Some AM stations change the direction of their antenna array (electronically) to adjust for daytime and nighttime differences in propagation, so they don't interfere with other stations on the same frequency. As an example, the wiki entry for
CKLW in Windsor talks about this:
For the station to be heard as far west as Arkansas, Iowa and Texas is impressive, given the station is not a "clear channel" Class A station, and has an extreme northward/eastward nighttime directional signal in order to protect stations on 800 kHz in Ciudad Juárez (clear channel XEROK-AM across the river from El Paso, Texas). A station in Bonaire in the Netherlands Antilles (PJB3, Trans World Radio), provided severe interference to CKLW during its Big 8 years and beyond, operating with 525,000 watts of power. CKLW was and is under no obligation to protect Bonaire, as PJB signed on long after North American allocations were settled and the Netherlands Antilles did not honour international agreements.
During CKLW's Top 40 heyday, because of its nighttime directional pattern, the station was frequently heard in Scandinavia, but was often rendered unlistenable just a few hundred miles to the west and south of Detroit because of interference from the Juarez and/or Bonaire stations. Nevertheless, the current news/talk format enjoys good ratings in Windsor, though it now hovers near the bottom of the Detroit Arbitron reports.