The response is specific to nestlings and appears within days of hatching. Chicks regurgitate an orange, strongly malodorous liquid over themselves and the cavity when a hand or predator reaches in. The fluid is unpleasant enough to repel at least some mammalian predators directly.
Experimental work published in 2014 suggested a second function. Adults arriving at a nest that carried the odour showed increased caution, delaying entry and behaving as though a predator had recently been present. On that reading the vomit is both a chemical deterrent and a signal to parents, which is unusual: most nestling defences work on the predator alone.
The species declined sharply across western and central Europe through the twentieth century with the loss of old hollow trees and insect-rich farmland, though the global assessment has since improved to Least Concern following eastern population data and nest-box programmes. Whether nest boxes sustain populations without habitat recovery is not yet demonstrated.