Can radars detect small drones?
Depends entirely on the drones. If the radar system is known and the drone is designed not to be detected by this particular radar system, then the drone can only be detected in exceptional circumstances. Small size would be one way, but in order to be effective, big is better.

Detection has many aspects, and one of them is how soon detection happens. If all you need is to stay undetected while you approach the target, only the frontal aspect needs to be considered. If, however, you want to stay undetected for the whole mission, the task becomes much, much harder.

The radar systems in operation during the Eighties along the Iron Curtain could already detect single, large birds. Adding computerized signal processing and networking of several radar stations has improved detection performance many times over since then. See this question for more details.