Before the end of February, the nature reserve’s rooks will have started collecting building materials for their nests. Here is a video that shows us what kind of behaviour to look out for:
Video by Film Studio Aves;
Header picture (CC0) pixabay.com
Southwick Country Park Nature Reserve 
Just in the last few days we noticed 8 rooks gathering at the top of a tree near our house early in the morning waiting for us to feed the birds. We never usually see rooks here are they likely to be nesting nearby?
Rooks are usually ground feeders; they use their beaks to probe for invertebrates in the soil. In this freezing weather they can’t do that so I expect they have come to your bird feeder instead. They prefer grain to seeds and will take mealworms if you have them but in these difficult times, they will take what they can get.
I didn’t really answer your question: sorry. Rooks have ancestral nesting sites; somewhere around, there will a rookery that has probably been there for years, if not decades. They are noisy places, difficult to miss.