The Sylviids are small to medium-sized birds. It comprises numerous genera, of which only two exhibit hybridization, namely Sylvia and Paradoxornis.
Most work focused on the Blackcap (S. atricapilla). First, experimental crosses of European migratory birds and African partially migratory birds uncovered the genetic basis of migration-related traits, such as migratory restlessness (Berthold & Querner, 1981), testes cycle (Berthold & Querner, 1993), winter moult (Berthold, Mohr & Querner, 1994), and wing morphology (Fiedler, 2005).
Later on, the focus turned towards the migratory divide of Central European Blackcaps. Some birds migrate in a south-western (SW) direction, while others prefer a south-eastern (SE) migration route. The orientation of F1-offspring was intermediate compared to the parental species (Helbig, 1991a). Surprisingly, some hand-reared birds from the contact zone choose a north-western (NW) orientated migration route (Helbig, 1991b). This new migration route, with a wintering quarter in Britain, was confirmed by ring recoveries and experiments in captivity (Berthold et al., 1992; Helbig, 1994; Helbig, 1996; Helbig et al., 1994).
Several genetic studies compared populations with the different migration strategies. Genetic divergence in sympatry exceeded that of allopatric Blackcaps and is associated with phenotypic divergence. This suggests that restricted gene flow accelerates the evolution of adaptive phenotypic divergence (Rolshausen et al., 2009). When comparing SE-SW migration with NW-SW migration, one expects higher divergence in the former because this migratory divide is older. However, Mettler et al. (2013) found a lack of genetic differentiation in the SE-SW migratory divide, while the NW-SW migratory divide showed significant genetic differentiation. This result suggests that there is a stronger isolating mechanism in the latter migratory divide. Rolshausen et al. (2010) investigated whether allochrony (differential arrival at breeding grounds) might be a strong isolating mechanism, but they could not find evidence for this hypothesis.
The Spectacled Warbler (S. conspicillata) is separated into 14 populations and comprises two distinct genetic groups. Coalescent analyses showed significant gene flow between these groups (Illera et al., 2014).
Berthold, P., Helbig, A. J., Mohr, G. & Querner, U. (1992). Rapid Microevolution of Migratory Behavior in a Wild Bird Species. Nature 360, 668-670.
Berthold, P., Mohr, G. & Querner, U. (1994). Inheritance of a Population-Specific Molt by Cross-Breeding Blackcaps (Sylvia-Atricapilla) of Different Populations. Journal Fur Ornithologie 135, 233-236.
Berthold, P. & Querner, U. (1981). Genetic-Basis of Migratory Behavior in European Warblers. Science 212, 77-79.
Berthold, P. & Querner, U. (1993). Genetic and Photoperiodic Control of an Avian Reproductive-Cycle. Experientia 49, 342-344.
Fiedler, W. (2005). Ecomorphology of the external flight apparatus of blackcaps (Sylvia atricapilla) with different migration behavior. Bird Hormones and Bird Migrations: Analyzing Hormones in Droppings and Egg Yolks and Assessing Adaptations in Long-Distance Migration 1046, 253-263.
Helbig, A. J. (1991a). Inheritance of Migratory Direction in a Bird Species – a Cross-Breeding Experiment with Se-Migrating and Sw-Migrating Blackcaps (Sylvia-Atricapilla). Behavioral Ecology and Sociobiology 28, 9-12.
Helbig, A. J. (1991b). Se-Migrating and Sw-Migrating Blackcap (Sylvia-Atricapilla) Populations in Central-Europe – Orientation of Birds in the Contact Zone. Journal of Evolutionary Biology 4, 657-670.
Helbig, A. J. (1994). Genetic-Basis and Evolutionary Change of Migratory Directions in a European Passerine Migrant Sylvia-Atricapilla. Ostrich 65, 151-159.
Helbig, A. J. (1996). Genetic basis, mode of inheritance and evolutionary changes of migratory directions in palearctic warblers (Aves: Sylviidae). Journal of Experimental Biology 199, 49-55.
Helbig, A. J., Berthold, P., Mohr, G. & Querner, U. (1994). Inheritance of a Novel Migratory Direction in Central-European Blackcaps. Naturwissenschaften 81, 184-186.
Illera, J. C., Palmero, A. M., Laiolo, P., Rodriguez, F., Moreno, A. C. & Navascues, M. (2014). Genetic, Morphological, and Acoustic Evidence Reveals Lack of Diversification in the Colonization Process in an Island Bird. Evolution 68, 2259-2274.
Mettler, R., Schaefer, H. M., Chernetsov, N., Fiedler, W., Hobson, K. A., Ilieva, M., Imhof, E., Johnsen, A., Renner, S. C., Rolshausen, G., Serrano, D., Wesolowski, T. & Segelbacher, G. (2013). Contrasting Patterns of Genetic Differentiation among Blackcaps (Sylvia atricapilla) with Divergent Migratory Orientations in Europe. Plos One 8.
Rolshausen, G., Hobson, K. A. & Schaefer, H. M. (2010). Spring arrival along a migratory divide of sympatric blackcaps (Sylvia atricapilla). Oecologia 162, 175-183.
Rolshausen, G., Segelbacher, G., Hobson, K. A. & Schaefer, H. M. (2009). Contemporary Evolution of Reproductive Isolation and Phenotypic Divergence in Sympatry along a Migratory Divide. Current Biology 19, 2097-2101.