Brood Size and the Use of Time and Energy by the Phainopepla

Time and energy budgets were calculated for 16 Phainopeplas (Phianopepla nitens) nesting in the coastal woodlands of southern California. Brood size differed 50%; 4 pairs raised 3 young and 4 pairs raised 2 young. Compared to pairs feeding 2 nestlings, pairs feeding 3 nestlings spent an average of 42% more time flycatching, 16% more time foraging at fruiting shrubs, and 22% more of their active day in flight. Daily energy expenditure of pairs feeding 3 nestlings was increased a surprisingly small amount compared to that of pairs feeding only 2 young–a mean of 7%, or 12 kJ. Over the entire period from egg synthesis to fledging, total parental energy expenditure is estimated to increase only 4.7% associated with a 50% increase in brood size. The Phainopepla may exhibit a unique annual cycle; it apparently breeds twice annually, once in its summer range (where the present study was conducted) and once in its winter range in the Sonoran Desert. Using data from this study plus those from a previous one, the 2 breedings were compared in terms of parental energy expenditure from egg—laying until fledging and parental harvesting requirements during the incubation and nestling periods. (Harvesting requirements = parental energy expenditure + the energy requirements of any young being fed.) Remarkable convergences are seen in the reproductive energetics of Phainopeplas breeding in the 2 habitats. Regardless of the striking differences between the 2 seasonal ranges in parental energy expenditure, habitat, daylength, and social system used, a Phainopepla incubating or feeding 2 nestlings in the coastal woodlands has a mean harvesting rate (kilojoules/hour active) virtually identical to that of a bird in a comparable stage of breeding in the desert. Similarly, total parental energy expenditure from egg synthesis until fledging averages only 2.6% different between populations breeding in the 2 areas, when calculated on an energy expended per young fledged basis.