Speciation
Speciation is the evolutional process that is characterized by a single species lineage split into two more lineages of species.
Example of a species likely to undergo a speciation event in future is the Central European blackcap. According to scientists, these birds used to spend wintertime in temperate Spain and this has been observed until 1960. However, in the last half century, some birds have opted spending the winter in Britain because of the ready food awaiting them there. The blackcap birds that carried the type of genes that made them ply to the northwest( to Britain) instead of southwest( to Spain) survived and went back to for the summer in Central Germany and scientists have noticed an increase of the birds wintering in northwest pegged at 10% of the total population.
The reproductive isolating mechanism that could lead to this speciation event will be the shift in the availability of the mating partners. The birds that ply the northwest route take a shorter time compared to their counterparts that take the southwest route and therefore the birds taking the northwest route take a shorter time to go back (Ulf). The birds select a mate for the season after arriving at the breeding grounds and since those taking the southwest route will not have arrived because of the longer distance, the birds will tend top mate with the ones taking the northwest route. The same case will apply to the birds taking the southwest too, whereby they will select the mates that ply the same route since the ones taking the northwest route will already be taken and will not be available.
Conclusion
The generation brought born by the birds taking the southwest route will be adapted to the longer distances and scientist have witnessed subtle differences where they have pointier and rounder wings to cater for increased maneuverability while the northwest birds travelling the northwest bound have rounder wings and longer beaks compared to the their southwest bound counterparts.

Work cited
Ulf, Dieckmann. Adaptive speciation. New York: Cambridge University Press, 2004.