The Red-rumped Swallow data here are a bit deceptive. In fact, the birds seem to turn up occasionally throughout the winter. I often see them in November and December (counts of up to 10 in Mai Po Village last December), and have seen nest-building activity at that time.
My impression is that Red-rumped tends to remain in the general area through the winter, and returns to the breeding site to repair nests. More birds (first-time nesters?) then turn up later in the spring, often not until April-May.