One of the most striking examples is the difference between the summer plumage and the winter plumage of some ptarmigan Lagopus species which permits a seasonally efficient camouflage throughout the year. Similar to the effect in age-dependent plumages, there are also transitional plumage stages consisting of feathers of two different, although recurringly molted, feather generations.
As described above, patterns of phenotypic variation can be complex and causally overlapping. To this adds the fact that in some birds, apparently as an adaptation to quickly changing resources within habitats, there is a corresponding swift eco-morphological response between and within species, as most prominently described for Darwin’s finches (Grant and Grant 2014). The so-called resource polymorphism has also been studied extensively in Pyrenestes seedcrackers (Fig. 4.1) in which the size and structure of their bills differ at the same time between individuals of a single population that have adapted to different food items (Smith 1990).
The preceding paragraphs show that morphological variability concerns different manifestations of external differentiation on individual, population, or geographic level and that sometimes several forms of variation might even overlay. Collectively, they represent the phenomenon of phenotypic plasticity. Premising the idea that selection acts on the phenotype, it is important to keep in mind that character variation as such is not a result of evolutionary adaptation in the first instance rather than a mean to flexibly enhance the evolutionary success of a population by stochastically offering variants of morphological traits for selective processes. The phenotype resulting from natural selection of advantageous traits may then represent a particular fraction of previous generations’ character variation and again might have its very own range of variation. As phenotypes do also reflect a response to local or temporal ecological conditions, which are not fixed over time themselves, a great deal of morphological plasticity in time can be likewise observed within or between populations and species (e.g., Grant and Grant 2014). Ultimately, shifts in character variation between populations can (but do not have to) lead to evolutionary distinct units that might carry the germ of new species (cf. Yousefi et al. 2017). Therefore, particularly if preserved specimens are part of studies of avian speciation, researchers have to carefully select the geographic and temporal origin of the specimens.