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Common Cuckoos (Cuculus canorus) are obligate brood parasites that lay their eggs in nests of other species and use these hosts to raise their parasitic offspring. Two key adaptations that increase their reproductive success are (1) the capacity for cuckoos to lay large numbers of eggs and thereby parasitize many nests per year, and (2) the ability of cuckoo eggs to hatch before those of hosts, enabling cuckoo nestlings to evict host eggs and eliminate competition for food. Producing more eggs is generally associated with reduced investment of nutrients and energy reserves per egg, which in turn is associated with shorter incubation periods both within and between species. We hypothesized that Common Cuckoos deposit reduced energy reserves into their eggs than do their hosts to facilitate both (1) and (2). To test these hypotheses, we compared the concentration of yolk lipids (per wet yolk mass) between …