184 WET GROUND COMMUNITIES 
invade, the girdle of marsh which is the nesting site of the birds men- 
tioned moves farther and farther toward the center of the pond or lake, 
the former positions being occupied by shrubs, such as buttonbush or 
willow, or in some cases by prairie. Such a situation is in unstable 
equilibrium. 
Turning to the margins of ponds, lakes, and rivers, we note that 
at the beginning we often have the bare sand. This is first occupied 
Fic. 142.—The long-billed marsh wren’s nest. The nest unopened. 
Fic. 143.—The nest torn open showing the eggs. 
by reeds and sedges, and finally by shrubs. It is this reed and sedge 
group, or the buttonbush, that invades the swamp as it fills with bul- 
rushes and cattails. We note accordingly that the vegetation which 
appears on the shore invades the pond as it fills. The last stage of 
a pond is either a buttonbush swamp or a low prairie which we shall 
discuss in later chapters. 
