PATTERNS OF PREDATION ON PASSERINE NESTS IN MARSHES - EFFECTS OF WATER DEPTH AND DISTANCE FROM EDGE

We tested a hypothesis that the high densities of some passerines breeding in North American marshes result from greater safety of this habitat from predators as compared to upland habitats. We examined the relative importance of water depth and distance from the marsh edge in lowering predation rates on experimental nests with Blue-breasted Quail (Coturnix chinensis) eggs. In addition, using cameras we studied the role of water depth in determining the predator community. Our results showed that: (1) predation was lower in the marsh than in the adjacent upland; (2) predation rates decreased with increasing water depth in the marsh; (3) for the marsh nests, the distance to the marsh edge was relatively unimportant; (4) the diversity of nest predators decreased with increasing water depth; and (5) in the deep marsh areas, there was only one major predator, the Marsh Wren (Cistothorus palustris). We propose that the greater safety of deep-water marsh areas, the reduced complexity of the predator community, and the type of predators allowing effective nest defense by nest owners have played the key role in the evolution of reproductive strategies of marsh-nesting passerines.