A recent meta-analysis has indicated that environmental quality and variability can influence whether offspring begging and parental responses to these signals are motivated by offspring need or offspring quality. We create a model to verify and apply evolutionary logic to this hypothesis. We determine the ecological and social conditions under which species signal and respond to need in favorable environments, and to quality in poor environments. The environmental conditions that favor this shift are widest when signalling costs and differences in quality between offspring are moderate. Low relatedness between siblings coupled with high signalling costs, as well as moderate relatedness between siblings coupled with low signalling costs, allow for the shift between signals of need and signals of quality to occur in more volatile environments. Further, only species whose offspring are highly dependent on parents for survival are not expected to use both signals of need and of quality. Ultimately, this shift between signalling need and signalling quality is the result of high-quality offspring benefiting more from meagre amounts of parental provisioning, while low-quality offspring have most to gain when parents can contribute more substantially. We show that this differential benefit of resources depends substantially upon offspring fitness as functions of parental investments, a variable which has lacked both diversity and biological realism in previous theoretical approaches. We then use this work to reassess previous theory on signals of need and of quality.