Ecological energetics of the Kestrel:Field estimates of energy intake throughout the year

• Main Authors: Masman, D., Gordijn, M., Daan, S., Dijkstra, C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1986
• Subjects: Common vole; Microtus arvalis
• Online Access: https://hdl.handle.net/11370/3c907a11-d7b5-47ae-a1c5-c6a6ab10aa66; https://research.rug.nl/en/publications/3c907a11-d7b5-47ae-a1c5-c6a6ab10aa66; https://pure.rug.nl/ws/files/3397378/1986ArdeaMasman.pdf; https://pure.rug.nl/ws/files/100797804/Ardea_Masman_CD_86.pdf

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. 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, 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. 4. Water, fat, protein and ash contents of common voles analysed varied with season. 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 ...