The limits of species recognition: heterospecific song learning in pied flycatchers Maria Triantafyllidou Degree project in biology, Bachelor of science, 2016 Examensarbete i biologi 15 hp till kandidatexamen, 2016 Biology Education Centre and Department of Ecology and Genetics/Animal Ecology, Uppsala University Supervisors: Dr. Anna Qvarnström and Dr. David Wheatcroft ABSTRACT The closely related species pied flycatcher (Ficedula hypoleuca) and collared flycatcher (F. albicollis) co-occur on the Swedish island of Öland, where they compete over similar resources. The majority of male pied flycatchers have been found to incorporate elements of the collared flycatcher song in their repertoire. Given that birdsong is partly inherited and partly learned, the relative contribution of genetic predispositions versus acoustic stimuli varies across different species. The results show that in pied flycatchers, song acquisition is tightly correlated with imprinting, and can therefore be greatly influenced by heterospecific tutors in their surroundings, i.e. male collared flycatchers. I found that pied males are capable of not only memorizing collared song elements, but also producing them with high fidelity. Thus, I infer that pied flycatchers are characterized by a high degree of vocal plasticity. INTRODUCTION The importance of sexual signals in speciation It is largely recognized that sexual signals play a key role in mate recognition as they indicate species identity and mate quality. It has been increasingly appreciated that they also play a significant role in patterns of speciation (Slabbekoorn and Smith 2002, Ritchie 2007, Verzijden et al. 2012). That is linked with the fact that sex traits evolve quickly and are therefore likely to diverge among closely related species, eventually leading to reproductive isolation (Qvarnström et al. 2006, Kraaijeveld et al. 2011). Sex traits arise by sexual selection (named “intersexual selection” by Charles Darwin), which is the result of male competition for copulation with the opposite sex. Female mating discriminability, as male signaling, is complex and subject to evolution, and thereby there is a well-established tradeoff between female preferences for sexual signals and male signaling traits (Ritchie 1996, Michaelidis et al. 2006, Ron 2008). Given that diversification is a prerequisite for speciation, divergence either in male sexual signals or in female preferences represent cases in which pre-zygotic reproductive isolation gets promoted and consequently induces speciation (Ritchie 2007, Kraaijeveld et al. 2011, Verzijden et al. 2012). Signal evolution, thereby, acts as a driver of speciation (Ritchie 2007, Seddon and Tobias 2010). This tradeoff, upon which the elaborate courtship signaling system is based, is the driver of sexual se- lection. When learning is involved as an agent of sexual selection, various challenges are introduced in the system. Sexual imprinting, a process whereby mate preferences are under the influence of learning, renders the maintenance of species-specific signals particularly challenging (Verzijden et al. 2012). At 1 the same time, sexual traits themselves can also be affected by learning. Thus, learned sexual traits and sexually imprinted preferences constitute models useful in understanding the tradeoffs of attracting fe- males while at the same time avoiding interspecific matings. By having preferences and selecting their mate, females establish reproductive isolation. Keeping those female preferences species-specific is of paramount importance with regard to evolution and speciation. Even though they are normally easily maintained in traits such as plumage, they can easily break down in traits that are learned and acquired. One such trait is birdsong, which is species-specific and functions in species recognition (Emlen 1972). On that basis, maintaining song differences across species is greatly important yet significantly challenging. Bird song Song in birds is a unique and intricate trait that has received a lot of scientific attention and has led to various researches and proposed hypotheses over the years. It serves a dual purpose: female attraction with the ultimate goal of copulation, and territory defense. The latter ties back to the former, since the exclusive access to a territory is tightly correlated to female settlement and nesting (Nowicki and Searcy 2004). Birdsong constitutes a sexual signal and is therefore characterized by great variation not only across species but also across individuals. Given that a song is an elaborate conglomeration of numerous parameters (such as pitch, length, trill rate, etc.), interspecific and intraspecific variation is only to be expected. A major source of variation in birdsong is linked to geographic divergence, i.e. the result of the process of different populations adapting to distinct environments (Slabbekoorn and Smith 2002, Podos and Warren 2007). Females base their mate choice on those features that are reliably correlated with condition and quality of the male (Nowicki and Searcy 2004). Birdsong is a sexual signal that serves as an indicator of the signaler's quality and is therefore broadly used by females in the evaluation process. Assessment of vocal performance is based upon vocal capabilities which reflect a male's ability to provide benefits to the female. Such benefits can be either direct and associated with better parental care and access to a better territory, or indirect, such as the transfer of robust genes to the offsprings that will increase their chances for survival and reproductive success (Ballentine et al. 2004, Nowicki and Searcy 2004). It is now broadly accepted that birdsong is learned through cognitive, imprinting-like processes (Grant and Grant 1997, Cynx and von Rad 2001, Williams 2004), i.e. through exposure to a tutor's vocal performances during the early post-hatching stage of a bird's life which represents the sensitive 2 period for perceiving and storing information. Clayton (1989) performed a series of cross-fostering experiments showing that birds raised by a foster father have the capacity to learn and identically produce elements of their tutor's song despite the heterospecific nature of the song. On that account, bird song is a culturally inherited trait largely dependent on the hatchling's acoustic surroundings. Nonetheless, it cannot be disregarded that there are certain constraints on what can be learned, that are correlated with species-specific innate predispositions (Kroodsma and Miller 1982, Clayton 1989, Nowicki et al. 2001, Ballentine et al. 2004). In other words, birds inherit a song template from their parents which is later influenced by input from the environment. How much of this template is inherited and what kind of genetic constraints it introduces to the system are subject to discussion (Marler 1997, Beecher et al. 2010). There is a tendency, more scientifically described as “innate predispositions”, for individuals to learn and internalize the song of the species they belong to, i.e. the conspecific song (Marler 1997, Doupe and Kuhl 1999). Baptista (1996) showed that even cross- fostered individuals preferentially learn the song of conspecifics rather than the song of their foster parents. When considered in the context of evolution, it becomes clear that those predispositions have a well-defined evolutionary significance. Conspecific song preference is essential for species recognition which constitutes the main driver of reproductive isolation (Grant and Grant 1997). The degree of maintenance of reproductive isolation, thenceforth, will determine the speciation patterns among different species. Given that birdsong is the outcome of a learning process during which sexual cues are being imprinted and mating preferences begin to form, various challenges are inevitably introduced into the system. When this intricate imprinting process is based upon parental and thus conspecific tutors, assortative mating is promoted. When, however, random heterospecific individuals influence the learning process, this balance is disturbed (Slabbekoorn and Smith 2002, Verzijden et al. 2012). This is where mixed singing comes into the picture. Mixed singing Traits that are at least partly learned, such as birdsong, unlike inherited traits strictly constrained by the genotype, have the potential to be subject to rapid change (Irwin et al. 2001). Change can occur in the form of either divergence, in which case rapid evolution is induced (Verzijden et al. 2012), or con- vergence, i.e. when different species learn from one another (Cody 1973, Helb et al. 1985). A good example of such a change is song convergence in sympatric bird species, widely known by the term “mixed singing” (Helb et al. 1985). Mixed singing occurs when a bird from one species partly 3 or completely learns and produces elements from the song of another species with which it is closely related and lives in close proximity. Such cases are commonly observed in natural systems where relatively recently speciated species are brought into secondary contact and ecological similarity leads to sympatry. With the possibility of introgression ruled out (Haavie et al. 2004, Vokurková et al. 2013), all evidence points towards a case of vocal convergence induced by social interactions. Selective learning of the conspecific song translates to learning and producing acoustic cues that lie within the structural (frequency range, amount of modulations, etc.) and temporal (note length, tempo, etc.) patterning that