Test for physiological limitation to nutrient assimilation in a long-distance passerine migrant at a springtime stopover site

During northward migration, blackcaps arrive at stopover sites in Israel's Negev Desert with reduced masses of organs that are important in food digestion and assimilation. Blackcaps that stay to refuel (largely on fruits) do not gain mass rapidly until after 3 d at the stopover site. We hypothesized that (1) it may take several days to rebuild these reduced organs, (2) during this recovery interval high feeding rates might not be possible, and (3) this could be the basis for the absence of immediate body mass gain in blackcaps at stopover sites. To test predictions from this hypothesis we used an established fasting protocol to create a group of blackcaps with reduced intestinal and liver mass compared with ad lib. fed controls. Migrants were captured and caged in the laboratory, where they were habituated to a fruit mash diet for 8 d. One experimental group was then fasted 2 d, one was fed at a restricted level (one-third ad lib. food intake) for 4 d, and one was held as ad lib.-fed controls. The fasted and restricted birds were then allowed to feed again ad lib. Birds that were experimentally fasted progressively increased their daily assimilation rate and achieved the highest rate (one-third higher than controls) 3 d after the end of their fast. Birds that were restricted achieved high rates immediately once ad lib. food was provided. Increased assimilation rate was achieved via hyperphagia and not increased assimilation efficiency. The response of the fasted birds supports the hypothesis that there may be physiological constraints to the rate of refueling during migratory stopover.