Nest defense as parental care in the northern hobby (Falco subbuteo)

Intensity of nest defense against a human intruder was recorded for 42 male and 43 female Northern Hobbies (Falco subbuteo) from 1992 to 1994. Defense did not vary between repeatedly and rarely visited nests. Except during in cubation, intensity of nest defense by females was higher than that by male s. For both sexes, defense intensity increased from incubation to fledging, within the nestling stage, and from fledging to the first 10 days of the p ostfledging period. Intensity of nest defense was positively correlated wit h brood size in females, but not in males. Experiments with dummy nests sho wed that defense was effective in deterring nest predation, and that its ef fect was positively related to its intensity. Hobby nest defense was an ind ividually varying "plastic" trait, probably tuned to the reproductive value of the offspring. Parents apparently trade off the costs and risks of the behavior against the increasing likelihood of offspring survival.