Habitat variables affecting nest predation rates at small ponds: a case study of the Little Crake Porzana parva and Water Rail Rallus aquaticus

Capsule Nest survival of Water Rail and Little Crake nesting at small, midfield ponds was determined by vegetation density and proximity to the shore, respectively. Aims To determine which habitat variables affect nest predation for two rallid species inhabiting small ponds. Methods Habitat variables describing nest location (water depth, distance to the shore and distance to open water) and nest concealment (density and height of vegetation) were measured for natural and artificial nests. All nests were monitored to determine their fate and to identify nest predators. We used generalized linear models to assess nest survival rates and to evaluate the influence of particular variables. Results Little Crake nest success was negatively correlated with distance to shore (the land-water edge). The survival of Water Rail nests and artificial nests was positively correlated with vegetation density. During three breeding seasons, predation rates on Little Crake nests varied from 22% to 58%, and on Water Rail nests from 36% to 91%. The main predator of natural and artificial nests was the Marsh Harrier (responsible for about 50% of natural and artificial nest failures). Conclusion High Marsh Harrier predation rates (attacks from the air) may explain why water depth did not affect the survival rate of natural and artificial nests and why nest concealment was a factor positively influencing the nest success of the Water Rail and artificial nests. Little Crakes nest successfully closer to the shore, where vegetation is usually denser. Terrestrial mammals could more easily access Water Rail nests, which were situated in shallower water than Little Crake nests, and this may have contributed to the higher rate of nest loss in this species.