Complex sentence: Seldom found in large numbers except at a few favored wintering or staging sites where numbers sometimes reach 100 or more, plovers are more typically seen in pairs or in groups of 3 or 4.
Split into simpler sentences: Seldom found in large numbers except at a few favored wintering or staging sites, where numbers sometimes reach 100 or more. More typically seen in pairs or in groups of 3 or 4.