Roost availability may constrain shorebird distribution: Exploring the energetic costs of roosting and disturbance around a tropical bay

High tides force shorebirds from their intertidal feeding areas to refuges known as roosts.This paper explores the energetic costs of roost disturbance of great knot (Calidris tenuirostris)and red knot (C. canutus) at Roebuck Bay, North-western Australia, assessingdisturbance levels at different roost sites through direct observation and automaticradio-telemetry, and applying physiological equations and predictive roost choice modelsto estimate energetic costs of disturbance through a complete tidal cycle. The study areahad a variety of roosts, but use of each was constrained by conditions of tide and time.The roost most suitable for shorebirds on daytime high tides of intermediate height experiencedhigh levels of disturbance from both natural sources (birds of prey) and humans.Flight costs caused by disturbance at this site exceeded the costs of flying to and roostingat the nearest alternative roost, 25 km away. However, shorebirds did not roost at the alternatesite, possibly because of the risk of heat stress in a prolonged flight in tropical conditions.Increases in disturbance levels at just one of the roost sites of Roebuck Bay wouldincrease energetic costs substantially, and could easily reach the point at which feedingareas accessed from this roost cannot be used without incurring a net energy deficit. Roostavailability can therefore limit access to feeding areas and hence limit population size.Adequate provision and management of roost sites is accordingly an important considerationin conservation of sites used by coastal shorebirds.