Do fluctuating water levels alter nest survivorship in reservoir shrubs

ABSTRACT Reservoirs often have highly fluctuating water levels. The perimeters of these impoundments, which alternate between being exposed or inundated by water (drawdown zone), are used by nesting birds, but at the risk of nest submergence when water levels rise. For species that nest above the ground in shrubs, foraging and predation may also be affected by flooded habitat. Our objective was to clarify the net impact that habitat flooding has on nest survivorship at Arrow Lakes Reservoir, British Columbia, Canada. This reservoir typically shows a pattern of water management where water is stored during the spring snowmelt (increasing water levels) and released later in the year. Yellow Warblers (Setophaga petechia; n = 272 nests) and Willow Flycatchers (Empidonax traillii; n = 81 nests) nested in similar parts of the drawdown zone, but differences in their nesting behaviors, particularly timing of nesting, caused the flycatchers to experience more nest submergence. Flycatchers also nested on a floating...