Availability of water affects renewal of tissues in migratory blackcaps during stopover

Migrating blackcaps (Sylvia atricapilla) were used to test the predictions that (1) the rebuilding of the digestive tract, as reflected by mass-specific consumption of food on the first 2-3 days of a stopover, is faster in birds with access to drinking water than in birds without, and (2) that adipose tissue and pectoral muscles grow faster and to a greater extent in birds with unlimited access to water. We simulated migratory stopover in two experiments. In Experiment I, each of 31 birds was randomly assigned to one of three experimental groups for 6 days. Along with mealworms (∼64% water) ad libitum, Group 1 received drinking water ad libitum; Group 2 had 0.5 h/day access to water; and Group 3 had no access to water. In Experiment II, 30 birds were offered a mixed diet for insectivorous birds (∼33% water) ad libitum for 6 days, while randomly assigned to two groups: (1) Water ad libitum-control; and (2) 30 min access to water twice a day. We measured lean mass and fat mass using dual energy X-ray absorptiometry, as well as body mass (m(b)), pectoral muscle index (PMI), and daily intake of food and water. Mean daily water intake was significantly different among the groups in both experiments. However, the availability of drinking water positively affected the rates of gain of lean and fat mass only in birds fed with the mixed, relatively dry diet. Furthermore, mass-specific daily food intake was affected by the availability of drinking water only in the mixed diet experiment, in which birds with unlimited access to drinking water reached an asymptote, 1 day earlier than birds in the water-restricted group. We suggest that in birds consuming diets with low water content, the lack of sufficient drinking water may result in slower rebuilding of the digestive tract, or may influence biochemical processes in the gut that result in slower growth of tissue. Although blackcaps obtained sufficient water from preformed and metabolic water to renew lost tissues when eating mealworms, given access to water, the birds drank prodigiously. Our results also suggest that if drinking water is unavailable to migrating blackcaps, their choices are restricted to water-rich foods, which may constrain their rate of feeding and thus the rate at which they deposit fat. Consequently, drinking water may have an important influence on birds' migratory strategies with respect to habitat selection, use of energy, and the saving of time.