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 eggs of 3 cuckoo gentes and their respective host species: Great Reed Warblers (Acrocephalus arundinaceus), Common Redstarts (Phoenicurus phoenicurus), and Reed Warblers (A. scirpaceus). Yolk lipids provide the bulk of energy required for embryonic development and can also serve structural and cell-signalling functions. As a general pattern, cuckoo eggs contained a lower concentration of energy-reserve lipids than eggs of their respective hosts, but not structural or cell-signalling lipids. When controlling for their heavier eggs and yolks, Common Cuckoo eggs had an estimated lower amount of energy reserve lipids for their size than host eggs. Our findings suggest a potential role of yolk lipid composition in facilitating (1) and (2) and advocate the need for further research in this area. We also highlight the potential problems of using either concentration or total yolk mass alone to compare maternal investment across taxa in comparative studies.