Read the extract given below and answer the question that follows:

Even today the Gujjar Bakarwals of Jammu and Kashmir are great herders of goats and sheep. Many of them migrated to this region in the nineteenth century in search of pastures for their animals. Gradually, over the decades, they established themselves in the area and moved annually between their summer and winter grazing grounds. In winter, when the high mountains were covered with snow, they lived with their herds in the low hills of the Siwalik range. The dry scrub forests here provided pasture for their herds. By the end of April, they began their northern march for their summer grazing grounds. Several households came together for this journey, forming what is known as a kafila. They crossed the Pir Panjal passes and entered the valley of Kashmir With the onset of summer, the snow melted and the mountainsides were lush green. The variety of grasses that sprouted provided rich nutritious forage for the animal herds. By the end of September, the Bakarwals were on the move again, this time on their downward journey back to their winter base. When the high mountains were covered with snow, the herds were grazed in the low hills.

When did Gujjar Bakarwals migrate to Jammu and Kashmir?