Jeffrey Podos

Correlated evolution of morphology and vocal signal structure in Darwin's finches

Speciation in many animal taxa is catalysed by the evolutionary diversification of mating signals. According to classical theories of speciation, mating signals diversify, in part, as an incidental byproduct of adaptation by natural selection to divergent ecologies, although empirical evidence in support of this hypothesis has been limited. Here I show, in Darwins finches of the Galápagos Islands, that diversification of beak morphology and body size has shaped patterns of vocal signal evolution, such that birds with large beaks and body sizes have evolved songs with comparatively low rates of syllable repetition and narrow frequency bandwidths. The converse is true for small birds. Patterns of correlated evolution among morphology and song are consistent with the hypothesis that beak morphology constrains vocal evolution, with different beak morphologies differentially limiting a birds ability to modulate vocal tract configurations during song production. These data illustrate how morphological adaptation may drive signal evolution and reproductive isolation, and furthermore identify a possible cause for rapid speciation in Darwins finches.

A PERFORMANCE CONSTRAINT ON THE EVOLUTION OF TRILLED VOCALIZATIONS IN A SONGBIRD FAMILY (PASSERIFORMES: EMBERIZIDAE)

Behavioral evolution can be influenced by constraints, for example, of phylogeny and performance. In this paper I describe a pattern in the evolution of birdsongs that may reflect a constraint on vocal performance. Trilled vocalizations from 34 species of songbirds (Passeriformes: Emberizidae) were analyzed. Two acoustic variables, trill rate and frequency bandwidth, were measured for different trill types. In most species, maximal values of frequency bandwidth were found to decrease with increasing trill rates. Further, trills with low trill rates exhibited wide variance in frequency bandwidth, and trills with high trill rates exhibited only narrow frequency bandwidths. The bounded nature of this pattern suggests that performance constraints have limited the evolutionary diversification of trills. In particular, I explore the role of constraints associated with vocal tract modulations during song production and evolution. Identification of this constraint may enhance our ability to explain particular patterns of trill evolution.

Female mate choice based upon male motor performance

Our goal in this essay is to review the hypothesis that females choose mates by the evaluation of male motor performance. We define motor performance as vigour, the ability to perform energetically expensive acts repeatedly, or as skill, the ability to perform difficult motor tasks well. Motor performance reflects most aspects of whole-organism performance that relate to survival, and thus should indicate, more reliably than ornaments do, individual male genetic quality and/or developmental history. Male sexual displays in many animal taxa contain elements of vigour and/or skill, and accumulating evidence suggests that females choose mates in nature based upon their evaluations of male motor performance. We note that male ornaments in many species are accompanied by conspicuous motor display, and we propose that ornaments often arise secondarily as a way to enhance the apparent skill or vigour of male motor performance. More and better methods to measure male vigour and skill are needed, as well as additional studies on the abilities of females to make discriminations of this type.

The Evolution of Geographic Variation in Birdsong

Publisher Summary This chapter outlines the evolution of geographic variation in birdsong. Studies of vocal signals in birds offer potentially useful opportunities for empirical tests of the relationships among geographic signal divergence, reproductive isolation, and speciation. The goal of the chapter is to evaluate, from both empirical and conceptual perspectives, the factors that facilitate the evolution of geographic variation in bird vocalizations. The chapter focuses on the evolution of song dialects and emphasizes the functional hypotheses to explain their evolution. An alternative set of hypotheses suggests that the song features may diverge through by‐product scenarios, in which selection for non-recognition functions drives incidental changes in song structure, and geographic variation. The chapter also describes the scenarios by which songs may diverge indirectly through selection on components of the vocal apparatus such as body size and beak form and function.

Possible human impacts on adaptive radiation: beak size bimodality in Darwin's finches

Adaptive radiation is facilitated by a rugged adaptive landscape, where fitness peaks correspond to trait values that enhance the use of distinct resources. Different species are thought to occupy the different peaks, with hybrids falling into low-fitness valleys between them. We hypothesize that human activities can smooth adaptive landscapes, increase hybrid fitness and hamper evolutionary diversification. We investigated this possibility by analysing beak size data for 1755 Geospiza fortis measured between 1964 and 2005 on the island of Santa Cruz, Galápagos. Some populations of this species can display a resource-based bimodality in beak size, which mirrors the greater beak size differences among species. We first show that an historically bimodal population at one site, Academy Bay, has lost this property in concert with a marked increase in local human population density. We next show that a nearby site with lower human impacts, El Garrapatero, currently manifests strong bimodality. This comparison suggests that bimodality can persist when human densities are low (Academy Bay in the past, El Garrapatero in the present), but not when they are high (Academy Bay in the present). Human activities may negatively impact diversification in ‘young’ adaptive radiations, perhaps by altering adaptive landscapes.

On the relationship between, and measurement of, amplitude and frequency in birdsong

A growing number of studies ask whether and how bird songs vary between areas with low versus high levels of anthropogenic noise. Across numerous species, birds are seen to sing at higher frequencies in urban versus rural populations, presumably because of selection for higher-pitched songs in the face of low-frequency urban noise, or because birds can avoid masking directly by shifting to higher-frequency sounds (Fernandez-Juricic et al. 2005; Slabbekoorn & den Boer-Visser 2006; Nemeth & Brumm 2009; Gross et al. 2010; Potvin et al. 2010). In addition to changing song frequency, birds are also reported to respond to increased background noise by singing at higher amplitudes (Brumm & Zollinger 2011). Nightingales, Luscinia megarhynchos, for example, sing with a higher sound pressure level in areas with intense traffic noise as compared to quieter locations (Brumm 2004). While frequencyand amplitude-based responses to ambient noise are often considered independently, the twomight also vary in tandem

Reproductive isolation of sympatric morphs in a population of Darwin's finches

Recent research on speciation has identified a central role for ecological divergence, which can initiate speciation when (i) subsets of a species or population evolve to specialize on different ecological resources and (ii) the resulting phenotypic modes become reproductively isolated. Empirical evidence for these two processes working in conjunction, particularly during the early stages of divergence, has been limited. We recently described a population of the medium ground finch, Geospiza fortis, that features large and small beak morphs with relatively few intermediates. As in other Darwins finches of the Galápagos Islands, these morphs presumably diverged in response to variation in local food availability and inter- or intraspecific competition. We here demonstrate that the two morphs show strong positive assortative pairing, a pattern that holds over three breeding seasons and during both dry and wet conditions. We also document restrictions on gene flow between the morphs, as revealed by genetic variation at 10 microsatellite loci. Our results provide strong support for the central role of ecology during the early stages of adaptive radiation.

Vocal mechanics in Darwin's finches: correlation of beak gape and song frequency

SUMMARY Recent studies of vocal mechanics in songbirds have identified a functional role for the beak in sound production. The vocal tract (trachea and beak) filters harmonic overtones from sounds produced by the syrinx, and birds can fine-tune vocal tract resonance properties through changes in beak gape. In this study, we examine patterns of beak gape during song production in seven species of Darwins finches of the Galápagos Islands. Our principal goals were to characterize the relationship between beak gape and vocal frequency during song production and to explore the possible influence therein of diversity in beak morphology and body size. Birds were audio and video recorded (at 30 frames s–1) as they sang in the field, and 164 song sequences were analyzed. We found that song frequency regressed significantly and positively on beak gape for 38 of 56 individuals and for all seven species examined. This finding provides broad support for a resonance model of vocal tract function in Darwins finches. Comparison among species revealed significant variation in regression y-intercept values. Body size correlated negatively with y-intercept values, although not at a statistically significant level. We failed to detect variation in regression slopes among finch species, although the regression slopes of Darwins finch and two North American sparrow species were found to differ. Analysis within one species (Geospiza fortis) revealed significant inter-individual variation in regression parameters; these parameters did not correlate with song frequency features or plumage scores. Our results suggest that patterns of beak use during song production were conserved during the Darwins finch adaptive radiation, despite the evolution of substantial variation in beak morphology and body size.

Evolution of bite force in Darwin's finches: a key role for head width

Studies of Darwins finches of the Galápagos Islands have provided pivotal insights into the interplay of ecological variation, natural selection, and morphological evolution. Here we document, across nine Darwins finch species, correlations between morphological variation and bite force capacity. We find that bite force correlates strongly with beak depth and width but only weakly or not at all with beak length, a result that is consistent with prior demonstrations of natural selection on finch beak morphology. We also find that bite force is predicted even more strongly by head width, which exceeds all beak dimensions in predictive strength. To explain this result we suggest that head width determines the maximum size, and thus maximum force generation capacity of finch jaw adductor muscles. We suggest that head width is functionally relevant and may be a previously unrecognized locus of natural selection in these birds, because of its close relationship to bite force capacity.

Chapter 5 Vocal Performance and Sensorimotor Learning in Songbirds

Abstract This chapter addresses the interplay of vocal performance, sensorimotor learning, and vocal evolution in songbirds. Vocal performance is increasingly recognized as an influential factor in song evolution, particularly with respect to vocal output, song consistency, and trill structure. We argue here that a comprehensive understanding of vocal performance requires attention to sensorimotor learning, a developmental phase during which birds attempt to reproduce song models memorized earlier in life. New research indicates that birds calibrate song structure during sensorimotor ontogeny in order to best match their own vocal performance capacities. Because of this relationship, performance-related features may provide reliable indicators of male quality as manifest during sensorimotor learning. We review evidence in support of the “developmental stress” hypothesis and propose that this hypothesis be expanded to also consider vocal features crystallized during sensorimotor learning. We suggest avenues for future research that document relationships between vocal performance, morphology, and song learning programs.