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The critical assumption of optimal foraging theory is that foragers with high net rates of energy gain have higher fitness than foragers with low net rates of energy gain. To test this assumption 60 pairs of zebra finches, Taeniopygia guttata, were randomly assigned to four experimental treatments in which their rate of energy gain was altered by increasing search time, without changing the amount of food available or the amount of food consumed. Reproductive success was monitored for the lifetime of the birds. Reproductive success decreased as the net rate of energy gain decreased. Decreased reproductive success was caused by a small decrease in brood size, an increase in the time intervals between broods, and increased adult mortality. The result that reproductive success is positively correlated with the rate of energy gain while foraging supports the prevalent use of energy measurements to predict fitness.