Nest defence by woodpeckers from inside vs. outside the cavity against the intruder

Cavity nests are safer than open nests and they uniquely allow a defence from inside. We studied the rates of nest defence from inside/outside the cavity with respect to nesting stage and number of responding parents in the Great Spotted Woodpecker (Dendrocopos major) against its dominant nest intruder, the Common Starling (Sturnus vulgaris). Undisturbed behaviour before Starling exposure (a control level) included high inside cavity attentiveness during incubation that decreased in subsequent nesting stages, while outside cavity attentiveness remained low in all nesting stages. Both inside and outside cavity attentiveness mostly increased after Starling exposure (caged individual). During Starling exposure, the pattern of inside cavity attentiveness was similar in singly and pair-responding parents. In vulnerable stages of incubation and young nestlings, singly-responding parents predominantly defended the cavity from inside, but this was not simply a consequence of parent´s initial position; they defended the cavity from outside as late as in the stage of old nestlings, while increased outside cavity attentiveness in the stages of incubation and young nestlings was a product of mate presence. Moreover, parents that defended the nest from outside behaved more aggressively in the presence of a mate, regardless of nesting stage. We consider the inside defence a priority tactics of nest protection; intruders probably pose a threat particularly to unattended cavity. These findings call attention to different (context-dependent) effectiveness of inside and outside cavity defence, which are often combined into general nest defence scores.