TRADE-OFFS AND CONSTRAINTS ON EASTERN KINGBIRD PARENTAL CARE

Ten Eastern Kingbird (Tyrannus tyrannus) nests were observed for 97 h to determine age-related changes in parental care and to identify poss ible trade-offs and constraints on feeding, brooding, and vigilant beh avior. Feeding rate (trips/h) was related positively to nestling age a nd brood size, but related negatively to amount of time spent vigilant . Per capita nestling feeding rates (trips/nestling/h) were affected m ost strongly and negatively by brood size, precipitation, and time spe nt vigilant. Time spent brooding declined as nestlings aged and as air temperature rose, whereas the amount of time spent shading nestlings varied only (inversely) with cloud cover. Vigilance time averaged abou t 20% of each hour, was independent of age and brood size, but was rel ated negatively to amount of time spent in nestling maintenance (brood ing plus shading), the number of feeding trips made to nests, and nest visibility. Weather had major influences on feeding and brooding beha viors, but regardless of other factors, kingbirds appear to reserve ti me for vigilance. Parental behavior thus reflects the action of a larg e number of factors that require compromises in the apportionment of t ime to the feeding, maintenance, and protection of young.