FEMALE RESPONSE TO REDUCED MALE PARENTAL CARE IN BIRDS - AN EXPERIMENT IN TREE SWALLOWS

In biparental species, parental-investment theory generally predicts a n inverse relationship between the level of parental care provided by each parent and incomplete compensation by one parent in response to r educed parental care by their partner. The factors that influence the magnitude of this compensation have rarely been examined in birds. For example, the level of compensation may differ between a widowed bird that receives no assistance from its partner and a mated bird whose pa rtner is still present but providing less than its normal share of par ental care. This study compares the compensatory response of female tr ee swallows (Tachycineta bicolor) without their mate's parental assist ance (when females are widowed) and with reduced male parental care (w hen males are handicapped by cutting some feathers). When compared wit h control females, experimental females compensated more in terms of n est visits for the absence than the reduction of male parental care. I n addition, widowed females had significantly reduced brood mass and f ledging success compared with control females. Although handicapped ma les reduced their nest-visit rate significantly, females with handicap ped mates did not significantly increase their nest-visit rate nor was there reduced brood mass or reduced fledging success at their nests. Total nest-visit rate was similar for all groups, yet widowed females fledged fewer and lighter young, suggesting that they brought less foo d per nest visit. We suggest that fledging success and measures of off spring quality are probably better indicators of the level of compensa tory parental care than nest-visit rate. We suggest that for widowed f emales the benefits of a relatively large compensatory response outwei ghed the costs; whereas, for females with a handicapped mate the benef its of higher feeding rates were not greater than the cost. The result s of this study help to explain the differences among experimental stu dies of compensatory parental care and point to a new method of testin g models of parental care.