EFFECT OF SHORT-TERM FEED RESTRICTION, REALIMENTATION AND OVERFEEDINGON GROWTH OF SONG THRUSH (TURDUS-PHILOMELOS) NESTLINGS

1. We examined the flexibility of the developmental programme of Song Thrush (Turdus philomelos) nestlings by subjecting them to food restri ction, and realimentation between 3-5 and 5-7 days of age, respectivel y, or overfeeding between 3 and 7 days of age. 2. Food restriction res ulted in a marked reduction of body mass and intestinal mass, but not pectoral muscle mass and tarsus length. Refed nestlings were unable to catch up with body mass of overfed young. None of the feeding regimes had much effect on the rate of maturation, measured here by the water content of muscle tissue. 3. Thus, the nestlings were unable to respo nd actively to changing food availability by slowing the pace of devel opment and subsequent compensatory growth. 4. Body-mass increments lev elled off with increasing energy intake, which suggests that overfed n estlings had reached physiological limits with respect to the rate of biomass production. Yet they did not grow faster than nestlings in the wild. 5. Growth of the young under natural conditions is therefore li kely to be limited by physiological or anatomical constraints, rather than food availability. This, along with the absence of the mechanisms that would increase the likelihood of surviving through short periods of food shortage, indirectly suggest that at least in some altricial bird species food shortages do not occur frequently enough to select f or responses to unpredictability of food resources.