Male birds of many species use conspicuous song and plumage displays in both courtship and territorial interactions. In some species, one or both of these signalling traits may not reach full adult maturity until a male’s second year of life. While the prevalence of delayed plumage maturation is well documented, delayed song maturation may be more difficult to detect. As a result, there are few studies which report age-based song differences between first-year and adult males. Additionally, despite the potentially large degree of variation of each trait within yearling males, little work has examined the benefits for young males who appear or sound more adult-like. Here, I investigate variation in both song and plumage displays of yearling male American redstarts (Setophaga ruticilla) as they relate to success during the breeding and non-breeding seasons. I first demonstrate a relationship between the degree of adult-like black plumage and both non-breeding season habitat quality in Jamaica and breeding season arrival date in Ontario. Previous studies have linked breeding season arrival date with winter habitat quality in adult males using stable-carbon isotope analysis. Together, these results suggest that variation in yearling male appearance may signal an individual’s competitive ability for high-quality resources. Next, I quantified the mate-attraction songs of both adult and yearling males and demonstrate a delayed maturation in this song type. I also present evidence of the potential benefits of expressing a more adult-like song by linking song structure with reproductive success in adult males. Finally, I demonstrate a potential relationship between the degree of adult-like song and plumage expression in yearling males, but not adult males. This work demonstrates that the delayed maturation of sexual signals may play an important role in the life-history of yearling male American redstarts, and highlights the need for in-depth analyses of individual variation of multiple sexual signals in this poorly-studied age class of birds. / Thesis (Master, Biology) -- Queen's University, 2009-09-25 10:42:19.794

While female mimicry and lower status signaling hypotheses of delayed plumage maturation have received much discussion in the literature, the experimental tests of these hypotheses have been infrequent. Those experimental tests often use a simulated intruder method with artificial model intruders rather than using live conspecific birds as intruders. Subadult male painted buntings (Passerina ciris) possess delayed plumage maturation where they appear visually identical to adult females during their first potential breeding season, while adult males are strikingly different in plumage coloration. Here I test the behavioral responses in a territorial population of painted buntings that exhibits extreme delayed plumage maturation using a simulated territorial intrusion experiment to measure territorial male behavioral response when presented with live caged intruders of both subadult and adult males. Territorial adult males were significantly more likely to initiate an attack and continue to attack caged adult male intruders than compared to caged subadult male intruders. This result supports both the female mimicry and status signaling hypotheses, and does not support the cryptic hypothesis. Additionally, in anecdotal observations, territorial males occasionally performed mating display behaviors to caged subadult male intruders. These results further suggest that territorial male painted buntings may identify subadult males as potential mates, supporting the female mimicry hypothesis for subadult males in this species. To what degree subadult males may benefit from DPM deserves further study.

<p>Studies of hybrid zones offer important insights into the process of speciation. Much of the knowledge to be gained is dependent on an accurate estimation of the strength of pre- and post-zygotic isolation between hybridizing taxa. My results demonstrate that hybridization can variously affect different components of fitness. In Ficedula flycatchers, late-breeding females may directly benefit from pairing with a heterospecific male by gaining access to superior territories. The hybrid offspring possess an immune system that is as equally well functioning as in the parental species (the collared, F. albicollis, and pied flycatcher, F. hypoleuca). However, I found that a severe reduction in fertility persists for at least three generations after the actual hybridization event. Combining all information about the reproductive success of hybridizing individuals and their descendents revealed that postzygotic isolation between flycatchers is very strong; hybridizing individuals leave almost no descendents. This thesis presents one of few comprehensive summaries of the selection for/against assortative mating in a natural hybrid zone. These findings suggest a central role for intrinsic postzygotic isolation as a reproductive barrier separating newly evolved bird species, and contrast previous suggestions that postmating isolation is the slowest of the reproductive barriers to evolve in birds.</p><p>Despite this strong selection against hybridization, pre-mating isolation is incomplete. Hybridization often results from females lacking conspecific partners, but appears to be also caused by errors in species recognition. Much of this error probably reflects the short period of time that pied flycatchers on Gotland and Öland have been in sympatry. Compared to collared flycatchers, pied flycatchers are poorer able to discriminate between conspecific and heterospecific song, and male pied flycatchers more often falsely signal their own identity through heterospecific song copying. However, despite colonising the study site from other sympatric populations and having very little gene flow from allopatry, collared flycatchers also possess traits (e.g. delayed plumage maturation) that increase their hybridization risk. Once pre-mating isolation is strong, the rarity of hybridization probably inhibits further selection against traits promoting interspecific mating, especially when such traits may be beneficial in other contexts. This thesis highlights complex interactions between factors affecting hybridization rate that would not be detected if such a study were not field-based. Furthermore, it showcases likely examples in nature of a number of theoretical objections to the evolution of pre-mating barriers between populations living in sympatry.</p>

Studies of hybrid zones offer important insights into the process of speciation. Much of the knowledge to be gained is dependent on an accurate estimation of the strength of pre- and post-zygotic isolation between hybridizing taxa. My results demonstrate that hybridization can variously affect different components of fitness. In Ficedula flycatchers, late-breeding females may directly benefit from pairing with a heterospecific male by gaining access to superior territories. The hybrid offspring possess an immune system that is as equally well functioning as in the parental species (the collared, F. albicollis, and pied flycatcher, F. hypoleuca). However, I found that a severe reduction in fertility persists for at least three generations after the actual hybridization event. Combining all information about the reproductive success of hybridizing individuals and their descendents revealed that postzygotic isolation between flycatchers is very strong; hybridizing individuals leave almost no descendents. This thesis presents one of few comprehensive summaries of the selection for/against assortative mating in a natural hybrid zone. These findings suggest a central role for intrinsic postzygotic isolation as a reproductive barrier separating newly evolved bird species, and contrast previous suggestions that postmating isolation is the slowest of the reproductive barriers to evolve in birds. Despite this strong selection against hybridization, pre-mating isolation is incomplete. Hybridization often results from females lacking conspecific partners, but appears to be also caused by errors in species recognition. Much of this error probably reflects the short period of time that pied flycatchers on Gotland and Öland have been in sympatry. Compared to collared flycatchers, pied flycatchers are poorer able to discriminate between conspecific and heterospecific song, and male pied flycatchers more often falsely signal their own identity through heterospecific song copying. However, despite colonising the study site from other sympatric populations and having very little gene flow from allopatry, collared flycatchers also possess traits (e.g. delayed plumage maturation) that increase their hybridization risk. Once pre-mating isolation is strong, the rarity of hybridization probably inhibits further selection against traits promoting interspecific mating, especially when such traits may be beneficial in other contexts. This thesis highlights complex interactions between factors affecting hybridization rate that would not be detected if such a study were not field-based. Furthermore, it showcases likely examples in nature of a number of theoretical objections to the evolution of pre-mating barriers between populations living in sympatry.