A table of new city pairs from Spirit Airlines’ (SAVE) earnings report on Wednesday (pdf) shows new flights the discount airline has added just since April, a torrid pace of growth focused heavily on Dallas-Fort Worth, an American Airlines fortress. Even more noteworthy, however, is that every domestic city on the new-flight roster—save for Myrtle Beach, Latrobe/Pittsburgh and Las Vegas—is a hub airport or “focus city” for players much larger than Spirit. The lesson? As the big guys consolidate, fares in their hub cities rise, leaving space for a niche player to court budget travelers. And while American, Delta (DAL) and United (UAL) are hardly nervous about Spirit’s expansion, you can bet they’re watching closely.