This thesis investigates the ecological and evolutionary implications of a recent secondary contact between two closely related bird species: collared (Ficedula albicollis) and pied (F. hypoleuca) flycatchers. Collared flycatchers started to colonize the Swedish island of Öland, where pied flycatchers were already present, in the late 1950s-early1960s. My major aims were to investigate which factors are acting against versus for long-term coexistence between the two species. Specifically, I investigated the relative importance of allopatric divergence, interspecific competition, hybridization and learning in promoting or inhibiting coexistence. The combined effects of interspecific competition and hybridization drives pied flycatchers towards local extinction in their preferred deciduous habitat. However, my results also show that pied flycatchers are better able to tolerate harsh environmental conditions. This trade-off between competitive ability and resilience in the face of harsh conditions facilitates a regional coexistence between the species. Coexistence is furthermore favoured by competition-mediated divergence in breeding habitat choice, timing of breeding and male breeding plumage colouration. Due to interspecific competition, male pied flycatchers are forced to breed in a more mixed forest type with a later peak in food abundance, which is accompanied by a divergence in breeding time between the two species. In areas shared with collared flycatchers, male pied flycatchers with brown plumage coloration, most divergent from that of collared flycatchers, are favoured by selection. In addition to facilitating coexistence, the observed shift in habitat occupancy increases reproductive isolation between the two species. By using cross-fostering experiments I demonstrate that natal habitat imprinting has the potential to additionally speed up habitat segregation. Finally I show that hybrid nestlings express an intermediate response to harsh environments, indicating that another aspect of ecological-based selection may be important in reproductive isolation between the species. In summary, my results show that adaptations during historic allopatry are important both in facilitating coexistence as well as in providing a foundation for further ecological divergence at secondary contact. This is of relevance today as many species are shifting their distributions in response to habitat disturbance and global warming.

Mining is a widespread industrial activity that in many cases, via mining wastes, leads to altered concentrations of metals in close vicinity to the mining activities. Metals in mining waste can have high toxicity and may persist in environments for long time periods. The presence of metals, such as lead (Pb), is known to contaminate and cause damage to nearby organisms and ecosystems. Birds are at risk of metal contamination and, since they are predators high up in the food chain, may face accumulation of metal in tissue over time, via consumption of contaminated prey. Small passerine species, such as the pied flycatcher (Ficedula hypoleuca), are suitable for studying metal contamination as they are ubiqutious, and high up in the food chain. Pied flycatchers feed exclusively on invertebrates, making the abundance, quality, and potential contamination of invertebrates interesting to study with regard to the health of the birds. Studies have shown that hemoglobin (Hb) levels in young pied flycatchers are reduced by high background levels of Pb.  This may be linked to prey availability and quality, as invertebrates are known to alter their composition, and contain higher Pb concentrations in Pb contaminated areas. Here, I investigated how invertebrate abundance and community composition, and pied flycatcher Hb concentrations (i.e. health), in reference and Pb contaminated areas, were related to each other. Invertebrate traps were set and sampled twice during the summer of 2018 to provide invertebrate data to the study. Bird Hb levels was acquired by taking blood samples from nearly fledged chicks in birdhouses placed in the different areas. I found a potential trend towards higher Hb levels in reference areas (p=0.110), suggesting that bird health is reduced by the presence of Pb, but this could not be explained by differences in invertebrate community composition or abundance (p>0.05). Hence, based on this study, high Pb concentrations in the soil does not directly, or indirectly via potential impacts on the prey community composition, influence the health of pied flycatchers.

<p>Abstrakt: Jag har undersökt hur älvreglering påverkar mängden knott (Tvåvingar: <i>Simuliidae</i>) längs flera norrlandsälvar och vilka konsekvenser denna påverkan får för den landlevande fågelfaunan. Studien visar att utbyggnad av älvar har en negativ effekt på mängden knott. Det fanns sju gånger högre tätheter av knott längs "orörda älvar" jämfört med utbyggda älvar.</p><p>Skillnaden i antal knott mellan älvtyperna var mycket större för knotthanar än för knotthonor, vilket förklaras av att honorna är rörligare eftersom de flyger och letar efter blod. Knott lever av blod från ett flertal däggdjur och fåglar och visar en stor variation i värd-specificitet mellan olika arter. Jag fann en högre frekvens av haemosporida blod parasiter (<i>Leucocytozoon</i>) i fåglar längs "orörda" Vindelälven jämfört med den reglerade Umeälven. Detta mönster stämmer väl överens med tätheten fågelparasiterande knott som är lägre längs Umeälven. Knott attackerade främst värdar som var stora och vanliga. Fågelsamhällets sammansättning skiljde sig mellan orörda och utbyggda älvdalar. Tätheten av icke insektsätande fåglar tenderade att minska, efter häcksäsongen, längs orörda älvar medan den ökade längs utbyggda älvar. Den insektsätande fågeln svartvit flugsnappare (<i>Ficedula hypoleuca Pallas</i>) uppvisade större häckningsframgång längs orörda älvar jämfört med utbyggda älvar. Detta är förmodligen en effekt av den större mängden insekter som finns längs orörda älvar.</p> / <p>The effects of river regulation on blackfly (Diptera: <i>Simuliidae</i>) abundances and consequences for the avifauna in terrestrial environments were studied along multiple rivers in northern Sweden. I found that impoundment of rivers has detrimental effect to blackfly abundances. The densities of large-river breeding blackfly species were several-fold higher along free-flowing than along regulated rivers. The difference in abundances was much larger in males than blood-seeking females. Blackflies attacked a variety of mammalian and avian hosts and showed different levels of host-specificity between species. I found higher prevalence of haemosporidian blood parasites (<i>Leucocytozoon</i>) in birds along the free-flowing Vindel River in comparison to the regulated Ume River, where the lower densities of ornithophilic blackflies were reduced. Blood-seeking blackflies predominantly attacked large and/abundant hosts.</p><p>Assemblages of birds differed between valleys of regulated and free-flowing rivers. Densities of noninsectivorous birds tended to decrease along free-flowing rivers in the post-breeding season, but increased along regulated rivers at the same time. Insectivorous European pied flycatcher (<i>Ficedula hypoleuca Pallas</i>) showed greater fledging success along free-flowing than regulated rivers, which probably reflects the higher insect abundances found along freeflowing rivers.</p>

Abstrakt: Jag har undersökt hur älvreglering påverkar mängden knott (Tvåvingar: Simuliidae) längs flera norrlandsälvar och vilka konsekvenser denna påverkan får för den landlevande fågelfaunan. Studien visar att utbyggnad av älvar har en negativ effekt på mängden knott. Det fanns sju gånger högre tätheter av knott längs "orörda älvar" jämfört med utbyggda älvar. Skillnaden i antal knott mellan älvtyperna var mycket större för knotthanar än för knotthonor, vilket förklaras av att honorna är rörligare eftersom de flyger och letar efter blod. Knott lever av blod från ett flertal däggdjur och fåglar och visar en stor variation i värd-specificitet mellan olika arter. Jag fann en högre frekvens av haemosporida blod parasiter (Leucocytozoon) i fåglar längs "orörda" Vindelälven jämfört med den reglerade Umeälven. Detta mönster stämmer väl överens med tätheten fågelparasiterande knott som är lägre längs Umeälven. Knott attackerade främst värdar som var stora och vanliga. Fågelsamhällets sammansättning skiljde sig mellan orörda och utbyggda älvdalar. Tätheten av icke insektsätande fåglar tenderade att minska, efter häcksäsongen, längs orörda älvar medan den ökade längs utbyggda älvar. Den insektsätande fågeln svartvit flugsnappare (Ficedula hypoleuca Pallas) uppvisade större häckningsframgång längs orörda älvar jämfört med utbyggda älvar. Detta är förmodligen en effekt av den större mängden insekter som finns längs orörda älvar. / The effects of river regulation on blackfly (Diptera: Simuliidae) abundances and consequences for the avifauna in terrestrial environments were studied along multiple rivers in northern Sweden. I found that impoundment of rivers has detrimental effect to blackfly abundances. The densities of large-river breeding blackfly species were several-fold higher along free-flowing than along regulated rivers. The difference in abundances was much larger in males than blood-seeking females. Blackflies attacked a variety of mammalian and avian hosts and showed different levels of host-specificity between species. I found higher prevalence of haemosporidian blood parasites (Leucocytozoon) in birds along the free-flowing Vindel River in comparison to the regulated Ume River, where the lower densities of ornithophilic blackflies were reduced. Blood-seeking blackflies predominantly attacked large and/abundant hosts. Assemblages of birds differed between valleys of regulated and free-flowing rivers. Densities of noninsectivorous birds tended to decrease along free-flowing rivers in the post-breeding season, but increased along regulated rivers at the same time. Insectivorous European pied flycatcher (Ficedula hypoleuca Pallas) showed greater fledging success along free-flowing than regulated rivers, which probably reflects the higher insect abundances found along freeflowing rivers.