Energetics and growth rate of northern shrike (Lanius excubitor) nestlings

Deserts are characterized by unpredictable food supplies, which can lead to a slow growth rate of nestlings. However, northern shrike males use prey from their caches to augment freshly caught prey in providing food for their mates and nestlings. Growth rate of nestlings was indeed similar to that predicted for passerines in temperate areas. However, metabolizable energy available in the cache amounted to only 7.2% of the total energy requirements of the nestlings or 4.2% of the total energy requirements of parents and nestlings during the nestling period. This suggested that other factors in addition to the cache were important in determining growth rate, including 1) an extremely low maintenance energy requirement of the nestling; 30% of that predicted for a bird of its body mass when it weighed 10 g, which gradually increased to 70% at 50 g. This allowed for more of the energy intake to be used for growth and also reduced foraging costs of males; 2) the relatively low amount of body energy retained as a fraction of metabolizable energy intake, 0.15-0.16, indicating that more water per unit growth was incorporated than in other passerines. Energy expenditure of adult males was substantially lower than that predicted for a passerine of similar body mass. Males collected 4.83 times the energy that they consumed in order to provide their mates and offspring with food. Yet, their ratio of field metabolic rate to basal metabolic rate was only 2.33, the lowest reported value for adults feeding altricial young. -from Authors