Rodents and rodenteaters

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 of 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, but with regional exceptions (Chitty 1950). In addition, vole cycles tend to lengthen northwards from about three years between peaks in temperate and southern boreal regions to 4-5 years in northern boreal regions (Figure 19.1). The amplitude of the cycles also increases northwards, from barely discernable cycles in some southern temperate regions, to marked fluctuations further north, where peak densities typically exceed the troughs by more than 100-fold (Hanski et al. 1991). 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 even greater, with peaks sometimes exceeding troughs by more than 1000-fold (Shelford 1945). In most places, the increase phase of the cycle usually takes 2-3 years and the crash phase occurs within one year. Importantly, the crash phase often occurs during spring and summer, which can cause widespread breeding failure among rodent predators (e.g. Lockie 1955, Maher 1970). Specialist rodent-eating birds

1Most of the rodent species eaten by specialist predators live in open areas (including openings in forest) and feed mainly on plant leaves, but other rodent species eat mainly seeds. To some extent, the populations of the seed-eating rodents may fluctuate in relation to tree-seed crops, giving them a connection with the seed-eating finches described in Chapter 18. Different species of open-country microtine rodents are involved, depending on their occurrence in particular regions, Microtus agrestis and M. arvalis being widely eaten in Europe, and M. pennsylvanicus in North America.

Figure 19.1 Index of Field Vole Microtus agrestis densities in spring, summer and autumn in Kielder Forest, northern England, over 15 years showing the regular peaks in numbers. Note that in most years vole densities increased from spring to summer (the owl breeding season), but in some years they decreased from spring to summer. From Petty (1999).

Figure 19.1 Index of Field Vole Microtus agrestis densities in spring, summer and autumn in Kielder Forest, northern England, over 15 years showing the regular peaks in numbers. Note that in most years vole densities increased from spring to summer (the owl breeding season), but in some years they decreased from spring to summer. From Petty (1999).

include various owls, diurnal raptors and skuas (jaegers). In general, these birds would have to shift their breeding areas by at least several hundred kilometres every few years if individuals were to breed under adequate food conditions every year, and avoid the lows.

Owls and other predators show two main types of response to fluctuations in their rodent food supply. One type is shown in resident species, which tend to stay in the same territories year-round and from year to year. While preferring rodents, they also eat other things. They can therefore remain in the same area through low rodent years, switching to alternative prey, but their survival may be lower, and their productivity much lower than in good rodent years (Newton 2002). The Tawny Owl Strix aluco, Ural Owl Strix uralensis and Barn Owl Tyto alba are in this category, responding to prey numbers chiefly in terms of the number of young raised (Southern 1970, Saurola 1989, Petty 1992, Taylor 1994). This type of response, shown by resident rodent feeders, produces a lag between prey and predator numbers, so that high predator breeding densities follow 1-2 years after good food supplies and low densities follow poor supplies. Prey and predator densities go up and down in parallel, but with the predator behind the prey (Newton 2002). The fluctuations in prey are reflected in the movement patterns of the predator, whose dispersal distances tend to be longer in poor rodent years (see later).

The second type of response is shown by 'prey-specialist' nomadic species, which concentrate to breed in different areas in different years, depending on where their food is plentiful at the time (Figures 19.2 and 19.3). Typically, individuals might

in Finland. Small dots - territorial pairs; large dots - nests. From Solonen (1986).

have 1-2 years in the same area in each 3-5 year vole cycle, before moving on when prey decline. Response to change in food supply is almost immediate (with no obvious lag), and the increases in numbers from one year to the next are often far greater than could be explained by high survival and reproduction from the previous year, so must also involve immigration. As in irruptive seed-eaters, such observations lead to the inference that year-to-year changes in local breeding densities are due primarily to movements - immigration or emigration - depending on food conditions at the time.

The Short-eared Owl Asio flammeus, Long-eared Owl A. otus, Northern Hawk Owl Surnia ulula and, to some extent, Snowy Owl Nyctea scandiaca and Great Grey Owl Strix nebulosa are in this category, as are the Common Kestrel Falco tinnunculus, Hen (Northern) Harrier Circus cyaneus and Rough-legged Buzzard Buteo lagopus in some regions. Their local breeding densities can vary from nil in low rodent years to several tens of pairs per 100 km2 in intermediate (increasing) or high rodent years. In a 47 km2 area of western Finland, for example, over an 11-year period, numbers of Short-eared Owls varied between 0 and 49 pairs, numbers of Long-eared Owls between 0 and 19 pairs, and Common Kestrels between 2 and 46 pairs, all in accordance with spring densities of Microtus voles (Figure 19.4, Table 19.1; Korpimaki & Norrdahl 1991). All these raptors were summer visitors to the area concerned, and settled according to vole densities at the time.

Other variations in local breeding densities recorded over periods of years for owls and raptors that exploit cyclic prey species are referred to in Table 19.1. Their fluctuations contrast with findings from other owls and raptors that depend on a wider range of prey species and show much more stable breeding densities from year to year (Newton 1979, 2003). The main points to emerge are that year-to-year fluctuations in breeding densities are typically very much greater in irruptive than in regular migrants, that the year-to-year fluctuations parallel food supplies at the time, and that (by inference) the primary proximate cause of the

Figure 19.3 Year-to-year changes in the densities of Snowy Owls Nyctea scandiaca in different parts of northern Canada, as judged from questionnaire surveys of trappers and other local residents. Each dot marks the centre of an area covered by an individual trapper. Large dots - marked increases from previous year; small dots - marked decreases from previous year. Changes in owl numbers generally matched those of lemming numbers, and changes were synchronised over much of northern Canada, with a few regional exceptions. From Chitty (1950).

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