World Leaf Warblers Research Articles

Positive correlations between pre‐ and post‐copulatory sexual traits in warblers

Theoretical models predict that investment in pre‐copulatory and post‐copulatory sexually selected traits should trade‐off. At the macroevolutionary scale, the majority of studies to date have focused on male weaponry as the target of pre‐copulatory sexual selection, but the trade‐off should equally apply to traits used to attract females, such as bird song and plumage. We studied the Old World leaf warblers (Phylloscopidae), a group of socially monogamous songbirds that experience relatively high levels of sperm competition. We examined the relationships between song duration and number of elements in the song with sperm length across 21 species, and between the same song variables and combined testes mass in a subset of these species (n = 10). Across species, these song variables and testes mass/sperm length were generally positively correlated, albeit not statistically significantly so or with borderline significance. In contrast to theory, we found no evidence for negative associations between pre‐ and post‐copulatory traits. We argue that this is a consequence of males of some species investing more into overall fertilization success (i.e. the sum of pre‐ and post‐copulatory sexual selection) than males of other species, and high fertilization success is achieved through investment into both mate attraction and sperm competition.

Early diversification of sperm size in the evolutionary history of the old world leaf warblers (Phylloscopidae).

Sperm morphological traits are highly variable among species and are commonly thought to evolve by post-copulatory sexual selection. However, little is known about the evolutionary dynamics of sperm morphology, and whether rates of evolutionary change are variable over time and among taxonomic groups. Here, we examine sperm morphology from 21 species of Old World leaf warblers (Phylloscopidae), a group of generally dull, sexually monochromatic birds, which are known to have high levels of extra-pair paternity. We found that sperm length differs markedly across species, spanning about 40% of the range observed across a larger selection of passerine birds. Furthermore, we found strong support for an 'early-burst' model of trait evolution, implying that the majority of divergence in sperm length has occurred early in the evolutionary history of this clade with subsequent evolutionary stasis. This large early divergence matches the early divergence reported in ecological traits (i.e. body size and feeding behaviour). Our findings demonstrate that rates of evolution in sperm morphology can change over time in passerine taxa, and that evolutionary stasis in sperm traits can occur even in species exhibiting characteristics consistent with moderate-to-high levels of sperm competition. It remains a major challenge to identify the selection mechanisms and possible constraints responsible for these variable rates of sperm evolution.

Rates of signal evolution are associated with the nature of interspecific communication

Some signals vary greatly, whereas others are remarkably similar across distantly related species. Here, we ask how the suite of receivers and information communicated correlates with signal evolution by comparing 2 different signals across the same set of species. Within the Old World leaf warblers (Phylloscopidae), each species utters 2 acoustically distinct alarm calls. The first, termed a call, is used in interactions with conspecifics as a well as during confrontations with predators and nest-parasitic cuckoos. The second, termed a call, is primarily used in the presence of nest-parasitic cuckoos. The rasp call precedes aggressive attacks on cuckoos and attracts surrounding heterospecifics that are also potential hosts. The general call attracts a wide range of species threatened by predators, including those that are not cuckoo hosts. Acoustic features of general calls evolve >5x faster than rasp calls. We argue that rasp calls show strong stasis because they have a restricted function as aggressive antiparasite signals, whereas multiple contexts and receivers have promoted divergence in general calls. These results support the idea that variation in the suite of receivers is a powerful force affecting signal evolution.

A test for community saturation along the Himalayan bird diversity gradient, based on within‐species geographical variation

The idea that ecological communities are unsaturated is central to many explanations for regional gradients in species diversity. We describe a test for differing degrees of saturation across a regional diversity gradient, based on within-species geographical variation in ecological attributes. If communities in species-poor regions are less saturated than communities in species-rich regions, species that straddle both regions should have broader niches in species-poor regions, exploiting resources that are consumed by other species in species-rich regions. We studied 10 species of Old World leaf warblers that range across the Himalayas. Elevational range and feeding method showed niche contractions in the species-poor north-west Himalayas with respect to the species-rich south-east Himalayas, whereas prey size did not vary geographically. Niche contractions are contrary to the expectation of character release in depauperate environments, as has been shown, for example in mainland-island comparisons. We show that arthropod abundances are likely a limiting resource, and that niche contractions are consistent with measurements of a narrowing of resource availability. Results suggest that north-western warbler communities are at least as saturated as the south-east and that lower resource diversity drives reduced species numbers.

Population Regulation and Character Displacement in a Seasonal Environment

Competition has negative effects on population size and also drives ecological character displacement, that is, evolutionary divergence to utilize different portions of the resource spectrum. Many species undergo an annual cycle composed of a lean season of intense competition for resources and a breeding season. We use a quantitative genetic model to study the effects of differential reproductive output in the summer or breeding season on character displacement in the winter or nonbreeding season. The model is developed with reference to the avian family of Old World leaf warblers (Phylloscopidae), which breed in the temperate regions of Eurasia and winter in tropical and subtropical regions. Empirical evidence implicates strong winter density-dependent regulation driven by food shortage, but paradoxically, the relative abundance of each species appears to be determined by conditions in the summer. We show how population regulation in the two seasons becomes linked, with higher reproductive output by one species in the summer resulting in its evolution to occupy a larger portion of niche space in the winter. We find short-term ecological processes and longer-term evolutionary processes to have comparable effects on a species population size. This modeling approach can also be applied to other differential effects of productivity across seasons.

The roles of time and ecology in the continental radiation of the Old World leaf warblers (PhylloscopusandSeicercus)

Many continental sister species are allopatric or parapatric, ecologically similar and long separated, of the order of millions of years. Sympatric, ecologically differentiated, species, are often even older. This raises the question of whether build-up of sympatric diversity generally follows a slow process of divergence in allopatry, initially without much ecological change. I review patterns of speciation among birds belonging to the continental Eurasian Old World leaf warblers (Phylloscopus and Seicercus). I consider speciation to be a three-stage process (range expansions, barriers to gene flow, reproductive isolation) and ask how ecological factors at each stage have contributed to speciation, both among allopatric/parapatric sister species and among those lineages that eventually led to currently sympatric species. I suggest that time is probably the critical factor that leads to reproductive isolation between sympatric species and that a strong connection between ecological divergence and reproductive isolation remains to be established. Besides reproductive isolation, ecological factors can affect range expansions (e.g. habitat tracking) and the formation of barriers (e.g. treeless areas are effective barriers for warblers). Ecological factors may often limit speciation on continents because range expansions are difficult in 'ecologically full' environments.

Bridging the Gap between Theory and Ecology in Evolutionary Models

Bridging the Gap between Theory and Ecology in Evolutionary Models

Unmelanized plumage patterns in Old World leaf warblers do not correspond to sequence variation at the melanocortin-1 receptor locus (MC1R).

Evolutionary changes in patterns and coloration of plumage are likely to represent a major mechanism for speciation among birds, yet the molecular basis for such changes remains poorly understood. Recently much attention has focused on the melanocortin-1 receptor (MC1R) as a candidate locus for determining the level and extent of epidermal melanin deposition. We tested the hypothesis that MC1R sequence variation is associated with interspecific variation in unmelanized plumage pattern elements in Old World leaf warblers (genus Phylloscopus). This genus is characterized by a variety of plumage patterns that nonetheless vary along similar lines. Species vary in the presence or absence of pale (unmelanized) pattern elements against a dark background, and these patterns are used in species recognition and courtship. We sequenced most of the MC1R coding region for eight Phylloscopus species, representing the full range of plumage patterns found in this genus. Although MC1R sequence varied among species, this variation was not related to melanin-based plumage variation. Rather, evolution of this locus in these birds appears to be conservative. Ratios of nonsynonymous to synonymous substitutions (dN/dS) were consistently low, suggesting that strong purifying selection has operated at this locus, and likelihood ratio testing revealed no evidence of variable selective pressures among lineages or across codons. Adaptive evolution at MC1R may be constrained by the adaptive importance of plumage pattern elements in this genus.

Cryptic species in the genus Phylloscopus (Old World leaf warblers)

In almost all ecological and evolutionary research, it is important to assess the number of species under study. Cryptic species, which are morphologically similar, present a special problem because they need to be identified through studies of behavioural or genetic variation. Here we review the important factors in the recent discovery of three species and elevation of nine previously known taxa to species status within the genus Phylloscopus (Old World leaf‐warblers) and we examine the case of three morphologically similar taxa (inornatus, humei and mandellii) that until recently were considered to be members of the single species Phylloscopus inornatus. We have identified several locations at which humei and inornatus coexist at high density, and the results of playback experiments, observations of interactions, and spectrogram analysis of vocalizations at these sites all indicate that there are significant behavioural differences and a lack of recognition between humei and inornatus. Estimated relationships within the species complex based on mitochondrial control region DNA sequences show that there is little variation within the geographic ranges of either humei or inornatus compared with the deep split between the two taxa. These observations support the division of humei and inornatus into separate species. The third taxon, mandellii, which is geographically separated from both humei and inornatus, is behaviourally and genetically more similar to humei. Under the biological species concept, it is difficult to determine the species status of allopatric taxa. Alternative species concepts, such as the phylogenetic and recognition concepts, are more easily applied to allopatric taxa but lead to differing conclusions over whether mandellii and humei are separate species. This confusion arises from the fact that the two taxa are in an early stage of allopatric divergence. The review of other recently designated species reveals that song divergence is of primary importance in their designation and that song variation, playback experiments and genetic analysis lead to similar conclusions regarding species status.

The Adaptive Significance of Colour Patterns in the Old World Leaf Warblers, Genus Phylloscopus

The great diversity of colour patterns in birds remains largely unexplained (Burtt 1986, Butcher and Rohwer 1989). Experimental and comparative studies have been used to identify some adaptive correlates, including intra-specific communication (Hill 1990, 1994, Slagsvold and Saetre 1991, Saetre and Slagsvold 1992, Marchetti 1993), camouflage (Gaston 1974, Burtt 1986), abrasion resistance (Burtt 1986), inter-specific signalling (Baker and Parker 1979, Gbtmark 1993, 1995, Saetre et al. 1993) and flushing out prey (Jablonski 1996). While we might expect plumage patterns to represent a compromise between all these and other selection pressures, two hypotheses have been recently proposed to explain the diversity of colour patterns within the genus Phylloscopus (Old World leaf warblers). Marchetti (1993) ascribes a role for intra-specific communication (i.e., signalling to potential mates or competitors) in the evolution of colour patterns, whereas Jablonski (1996) suggests that conspicuous coloration might be important for flushing prey. In this note we assess the role of prey-flushing in the Phylloscopus warblers using quantitative data on foraging and colour patterns. Jablonski (1996) proposed that wing, rump and tail patches could be important in prey-flushing in the Phylloscopus warblers. To support this idea he used qualitative reports on foraging behaviour culled from the literature, and a cross-species comparative study to demonstrate an association of conspicuousness with the use of the wings during foraging. However, Burtt (1986) in a comparative study of the New World warblers also found an association between plumage brightness and foraging movements, but interpreted it as being a consequence of intra-specific communication. For example, species which flycatch are prominently displaying their wings, and therefore the wings are suitable locations to place patterns used in intra-specific communication. From Jablonski's prey-flushing hypothesis we predict positive correlations between plumage brightness and only those flying movements that lead to chases after flushed prey, but from Burtt's intra-specific communication hypothesis we expect a positive correlation between plumage brightness and the number of all types of flying movements performed by the bird when feeding. There are therefore two purposes to this paper. First, we demonstrate a correlation between plumage brightness and foraging among the Phylloscopus warblers using more quantitative data than were used by Jablonski (1996). Second, we use our results to compare the predictions of the intra-specific communication and prey-flushing hypotheses. Phylloscopus species differ in the number of colour patches they have. In all species, colour patches occur as bright, unpigmented areas on the tips of certain feather tracts of the body. Some species are essentially unpatterned and have uniform dark upperparts except for an eye-stripe. Others have one or two patches on the wings, a crown stripe, a rump patch and/or white tail patches in nested combinations, so that, for example, every species with crown or rump patches always has a wing patch (see Marchetti 1993, Price and Pavelka 1996). We used three different measures of plumage brightness for a species: 1. the total number of colour patches, varying from 0-5 (Marchetti 1993). 2. the total brightness (% reflectance relative to a standard, as measured by reflectance spectroradiometry), of the colour patch at the tip of the feather taken from the tract of feathers which form the largest wing patch on those species which have a wing patch (Marchetti 1993). 3. the size of the unpigmented patch on this same feather (area without melanin; Price and Pavelka 1996). The three variables are correlated (Fig. 1, Table 1). Quantitative data on foraging are available for eight Phylloscopus species from Kashmir, India (Price 1991). Feeding methods were classified into four main types: standpick (a prey capture not involving flight), flypick (prey resting on a leaf or branch, observed while hopping, but taken in flight), flycatch (capture of prey, which was nearly always flushed), and hoverpick (prey

Evolution of Breeding Distributions in the Old World Leaf Warblers (Genus Phylloscopus)

Evolution of Breeding Distributions in the Old World Leaf Warblers (Genus Phylloscopus)

ECOLOGICAL DIVERSIFICATION AND COMMUNITY STRUCTURE IN THE OLD WORLD LEAF WARBLERS (GENUS PHYLLOSCOPUS): A PHYLOGENETIC PERSPECTIVE.

I investigated the historical basis for variation in regional species diversity. I used a molecular phylogenetic analysis within a single genus of birds (the Old World leaf warblers, genus Phylloscopus) in conjunction with ecological studies in Europe, the Himalayas, and Japan to evaluate the importance of historical events in shaping the present constellation of morphology and behavior in the three different regions. The relatively depauperate assemblages have different histories. In Japan, there was invasion of several lineages, which have more closely related species elsewhere in Asia, whereas in Europe there was also limited in situ speciation. Much of the structure of peripheral communities is attributable to invasions from species-rich Asia, with little in situ morphological diversification. Within the Phylloscopus there are several phylogenetic clades with nonoverlapping size distributions. Major ecological and morphological shifts occurred early in the history of diversification within the group, and rarely since.

Ecological Diversification and Community Structure in the Old World Leaf Warblers (Genus Phylloscopus): A Phylogenetic Perspective

Ecological Diversification and Community Structure in the Old World Leaf Warblers (Genus Phylloscopus): A Phylogenetic Perspective

Latitudinal Ranges of Teleost Fish in the Atlantic and Indo-Pacific Oceans

Latitudinal Ranges of Teleost Fish in the Atlantic and Indo-Pacific Oceans

Correlates of Wing Morphology with Foraging Behaviour and Migration Distance in the Genus Phylloscopus

Wing size and shape variation in thirteen species of Old World leaf warblers, genus Phylloscopus, were examined using wing formulae and principal components analysis. Data were taken from male individuals in breeding populations in Northern India and Japan. Species with longer wings migrate further and hover less than species with shorter wings. Species with more pointed wings migrate further and are more arboreal than species with rounder wings. Findings agree with those from previous studies on the genus Phylloscopus. Both foraging behaviour and migration distance have affected wing size and shape evolution in the genus.

Correction: Evolution of ecological differences in the Old World leaf warblers

IN Fig. 2a of the above letter the second and third rows were inadvertently interchanged. In addition, the sequence at position 15176 (ref. 1.) for both Sylvia and P. coronatus should read 'A' and not F.