“Monarchs from eastern North America winter each year in masse at a number (12+) of mountain-top locations in central Mexico, beginning in November and typically lasting through March. During this time, the staff of the monarch program at WWF-Mexico conducts field surveys of the size of these colonies. By mapping the outer perimeter of the colonies, the amount of surface area occupied by monarchs is determined for each colony.
Typically, the ‘size’ of the overwintering population is then assessed by summing the areas for all colonies […] While this method may seem crude, scientists have found it is extremely difficult to estimate actual monarch numbers at the colonies, because of their overwhelming abundance. Assessing the surface area has therefore been one of the simplest ways to track the wintering population” (MonarchNet 2016).