Individuals can benefit by varying their investment in offspring. The optimal amount of investment may vary in relation to both climatic conditions and social conditions (such as the number of carers for the offspring). However, the precise cues used by females to determine optimal maternal investment and the long-term consequences to offspring of variation in investment are poorly understood. Cooperatively breeding superb fairy-wrens, Malurus cyaneus, have previously been found to adapt maternal investment to both climatic and social conditions by varying their egg size. However, it is unknown what cues females use to manipulate their egg size or whether egg size is important for nestling growth and survival. Optimizing maternal investment is even more complicated for brood parasitic cuckoos. To avoid detection by the host, cuckoos lay eggs that mimic the size and appearance of the host eggs. Thus females must trade-off the benefits of investing in larger offspring against the risk of detection of the parasite young by the host. It is unknown if cuckoos, like their hosts, adapt their egg size to climatic and social conditions. In this seminar I will share my finding on these questions, which I have investigated using field work experiments and analysis of long-term data. Overall, this work looks at the importance of maternal investment and aims to gain a better understanding of what maternal investment is influenced by and in turn its implications for offspring.