Almost all of the internal organs of migrating birds undergo pronounced mass changes, but the digestive tract changes most and fastest. The masses of the small intestine and the liver may be reduced by as much as 50% during migratory flight, indicating extreme phenotypic flexibility. Birds must rebuild these organs during stopovers to facilitate rapid mass gain and fuel deposition for continuation of migration. Laboratory studies indicate that birds may vary mean gut retention time to maintain high digestive efficiency, even while intestine length is substantially reduced. We examined migratory blackcaps (Sylvia atricapilla) after they performed a northward flight across the Sahara Desert in spring, and we subjected them to a 5-d artificial stopover. Body mass (m(b)) changes over the 5-d period resembled the typical mass change pattern of blackcaps stopping over naturally, with a small increase on the first day and a subsequent peak in the rate of m(b) gain on day 2 of the stopover. By day 5 of the stopover, the rate of m(b) gain had decreased to the point that it was not significantly different from that on day 1, presumably because the digestive tract had been rebuilt by this time. The same pattern was observed for food intake rates and amount of excreta produced, while digestive efficiency remained constant throughout the five experimental days. In contrast, mean retention time on stopover day 1 was significantly higher than it was on day 5 (106 min vs. 53 min; P = 0.031). Thus, after a sustained migration period, during a stopover blackcaps apparently compensate for reduced digestive tract size by processing food for twice as long, thereby maintaining high digestive efficiency.