Owls.1. Newton, I. 2002. Population limitation in Holarctic Owls. Pp. 3-29 in ‘Ecology and conservation of owls’, ed. I. Newton, R. Kavenagh, J. Olsen & I. Taylor. CSIRO Publishing, Collingwood, Australia. POPULATION LIMITATION IN HOLARCTIC OWLS IAN NEWTON Centre for Ecology and Hydrology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire PE28 2LS, United Kingdom. This paper presents an appraisal of research findings on the population dynamics, reproduction and survival of those Holarctic Owl species that feed on cyclically-fluctuating rodents or lagomorphs. In many regions, voles and lemmings fluctuate on an approximate 3–5 year cycle, but peaks occur in different years in different regions, whereas Snowshoe Hares Lepus americanus fluctuate on an approximate 10-year cycle, but peaks tend to be synchronised across the whole of boreal North America. Owls show two main responses to fluctuations in their prey supply. Resident species stay on their territories continuously, but turn to alternative prey when rodents (or lagomorphs) are scarce. They survive and breed less well in low than high rodent (or lagomorph) years. This produces a lag in response, so that years of high owl densities follow years of high prey densities (examples: Barn Owl Tyto alba, Tawny Owl Strix aluco, Ural Owl S. uralensis). In contrast, preyspecific nomadic species can breed in different areas in different years, wherever prey are plentiful. They thus respond more or less immediately by movement to change in prey-supply, so that their local densities can match the local food-supply at the time, with minimum lag (examples: Short-eared Owl: Asio flammeus, Long-eared Owl A. otus, Great Grey Owl Strix nebulosa, Snowy Owl Nyctea scandiaca). Some owl species that exploit sporadic food-supplies move around mainly within the breeding range (examples: Tengmalm’s Owl Aegolius funereus, Northern Hawk Owl Surnia ulula). In other species, part of the population migrates to lower latitudes for the winter, thereby avoiding the worst effects of snow cover, but returns to the breeding range each spring, settling wherever voles are plentiful (examples: Short-eared Owl Asio flammeus, Long-eared Owl A. otus). In all these species, as well as in the hare-eating Great Horned Owl Bubo virginianus, foodsupply affect every aspect of demography, including age of first breeding, reproduction (proportion of pairs laying, hatching and fledging young, clutch and brood sizes), juvenile and adult survival, natal and breeding dispersal, and winter irruptions. In eastern North America, irruptions of Snowy Owls Nyctea scandiaca document since 1880 have occurred every 3–5 years, Owls•book 24/04/2002 03:05 PM Page xvi Population limitation in owls 3 between years. Such marked changes in food-supply affect the reproductive and survival rates of owls, as well as their movements, which in turn can bring about rapid changes in their local densities. Cycles in prey numbers Two main systems are recognised: (1) an approximately 3–5 year cycle of small (microtine) rodents in the northern tundras, boreal forests and temperate grasslands; and (2) an approximately 10-year cycle of Snowshoe Hares Lepus americanus in the boreal forests of North America (Elton 1942, Lack 1954, Keith 1963). The numbers of certain grouse species also fluctuate cyclically, in some regions in parallel with the rodent cycle and in others in parallel with the hare cycle (Hörnfeldt 1978, Keith & Rusch 1988). Populations of microtine rodents do not reach a peak simultaneously over their whole range, but the cycles may be synchronised over tens, hundreds or many thousands of square kilometres, out of phase with those in more distant areas. However, peak populations may occur simultaneously over many more areas in some years than in others, giving a measure of synchrony, for example, to lemming cycles over large parts of northern Canada, with few regional exceptions (Chitty 1950). In addition, the periodicity of vole cycles tends to increase northwards from about three years between peaks in temperate and southern boreal regions, increasing to 4–5 years in northern boreal regions. The amplitude of the cycles also increases northwards from barely discernible cycles in some temperate regions to marked fluctuations further north, where peak densities typically exceed troughs by more than 100-fold (Hansson & Henttonen 1985,Hanski et al. 1991). Further north, on the tundra, the periodicity of lemming cycles is in some places even longer (5–7 years between peaks on Wrangel Island, Menyushina 1997), and the amplitude is even greater, with peaks sometimes exceeding troughs by more than a thousand-fold (Shelford 1945). In most places, the increase phase of the cycle usually takes 2–3 years, and the crash phase 1–2 years. Importantly, the crash phase often overlaps with spring and summer, a time when owls and other rodent predators are breeding. In research projects, the numbers of rodents in an area are usually monitored by regular trapping programmes, measuring the numbers caught per unit effort (such as ‘trap days’), or less directly by counting the numbers of signs (such as droppings, runs or cut grass stems) per unit area. Different measures of rodent abundance taken at the same dates in the same area are usually closely correlated with one another, giving reassurance over the validity of the different indices (e.g.Hansson 1979, Petty 1999). An example of results from the same Field Vole Microtus agrestis population trapped three times each year over a period of years is shown in Fig. 1. Peaks in abundance occur at regular intervals of 3–4 years, but the height of the peaks and the depths of the troughs vary from one cycle to the next.Moreover, the trend in numbers at particular seasons can vary from one year to another. In some springs, when owls are breeding, they face an increasing food-supply, whereas in other springs, as mentioned above, they face a sharply decreasing food-supply. As expected, these contrasting situations have markedly different effects on owl breeding success (see below). An overall rodent density of less than 2–4 individuals per hectare (or two captures per 100 trap-nights) has been estimated as the threshold in prey density below which rodent-eating birds of prey do not breed (Hagen 1969, Potapov 1997), but this figure could well vary between areas and between species. The longer hare cycles have been less studied, but peaks in numbers can exceed troughs by 2 Ecology and Conservation of Owls at a mean interval of 3.9 years (SE O.13). In periods when information on lemmings was available from breeding areas, mass emigration of owls coincided with crashes in lemming numbers. Similar periodicity has been noted in the movements of some other owl species in both North America and Europe. In most (but not all) irruptions, juveniles predominated. Irruptions of Great Horned Owls (and Northern Goshawks Accipiter gentilis) in North America have occurred for 1–3 years at a time, at approximately 10-year intervals, coinciding with known lows in the hare cycle. While food-supply is the primary limiting factor, nest-site shortages, adverse weather and other secondary factors can sometimes reduce owl breeding densities and performance below what food-supply would permit. INTRODUCTION Studies on Holarctic owls have contributed greatly to our understanding of the processes of population limitation in birds. About 33 different owl species breed in this region, 14 in the Palaearctic, 12 in the Nearctic, and a further seven in both regions. Nearly half of these species feed largely or entirely on microtine rodents (lemmings and voles), two on lagomorphs (rabbits and hares), one on fish, and the rest mainly on insects or other invertebrates. In this paper, I shall concentrate on the rodent and lagomorph feeders, partly because they have been better studied than the others, but also because they provide some of the best evidence available among birds for the role of food-supply in influencing densities and performance. Other factors important in the ecology of these owls include winter snow cover and nest site availability, the effects of which vary with the hunting methods, life style and dietary range of the species themselves (Korpimäki 1992). I shall be concerned only with the limitation of numbers within areas of suitable habitat, and not with the effects of habitat loss and fragmentation, which, although important in conservation, are outside the scope of this review (but see Lande 1988, Lamberson et al. 1992, La Haye et al. 1994, Redpath 1995). Some familiar aspects of owl biology influence the way in which owls respond to food conditions, and are affected by shortages and other adverse factors, such as snow. Their acute hearing, and ability to see in poor light, enable owls to hunt nocturnal mammals hidden under ground vegetation or snow, in a way that diurnal raptors cannot, giving them a particular advantage at high latitudes in winter. Secondly, most species nest mainly or wholly in cavities which protect them to some extent against predation, while others defend their nests aggressively. In consequence, nest predation levels are often low compared with other birds (although exceptions occur, see later). Thirdly, clutch sizes in many species are large, and very variable, so that owls can take advantage of good food conditions when they occur. Most also start incubating from the first or second egg, so that hatching is asynchronous and broods typically contain young of different sizes. This in turn provides a means of rapid brood reduction if food becomes scarce, for the smallest young dies first, followed by the second smallest and so on. THE PREY Most of the mammal species eaten by owls are ground dwelling, hidden under thick grass or other low vegetation, and are active mainly at night (though some mainly or also by day).