Can radars detect small drones? I've just started working with weather radars and such. With weather radars, it is possible to detect bird migrations with a specific algorithm and a lot of information about birds, but it would be a problem to detect a single bird (depending on the bird size) because of the radar's resolution. 

I am wondering, if you have a place or a secret base that you don't really want the population to see whats in that area and take photos of the inside of the area or spy, is there a way to detect small drones with radar signals if you pack a few radars around your place or military base? If so, how?
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.