Adjustments in body composition over the annual cycle have been documented in many organs and muscle groups. Here we consider the nature and significance of intra-organ variation in the eared grebe Podiceps nigricollis stomach, a large and variable organ that can weigh > 30 g when birds are staging, drop to 8–11 g before setting off, or to as little as 6.6 g after a several-day migration. Weight reduction in association with migration is conventionally regarded as an adaptation to reduce wing loading and flight costs. This interpretation applies to the premigratory reduction of the entire stomach. However, it does not fit the differential in-flight reduction of the proventriculus, because grebes require a large proventriculus to initiate digestion, and its smaller size when they need to rebuild the entire stomach and resume feeding quickly is opposite that expected in a functional context. We view the reduction of the proventriculus as a non-adaptive response, or spandrel, stemming from its intrinsically higher turnover rate. Starving birds, like migrants, also need to digest food quickly. In their case, the proventriculus is maintained as body weight declines and the gizzard is sacrificed. Mechanisms by which individual organisms achieve different responses to similar challenges, including starvation, merit further investigation.