A decline in size is increasingly recognised as a major response by ectothermic species to global warming. Mechanisms underlying this phenomenon are poorly understood but could include changes in energy balance of consumers, driven by declines in prey size coupled with increased energy demands due to warming. The sardine Sardina pilchardus is a prime example of animal shrinking, European populations of this planktivorous fish are undergoing profound decreases in body condition and adult size. This is apparently a bottom-up effect coincident with a shift towards increased reliance on smaller planktonic prey. We investigated the hypothesis that foraging on smaller prey would lead to increased rates of energy expenditure by sardines, and that such expenditures would be exacerbated by warming temperature. Using group respirometry we measured rates of energy expenditure indirectly, as oxygen uptake, by captive adult sardines offered food of two different sizes (0.2 or 1.2 mm items) when acclimated to two temperatures (16 °C or 21 °C). Energy expenditure during feeding on small items was tripled at 16 °C and doubled at 21 °C compared to large items, linked to a change in foraging mode between filter feeding on small or direct capture of large. This caused daily energy expenditure to increase by ~10 % at 16 °C and ~40 % at 21 °C on small items, compared to large items at 16 °C. These results support that declines in prey size coupled with warming could influence energy allocation towards life-history traits in wild populations. This bottom-up effect could partially explain the shrinking and declining condition of many small pelagic fish populations and may be contributing to the shrinking of other fish species throughout the marine food web. Understanding how declines in prey size can couple with warming to affect consumers is a crucial element of projecting the consequences for marine fauna of ongoing anthropogenic global change.