SUMMARYDiving endotherms inhabiting polar regions face potentially high thermoregulatory costs. Unless properly insulated, these animals will lose vast amounts of heat when diving in cold water, which has to be balanced by heat production. Heat generated as a by-product of digestion (heat increment of feeding, HIF) or from exercising muscles might be important in maintaining thermal balance under such conditions, as it would reduce the need for shivering thermogenesis. Recording the rate of oxygen consumption(V̇O2),respiratory exchange ratio (RER), and stomach temperature, we studied the magnitude and duration of HIF in seven double-crested cormorants(Phalacrocorax auritus) following the voluntary ingestion of a single herring (Clupea pallasi) while birds rested in air. Conducting trials at thermoneutral (21.1±0.2°C) and sub-thermoneutral temperatures(5.5±0.7°C), we investigated the potential of HIF for thermal substitution. After the ingestion of a 100 g herring at thermoneutral conditions, V̇O2was elevated for an average of 328±28 min, during which time birds consumed 2697±294 ml O2 in excess of the resting rate. At sub-thermoneutral conditions, duration (228±6 min) and magnitude(1391±271 ml O2) of V̇O2elevation were significantly reduced. This indicates that cormorants are able to use the heat generated as by-product of digestion to substitute for regulatory thermogenesis, if heat loss is sufficiently high. Altering meal size during sub-thermoneutral trials, we also found that HIF in cormorants was significantly greater after larger food intake. Based on these experimental results, a simple calculation suggests that substitution from HIF might reduce the daily thermoregulatory costs of double-crested cormorants wintering in coastal British Columbia by ∼38%. Magnitude of HIF and its potential for thermal substitution should be integrated into bioenergetic models to avoid overestimating energy expenditure in these top predators.