The Hajdučki and Rožanski Kukovi are a field of limestone towers in the northern Velebit, a landscape of pale spires, pinnacles and karst pavements that look like a city abandoned by giants. They sit in the Northern Velebit National Park, a long mountain range that parallels the Adriatic coast and shelters the Croatian interior from the sea. The sea is close, and the contrast is part of the appeal: bare white stone and wind above, blue water far below.
You will not ride all the way in. Access is on foot from the road, and the road itself is a long way from anywhere. The nearest places in the data are on the coast and the island of Rab, but the practical bases are Senj, a historic port town of around 5,500 people, and Gospić inland. Senj is also where the Velebit meets the sea, and the main coastal road, the Magistrala, is a famous ride in its own right, with the Adriatic on one side and the Velebit on the other.
Be honest about the surface and the weather. Roads that climb into the range from the coast are steep and narrow, with a lot of hairpins, and some of the upland tracks are unsealed and closed in winter. The bura, a cold, gusty wind that comes down from the mountains onto the Adriatic, can be violent enough to close the coastal road to motorcycles. Check the forecast. In summer the mountains offer relief from the heat, and the best months for the upland roads are generally late spring through early autumn.
The Velebit is a place of rare plants and wildlife. Its limestone supports endemic species, among them the Velebit degenia, a yellow-flowered cabbage relative found nowhere else, and the forests hold bears and lynx, though you are unlikely to see either. In the old days the region was a frontier between Venetian and Ottoman influence, and the hajduks of the name were outlaw bands who lived in these hills and took on the authorities of the day. Carry water and a layer, fill up before leaving the coast, and keep an eye on the sky.