We studied growth in the Green-rumped Parrotlet (Forpus passerinus), a small Neotropical parrotlet that has an unusually long nestling period (28–35 days) and feeds nestlings nutritionally poor seeds. We asked why nestlings grow slowly even though nest predation is their main mortality factor. We examined nutrient accretion and the development of thermoregulation and computed an energy budget until fledging. We described three periods of growth: (1) days 1–13, when peak accretion for sodium, lipids, and energy occurred; (2) days 14–22, when nestlings became endothermic and accretion for proteins, calcium, and phosphorus peaked; and (3) days 23–30, when nestlings reached 90% of the asymptotic values of body components. The deposition rate of lipids (logistic constant K = 0.33) was higher than that of protein (K = 0.19), ash and calcium (K = 0.20), and, particularly, phosphorus (K = 0.14). In concordance with their slow growth, nestlings became endothermic only 10 days before fledging, considerably later than expected from asymptotic mass. Total metabolized energy during the nestling period was 1,133.3 kJ, of which 59.2% accounted for resting metabolic rate and 26.2% for activity. Growth efficiency, the proportion of total metabolized energy allocated to tissue deposition, was 14.5%, one of the lowest reported for altricial nestlings. We argue that nestling growth was not energy-limited and that limiting nutrients were deposited slowly as a result of restrictions imposed by low availability of protein and phosphorus in the diet. We propose that slow growth and poor diets are interrelated in Green-rumped Parrotlets and other Psittaciformes, but the direction of the causal relationship between them is unclear.