Sex allocation in response to maternal condition: Different tactics of care-giving by male and female zebra finches

While behavioral ecologists have studied sex allocation for 30 yr, almost nothing is known about the roles of male vs. female parents in making secondary allocation decisions in species with bi-parental care. To investigate possible sex differences in tactics in zebra finches, we manipulated the condition of one cohort of females by trimming their flight feathers; a second cohort of females and all males were left intact. Focal observations of feeding behavior revealed that manipulated females did not reduce their feeding time, but rather focused their efforts on rearing sons, while control females allocated effort to rearing daughters. Males mated to manipulated females tended to focus effort to daughters, while control males showed no consistent response. Manipulated females experienced a lower reproductive rate, primarily because of a low daughter rate, relative to control females. Manipulated females also laid smaller clutches and had longer inter-clutch intervals, which suggest that their ability to metabolize and allocate essential proteins from pectoral muscle mass during the egg-laying period was impaired. To our knowledge, this study is the first to show that parents of both sexes make active sex allocation decisions when the physiological condition of one parent is manipulated; males do not simply adopt female allocation patterns, but make complementary adjustments. Our results have implications for interpretation of handicapping studies that seek to understand the dynamics of inter-sexual negotiations of care-giving. Investigation of sex differences in sex allocation tactics in species with limited dietary protein, such as granivorous birds, is a potentially important direction for future research.