Lack of avian predators is associated with behavioural plasticity in nest construction and height in an island songbird

Orange-crowned warblers, Leiothlypis celata sordida, breeding on the California Channel Islands exhibit remarkable variation in their nest structure and placement, providing an intriguing exception to the general pattern that avian nest structure and nest site selection are highly conserved characters. We examined nest construction at both the population and individual scale to test whether warblers on Santa Catalina Island change their nest construction in response to nest height. At the population level, warblers built both lighter, grass-dominated ground nests and heavier off-ground nests that contained more ridged materials and less grass. The probability of nest success was significantly and positively correlated with nest height. At the individual level, we found the same individuals were capable of building on- and off-ground nests between nesting attempts within the same season. However, nest construction was highly variable among individuals and not significantly correlated with nest success after controlling for nest height. We suggest this observed behavioural plasticity in nest construction and nest height is a hierarchical response to the absence of avian predators. Reduced risk from avian predators appears to allow the warblers to use a variety of nest sites, thereby necessitating increased flexibility in nest construction