Threat predictability influences seaside sparrow nest site selection when facing trade-offs from predation and flooding

Habitat selection trade-offs between avoiding predation and gaining energy are well studied, but similar trade-offs resulting from multiple threats to survival remain poorly understood. We studied how seaside sparrows, Ammodramus maritimus, approach nest site selection decisions to avoid threats to nesting success from predation and tidal flooding. Along a nest height gradient, nest site selection to avoid tidal flooding (i.e. placing nests higher) may make seaside sparrow nests more vulnerable to predation, and selection to avoid predation may make nests susceptible to flooding. We monitored nesting success rates and nest site selection decisions of breeding seaside sparrow pairs at five sites near Brunswick, Georgia in April-July of 2013-2015. We found that seaside sparrows encountered a nest site selection trade-off along a gradient of nest height. Nest height had an effect on survival probability during each of our study years, with positive effects of nest height on predation probability and negative effects of nest height on flooding probability observed in some years. Sparrows dealt with this trade-off by altering their nest site selection in relation to a threat's predictability; low within-season variability in predation risk and high information about predator presence in open marshes make predation risk more predictable than the magnitude of tidal flooding, which is governed by unpredictable and variable winds. Sparrows responded to predictable predation threats by nesting at lower nest heights in years with higher predation risk, but sparrows responded to unpredictable flooding threats by placing nests higher off the ground but only following nest failure from flooding. Understanding decision making through a lens of threat predictability could provide a useful approach for studies of other animals' habitat selection trade-offs. (C) 2016 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.