Brood defense and brood size in the great tit (Parus major): a test of a model of unshared parental investment

Abstract

To understand the interaction of the many contextual variables that affect parental behavior a number of static optimality models have been developed. Among these the one by Lazarus and Inglis (1986) is the only one to specifically predict the magnitude of unshared parental investment (PI), i.e., of parental care that carries a cost to the parent and that benefits all current offspring equally because it cannot be divided among them. We investigated specifically how parent great tits (Parus major) gear their brood defense, a form of unshared PI, to the size of the brood at stake and to the risk incurred as a function of the type of predator. The predators used were dummies of the great spotted woodpecker (Picoides major) and of the tawny owl (Strix aluco). Normally, adults can approach the woodpecker with impunity; it had inflicted heavy losses to nestlings of the study populations of great tits near Wolfsburg, Lower Saxony. Parent great tits whose brood had been artificially reduced to two young responded to the dummy with less defense than did a control group with their pre-test brood size left intact. The nature of defense was qualitatively the same as that elicited by a live woodpecker. Parents confronting the owl near their brood decreased their response with an artificial reduction in brood size much less. Because the owl used poses a serious risk to the defenders, as compared to the woodpecker, the result lends powerful support to the “total loss” version of the model of unshared PI; it predicts brood size to affect unshared PI more strongly when there is less risk to the parent. This interpretation is correct to the extent that one premise of the model, namely that of uncompromised parentage, can be relaxed; great tit broods contain a sizeable number of extrapair young. Males defended their brood more strongly than did females. Sex and brood manipulation added up linearly when affecting defense level, i.e., there was no interaction.