ECOLOGICAL ENERGETICS OF THE LONG-EARED OWL (ASIO OTUS) H. WIJNANDTS Zoological Laboratory, University of Groningen, Kerklaan 30,9751 NN Haren, The Netherlands CONTENTS 9.1. Introduction........................................... 61 9.2. Annual cycle in body weight.... 62 1. Introduction................................................... 1 9.3. Body weight and flight costs: theory............. 64 1.1. Background........................................... 1 9.4. Direct observations of food consumption in 1.2. Organization ofthe paper.......................... 2 the reproductive period......... 64 1.3. Study area.... 3 9.5. Body weight changes during breeding and 1.4. Housing and general techniques.... 4 parental energy budget............................. 67 2. Natural History of the Long-eared Owl................. 4 10. Moult........................................................... 68 2.1. Winter.................................................. 4. 10.1. Introduction ~.............. 68 2.2. Home range during autumn and winter......... 7 10.2. Methods................................................ 68 2.3. Breeding phenology............. 8 10.3. Feather number and plumage weight............ 69 2.4. Breeding densities................................... 11 10.4. Moult of flight feathers............................. 70 2.5. Resume 11 10.5. Moultof body feathers.............................. 74 3. The diet......................................................... 12 10.6. Moult and the energy budget...................... 75 3.1. Literature review..................................... 12 11. Final discussion: the economic hunter.. 77 3.2. Methods................................................ 12 12. Acknowledgements.......................................... 80 3.3. Prey spectrum 13 13. Summary....................................................... 81 3.4. Repercussions of the vole cycle................... 14 14. References..................................................... 83 3.5. Comparison with other areas 15 15. Samenvatting.................................................. 88 4. Metabolized energy for free-living throughout the year.............................................................. 17 4.1. The approach......................................... 17 1. INTRODUCTION 4.2. Daily pellet production............................. 17 4.3. MEC and the relationship between pellet 1.1. BACKGROUND weight and the weight of the ingested prey..... 24 4.4. GEl and ME outside the breeding period...... 26 With the growing interest in cost-benefit anal­ 5. Seasonal variation in metabolic rate..................... 30 yses the study of energy flow in the life of birds 5.1. Introduction. .......................................... 30 has entered a new phase, where the central 5.2. Methods... 30 5.3. Results.................................................. 31 problem is the interaction between time and en­ 5.4. Discussion.... 35 ergy as limiting factors determining the behav­ 6. Metabolized energy under caged conditions........... 38 iour of the individual (King 1974). Until recent­ 6.1. Introduction........................................... 38 ly such studies were severely hampered on the 6.2. Methods................................................ 39 6.3. Results.................................................. 39 income side because of the shortcomings of 6.4. Discussion 40 standard techniques when applied to the field 7. Balancing the energy budget for free-living owls situation, but a number of new avenues have in winter........................................................ 44 7.1. Introduction........................................... 44 opened up (Gessaman 1973, King 1974, Eb­ 7.2. Telemetry techniques 44 binge et aI. 1975, Kendeigh et aI. 1977, Wals­ 7.3. Durationand distribution of hunting flight..... 46 berg 1980). Raptorial birds offer advantages pe~ 7.4. Power requirement for flight.. 48 7.5. Balancing the budget... 48 culiar to their way of life, and from an early date 8. Reproduction................................................. 49 attempts were made to quantify energy intake 8.1. Introduction........................................... 49 for free-living birds. Graber (1962) relied on de­ 8.2. Methods 49 ductions from pellet analysis to reconstruct daily 8.3. Gonadal growth and egg production...... 50 8.4. Energy cost of incubation and brooding........ 51 energy intake for three owl species, but as we 8.5. Growth of the owlets................................ 53 shall see several inferences basic to his method 8.6. Changes in body composition with age.......... 54 require reVISIOn. More recently Tarboton 8.7. Basal metabolic rate and temperature regu- lation in the owlets................................... 55 (1978) and Koplin et aI. (1980) presented fig­ 8.8. Efficiency of digestion and metabolized ures on daily energy intake in kites and kestrels energy in the owlets................................. 56 where these pitfalls have been avoided, but 8.9. An energy budget for growth ; 58 their data cover only the winter period. The on­ 8.10. Discussion 59 9. Parental e[)ergetics and the annual cycle of body ly data set for the entire annual cycle concerns weight , 61 the Common Buzard, Buteo buteo, but an inde- Ardea 72 (1984): 1-92 2 ECOLOGICAL ENERGETICS OF THE LONG-EARED OWL [Ardea 72 INTAKE EXPENDITURE DIET ® PELLET Q PRODUCTION I§ , , , , , ~ MOULT ®I , , , REPRODUCTION , , PREY DELIVERIES (NESTING PERIOD) Fig. 1. Schematical overview of the energy budget of the Long-eared Owl, with methods followed in the estimation of food intake of free-living birds (left) and the components of energy expenditure (right). Numerals refer to the chapters (or chapter sections) iI,l which the relevant material is presented; the bar graph shows the approximate values for the Long-eared Owl outside the breed­ ing season. pendent check on the reliability of the method is by our group in a follow-up study on the kestrel not included (Sylven 1982). (Rijnsdorp et at. 1981, Daan et at. in prep.). The aim of the present study was, to measure the daily gross energy intake (hereafter abbre­ 1.2. ORGANIZAnON OF THE PAPER vi~ted GEl) throughout the annual cycle in the The concept of energy budgets as here em­ Long-eared Owl, Asia atus, by means of field ployed involves the energy intake of individual and laboratory measurements of pellet produc­ birds (as determined from food consumption tion, the analysis of pellets collected in the field, and utilization) as one side of the budget, and and during the reproductive period indirect ob­ the partitioning of this income over the various servation at the nest. Secondly, on the side of categories of energy expenditure on the other energy expenditure, a partitioning over the var­ (see Fig. 1, showing also the sequence of chap­ ious functions was aimed at by a combination of ters). field and laboratory techniques. The central as­ Chapter 3 covers the composition of the ,diet sumption is that the energy-demanding func­ and its seasonal variation, based on analysis of tions (especially reproduction and moult) will be pellets collected in the field. Estimation of the so timed in the ann.ual cycle to result in an opti­ amount of food eaten daily is presented in chap­ mal seasonal allocation of energy (King 1974, ter 4, relying on deductions from a study of the Murton & Westwood 1977). The logical next rate of daily pellet production throughout the step, bringing this quantification to bear on the year (4.2. and 4.3.) supplemented by the direct timing of individual pairs, demands more de­ observation of prey delivery at, the nest for the tailed information on hunting success and forag­ reproductive period (9.4.). When converted in ing cost than can feasibly be collected on noctur­ terms of energy (kJ = kilo-Joules) these data nal hunters such as owls, and is being pursued provide the daily gross energy intake (GEl). 1984] ECOLOGICAL ENERGETICS OF THE LONG-EAREp OWL 3 Not all ofthis energy can be metabolized by the 1.3. STUDY AREA birds, and the metabolizable energy coefficient Most of the field work was done in 'the province of Gro­ (MEC) was determined in trials with captive ningen and the northern part of Drente, situated in the birds, where food intake and also excretion north-eastern part of the Netherlands (see Fig. 3A). This whole area is generally flat and can be divided in four were quantified (4.3.). Taken together these characteristic parts. The northern part of Groningen is data provide the annual pattern in daily metab­ mainly agricultural land with very few wood lots. Only close olized energy (MEfield) for free-living owls to farms and villages are some small plantations found. The (4.4.). soil is mainly marine clay. The eastern and south-eastern part of Groningen is a typical fen-peat landscape. Outside Turning now to the routes of energyexpendi­ the villages the land is largely farmland. There are' some ttire, chapter Scovers basal metabolic rate plantations, from a few to less than 100 ha. Scattered over the whole area are many locations where natural g<ts is (BMR) and energy devoted to temperature reg­ brought to the surface, each up to a few ha in size and illu­ ulation (TR), in both cases based on mea­ minated at night. surements of the oxygen consumption of indi­ The landscape to the south of the city of Groningen and viduals held at constant temperatures. Two oth­ the northern part of Drente is much more wooded, but it is still mainly farmland (with an emphasis on dairy farming). er