Time and energy budgets of Cassin's sparrow ( Aimophila cassinii) during the breeding season: evaluation through modelling

Abstract

The elusive nature of Cassin's Sparrow ( Aimophila cassinii) has hampered studies of its basic biology. In this paper, we examine the bird's behavioral and energetic ecology within a systems analysis framework. First, we present an individual energetics model for Cassin's Sparrow. A computer simulation of the model is then used to quantify time and energy budgets of birds during five phases of their breeding cycle. Attention is focused on (1) evaluating the relative costs of reproductive events and their seasonal correlates, (2) demonstrating the effectiveness of a systems approach in time/energy budget analysis, and (3) demonstrating how these modelling techniques can extend our capacity to reason about obscure behaviors in secretive species. Salient results of the behavioral analysis are: (1) Male Cassin's Sparrows acquire territories by singing from prominent perches and engaging other males in song duels; the distinctive male song flights, in contrast, are important in establishing pair-bonds. (2) Division of parental responsibility is a prominent feature of Cassin's Sparrow natural history; females assume primary responsibility for care of the young, males are responsible for acquiring and maintaining territories and repelling predators. (3) Males show diurnal and interphasic variation in activity patterns; significantly more singing occurs in morning hours ( P < 0.05), and birds spend significantly more time perching once nesting begins ( P < 0.01). (4) Females spend a seasonal average of 95% of daytime hours in activities on the ground. The energetic analysis indicates that: (1) basal and thermostatic power comsumption preempt a seasonal average of 59% of the total daily energy expenditure ( H TD) of Cassin's Sparrow; (2) H TD in males varies from 59.1 kJ (incubation phase) to 118.9 kJ (pre-incubation phase) and in females 57.8 kJ (nestling care phase) to 87.0 kJ (pre-incubation phase); (3) the seasonal mean H TD for males is 88 kJ and 70 kJ for females; (4) the breeding cycle is potentially long, and males invest heavily in maintaining territories and pair bounds until circumstances in the habitat permit successful nesting. The model predicts, among other things, that male participation in the feeding of nestlings may become an energetic necessity with clutch sizes larger than average.