In birds with biparental care, two parents cooperate to provide the appropriate amount of care for the young. Evolutionary stable strategy (ESS) models predict that cooperation can be stabilized when parents respond to reductions in care by their partners by increasing their effort, while not fully compensating for the reduction. To examine whether parents adjust their effort according to their partner's contribution and what cues the parents use in the bargaining process, we manipulated parental care in the Barn Swallow Hirundo rustica. Twenty-eight pairs were randomly assigned to three groups: (i) reduced male parental care (7 pairs), (ii) control (13 pairs), (iii) reduced female parental care (8 pairs). Parental care was manipulated by attaching small weights to the base of a bird's tail feathers. The manipulation successfully reduced parental provisioning in the handicapped birds, while still maintaining biparental care. Regardless of sex, however, handicapping of individuals led to no compensatory responses by the mates. The handicapped birds spent more time resting, causing lowered provisioning rates. Males with a handicapped female decreased their provisioning rates to guard the resting females against extra-pair males. Since the provisioning parents in the three groups seldom met at their nest, it is unlikely that parents monitor their partner's provisioning rate directly. We predicted that parents would adjust their provisioning rates according to the begging behaviors of their nestlings. However, no significant relationship was detected between the begging intensity (begging level and calling duration) and travelling time. Moreover, there were no significant differences in either begging level or calling duration among the three groups. Since various factors affect the provisioning rate in a handicapping manipulation, our data do not support the prediction derived from ESS models of biparental care.

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