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.

Age is often a neglected factor in ecological studies. However, age-related changes in reproduction and survival of organisms may strongly influence population dynamics. The Gotlandic population of collared flycatchers is a perfect system for studying age-related changes in the wild, as the exact age and reproductive history of most individuals is known. Collared flycatchers (Ficedula albicollis) on Gotland show the typical pattern of age-related changes in survival and reproductive success; both factors show an increase early in life and a decrease late in life. This thesis presents a broad study not only of age-related patterns of reproduction and immunity, but also proposes the mechanisms driving these patterns. My results show that in addition to survival probability and reproductive performance, reproductive costs and life-history trade-offs also change with progressing age. There is a significant increase in reproductive performance at the population level during first years of life which result from selection against low quality phenotypes. On the individual level this pattern is best explained by an optimization of reproductive effort. However, high quality individuals have higher reproductive success as early as their first breeding event and are long-lived. Thus, they seem to adopt a different strategy than lower quality, short-lived individuals. Differences in individual quality seem to be shaped by the developmental conditions experienced as nestlings. Fledglings with longer tarsi, but lower body mass become long-lived, high quality adults. Young individuals breeding for first time pay higher costs of reproduction. They also express a limited ability to reduce these costs by breeding in high quality territories when compared to middle-aged individuals. Young individuals seem to invest more into self-maintenance, whereas old individuals reduce the level of self-maintenance (measured as immune response) and redistribute their investment towards reproduction. Thus, old individuals are limited in their ability to reduce reproductive costs under favorable conditions, especially as they also senesce, which pattern is also shaped by individual quality. Variation in individual quality appears to have an strong effect on age-related survival probability, reproductive performance, reproductive costs, and even life-history decisions. Therefore, taking this factor into account in studies of life-history patterns is necessary to obtain reliable results.