ENERGETICS AND GROWTH-RATE OF NORTHERN SHRIKE (LANIUS-EXCUBITOR) NESTLING

Northern Shrikes (Lanius excubitor) breed in a variety of habitats, including deserts. Deserts are characterized by unpredictable food supplies, which can lead to a slow growth rate of nestlings. However, given that Northern Shrike males use prey from their caches to augment freshly caught prey in providing food for their mates and nestlings, we hypothesized that their nestlings do not have a slow growth rate, but one that is equivalent to that in other passerine nestlings from temperate areas. To test this hypothesis, we measured growth rates and energy use in Northern Shrike nestlings and fledglings. We also measured energy expenditure in two adult males that were attending nests. Growth rate of Northern Shrike nestlings was similar to that predicted for passerines in temperate areas and therefore our hypothesis was supported. However, metabolizable energy available in the cache amounted to only almost-equal-to 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. These included (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 to 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.