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Parasites can exert strong selection on hosts. Th roughout the year migrants are exposed to diff erent sets of parasites, which may aff ect life history traits such as migratory schedules. Here, we studied the relationship between parasite infection and arrival date of blackcaps Sylvia atricapilla to their breeding grounds in Germany throughout a period of six years (2007 – 2012). We used two data sets, one that included all blackcaps and one that included only recaptured birds. We assesed whether parasites infl uence spring arrival to breeding grounds, and for the recaptured data set, we analysed temporal variation in parasitism (i.e. infection status and parasitaemia) throughout the breeding season. We used both microscopy and PCR (a fragment of ∼ 479 bp of the mtDNA cyt b) to determine haemosporidian infection. Blackcaps were mostly infected with Haemoproteus parabelopolskyi (lineages SYAT01 and SYAT02). Infection status, but not parasitaemia, was constant through time for individual birds; meaning that once a bird is infected, it most likely will retain the infection for life. We found that infection by haemosporidian parasites has no relationship to arrival date in this blackcap population; however, infection by H. parabelopolskyi has a marginally signifi cant eff ect on arrival date of recaptured blackcaps, somewhat delaying their arrival to breeding grounds. Birds captured later in the season were more likely to be infected than those from early spring, and parasitaemia was frequently lower in birds captured earlier in the season compared to those captured later (summer).

Long-distance migrations influence the physiology, behavior, and fitness of migratory animals throughout their annual cycles, and fundamentally alter their interactions with parasites. Several hypotheses relating migratory behavior to the likelihood of parasitism have entered the literature, making conflicting, testable predictions. To assess how migratory behavior of hosts is associated with parasitism, we compared haemosporidian parasite infections between two closely related populations of a common North American sparrow, the dark-eyed junco, that co-occur in shared habitats during the non-breeding season. One population is sedentary and winters and breeds in the Appalachian Mountains. The other population is migratory and is found in seasonal sympatry with the sedentary population from October through April, but then flies (≥ 900 km) northwards to breed. The populations were sampled in the wild on the shared montane habitat at the beginning of winter and again after confining them in a captive common environment until the spring. We found significantly higher prevalence of haemosporidian parasite infections in the sedentary population. Among infected juncos, we found no difference in parasite densities (parasitemias) between the sedentary and migrant populations and no evidence for winter dormancy of the parasites. Our results suggest that long-distance migration may reduce the prevalence of parasite infections at the population level. Our results are inconsistent with the migratory exposure hypothesis, which posits that long-distance migration increases exposure of hosts to diverse parasites, and with the migratory susceptibility hypothesis, which posits that trade-offs between immune function and migration increase host susceptibility to parasites. However, our results are consistent with the migratory culling hypothesis, which posits that heavily infected animals are less likely to survive long-distance migration, and with the migratory escape hypothesis, which posits that long-distance migration allows host populations to seasonally escape areas of high infection risk.

Black-headed grosbeaks (Pheucticus melanocephalus) and rose-breasted grosbeaks (Pheucticus ludovicianus) are passerine bird species known to hybridize in the Great Plains of North America. Both extrinsic (environmental) and intrinsic factors (pre- and postzygotic reproductive isolation) have been credited for the generation and maintenance of the grosbeak hybrid zone, but little is known about the genetic characteristics of this hybrid zone. To investigate the stability and extent of the grosbeak hybrid zone, we constructed clines from both molecular sequence data (mtDNA, three autosomal intron loci, and one Z-linked locus) and morphological data (morphometric analyses and hybrid index scores) to determined zone centre and width. Hybrid zone centre and width were also determined for samples collected across the zone 40 years ago from morphological data. The present and past clines were compared and provided support for stability in hybrid zone location and width, and the evolutionary implications of this are discussed. Three models of hybrid zone maintenance were investigated to consider the influence of intrinsic and extrinsic factors on this zone. Our results suggest low hybrid frequencies, a stable zone location and narrow width, and reduced hybrid fitness over the past 40 years best categorize the grosbeak hybrid zone as a tension zone.

Migratory divides are thought to facilitate behavioral, ecological, and genetic divergence among populations with different migratory routes. However, it is currently contentious how much genetic divergence is needed to maintain distinct migratory behavior across migratory divides. Here we investigate patterns of neutral genetic differentiation among Blackcap (Sylvia atricapilla) populations with different migratory strategies across Europe. We compare the level of genetic divergence of populations migrating to southwestern (SW) or southeastern (SE) wintering areas with birds wintering in the British Isles following a recently established northwesterly (NW) migration route. The migratory divide between SW and SE wintering areas can be interpreted as a result of a re-colonization process after the last glaciation. Thus we predicted greater levels of genetic differentiation among the SW/SE populations. However, a lack of genetic differentiation was found between SW and SE populations, suggesting that interbreeding likely occurs among Blackcaps with different migratory orientations across a large area; therefore the SW/SE migratory divide can be seen as diffuse, broad band and is, at best, a weak isolating barrier. Conversely, weak, albeit significant genetic differentiation was evident between NW and SW migrants breeding sympatrically in southern Germany, suggesting a stronger isolating mechanism may be acting in this population. Populations located within/near the SW/SE contact zone were the least genetically divergent from NW migrants, confirming NW migrants likely originated from within the contact zone. Significant isolation-by-distance was found among eastern Blackcap populations (i.e. SE migrants), but not among western populations (i.e. NW and SW migrants), revealing different patterns of genetic divergence among Blackcap populations in Europe. We discuss possible explanations for the genetic structure of European Blackcaps and how gene flow influences the persistence of divergent migratory behaviors