Masman D., Gordijn M., Daan S. & Dijkstra C. (1986) Ecological energetics of the Kestrel: field estimates of energy intake throughout the year. ARDEA 74 (1): 24-39

1. Daily metabolizable energy intake (M) was estimated on 375 days of dawn to dusk observation of individual free living Kestrels in the different phases of the annual cycle. Prey species, meal frequency and meal duration were obtained from the behavioural protocols. The seasonal variation in energy content and assimilation quotient of the different prey types was established in the laboratory. 2. During winter (October-March) prey were primarily small mammals (95% common vole Microtus arvalis, 2% common shrew Sorex araneus). In summer (April-September) the diet was still dominated by the common vole (91 %) but the relative frequencies of common shrew (6%), songbirds (2%) and juvenile waders (1 %) increased (Table 1). 3. Common vole mass estimates were obtained from four sources: 1) cached voles, weighed by observers after finding the cache (n = 71), 2) Kestrel meal durations (n = 879), which were significantly correlated with the prey mass eaten (Fig. 1), 3) nest deliveries weighed during observations with weighing platforms in the nest-boxes (n = 239), and 4) voles collected in break-neck trap censuses in the study area (n = 1100). During summer the mean mass of voles caught by Kestrels was smaller than that of voles trapped. During reproduction (April-July) males treated the voles caught selectively. The voles eaten immediately after capture had a smaller mean body mass than voles delivered to the dependents or voles cached. Voles delivered to the female and/or the nestlings had a smaller mean body mass than voles cached. During winter, the vole population was non-reproductive and homogeneous in size. Mean body mass of voles eaten or cached by Kestrels and of voles trapped were not distinguishable (Fig. 2, Table 2). 4. Water, fat, protein and ash contents of common voles analysed varied with season (Table 4). The energy content per g dry matter did not show significant seasonal variations (21.4 ¦ se 0.1 kJ/g). The assimilation quotient for a common vole diet in summer was higher than in winter. These variations resulted in a metabolizable energy equivalent of 4.6 kJ/g fresh in winter and 4.2 kJ/g fresh in summer. 5. In the non-reproductive season metabolizable energy intake varied with time of day. Intake rate increased at the end of the active day. In contrast energy intake in the reproductive season was constant over the active period. 6. Daily metabolizable energy intake in winter showed large day-to-day variations, related to weather conditions since there are severe meteorological constraints on flight hunting (Fig. 4). Caching and retrieving prey buffers such variations. 7. Mean daily metabolizable energy intake varied through the annual cycle (Fig. 5, Table 6). In females the mean intake rate was highest during egg-laying (371.2 ¦ se 32.1 kJ/day), an elevation of 35% above the winter level. The mean intake rate of the males reached a maximum in the nestling phase (415.6 ¦ 42.3 kJ/day) an elevation of 52% above the winter level, and coinciding with the seasonal maximum in flight-activity. The minimum levels of intake and flight-activity (males and females during moult) also coincided. There was a correlation between daily energy intake rate and daily flight-hours, for male Kestrels tending nestlings (Fig. 6). On days with less flight-activity than average (4.6 h/day) male Kestrels seemed able to maintain a balance between energy intake and energy expenditure. On days with more than 4.6 flight-hours, the energy intake levelled off at a mean of 430.0 ¦ se 23.4 kJ/day. It is suggested that M in the latter condition was constrained by a limit to the total amount of food processed per day, which was insufficient to cover the energy costs. 8. The implications for the energy budget of seasonal variations in energy intake and time allocation are discussed. The Kestrel appears to use various options to meet seasonal peak energy demands: increased daily metabolizable energy intake, accumulation and mobilization of body reserves and changes in energy allocation to thermoregulation and activity.