ENERGY-EXPENDITURE DURING MOLT IN DIPPERS (CINCLUS-CINCLUS) - NO EVIDENCE OF ELEVATED COSTS

The energetic cost of molt in birds is generally considered to be high and is frequently regarded as being a main reason for the temporal se paration of molt from reproduction and other activities. We assessed t he energetic cost of the post-nuptial molt in dippers, Cinclus cinclus , using laboratory measurements of O-2 consumption and CO2 production of molting and nonmolting individuals, time-activity budgets in the fi eld, and the doubly labeled water technique on free-living, molting bi rds. Energy expenditures of molting dippers within their thermal neutr al zone in the laboratory (males, 92 kJ d(-1); females, 74 kJ d(-1)) w ere not significantly different from those of nonmolting individuals ( males, 85 kJ d(-1); females, 76 kJ d(-1)). Free-living, molting dipper s spent most of their active day resting (55%) and foraging along rive rbanks (39%). Flying and diving were infrequent, accounting for less t han 1% of their activity budget. Mean daily energy expenditures of fre e-living dippers during molt, obtained from time-activity laboratory e stimates and from doubly labeled water estimates, were in close agreem ent, with the latter averaging 177 kJ d(-1) for males and 162 kJ d(-1) for females. These rates of energy expenditure are lower than at any other time of the year, and even if normal levels of flying and diving are superimposed on daily energy expenditure during molt, energy expe nditure is elevated only to levels typical of males during mate guardi ng and females attending young. We conclude that molt in dippers does not elevate basal metabolic rare or daily energy expenditure and that the timing of molt in this species is therefore not constrained by the direct energetic costs of feather replacement. We propose several exp lanations for the timing of molt that may apply to dippers and other s pecies.