Low Vole Research Articles

Food selection by adult red foxes Vulpes vulpes during a water vole decline

Food selection by adult red foxes Vulpes vulpes was studied in a mountainous environment characterised by fluctuating populations of water voles. According to optimal foraging theory, the diets of adults and cubs should be similar during vole highs, with voles as staple prey. On the other hand, a difference should occur when water voles become scarce. Adults should then bring larger alternative prey to their cubs, and consume smaller prey at the capture site. Adult and cub scats were collected around breeding dens during a 6‐year period, from the peak to a near‐extinction phase of the water vole population, and analysed. As predicted by the optimal foraging theory, no significant difference was found between the diets of cubs and adults when voles were abundant. Furthermore, cubs consumed a higher proportion of large alternative prey, i.e. lagomorphs, than adults in the first year of low vole density. Unexpectedly, this difference did not persist the following years despite the water vole scarcity. The consumption of large alternative prey at the den decreased significantly, whereas the consumption of small alternative prey, i.e. invertebrates, increased. Consequently, the composition of prey brought to the den was similar to that consumed at the capture site. Food selection was probably affected by the unpredictability of large prey availability and accessibility. It is suggested that if small alternative prey proves less profitable to the cubs than large prey, a long‐lasting shortage of water voles could possibly become a factor regulating fox numbers in the area studied.

Contrasting phenotypic correlations in food provision of male Tengmalm's owls (Aegolius funereus) in a temporally heterogenous environment

We examined the food provision rate of male Tengmalm's owls,Aegolius funereus, during one 3 year vole cycle consisting of consecutive low, increase and peake vole years. The data were collected in the midnestling period when males provisioned the whole family. In the low vole year, males with a low loading index (g/cm2) of flying area fed their offspring more often than did males with a high loading index, whereas in the peak vole year the opposite trend was evident. Similar relationships were found in the food mass provisioned to the nest. In the increase vole year, male body size had no effect on feeding efficiency. In the peak vole year, when large voles are abundant, heavy males preyed on larger voles than were generally available in their territories, indicating that largeness may increase strike power in hunting attempts. In the low vole year, when breeding is costly due to food scarcity and extensive hunting area, small males are more economical fliers and efficient hunters than large males. The contrasting trends in correlations between male size and feeding efficiency in years of vole abundance versus scarcity suggest that no fixed phenotype may most efficiently cope with variable food supply.

Does feeding effort of Tengmalm's owls reflect offspring survival prospects in cyclic food conditions?

In an environment fluctuating in a predicatable manner with wide among-year variation in offspring mortality, fitness is largely influenced by the timing of reproductive investment. In vole-eating nocturnal Tengmalm's owls (Aegolius funereus), within-cycle variation in 1st-year survival of owlets is 3-fold as estimated by the recruitment probability of an offspring. It increases from the peak through the low to the increase phase of the vole cycle. We recorded prey delivery rates of males during a 3-year vole cycle using 4 h continuous recording at night. Males brought significantly more prey items per offspring in a low-vole year than in the increase and peak vole years. In the early night (10 p.m.-12 p.m.), males fed their families equally in the increase and peak vole years, whereas in the late night (0.01-2.00 a.m.) males reduced their feeding rate in the peak year but not in other years. Both prey number and prey mass per offspring were larger in the low and increase vole years than in the peak year, though in the peak phase food is most abundant. We suggest that feeding effort of site-tenacious, long-lived (mean lifespan 3.5 years) male owls culminates in the increase rather than in the peak phase of the vole cycle, because offspring survive better in the former phase.

Food limitation and social regulation in a red fox population

This study evaluates a conceptual model on functional and numerical response to short‐term fluctuating vole populations of a red fox (Vulpes vulpes L.) population in south‐central Sweden. The model assumes that this particular population is located in between socially regulated stable populations to the south and direct food‐limited populations to the north. The model predicts: (1) food availability as the primary factor for limiting fox numbers, causing reduced rates of reproduction and survival during years of low vole densities, and (2) density‐dependent regulation during years of increasing and high vole densities resulting in increased group sizes within territories of fixed dimensions. During 1973–1980 data were obtained from 1216 fox scats, 874 fox carcasses, 63 tagged foxes, nine radio‐collared females and from yearly den counts in an area of 130 km2, Eight predictions of the model were tested. These concerned the occurrence of small rodents in fox diet, fluctuations in the density of foxes, variations in the number of fox litters, the effect on reproduction of providing supplemental food during January–May, the proportion of vixens bearing a litter different years, dispersal of young males relative to that of young females throughout the vole cycle, and variations in mortality rates of young males and females. All tests were in favour of the conceptual model, and contradictory to alternative models.

Dietary shifts, niche relationships and reproductive output of coexisting Kestrels and Long-eared Owls.

Food samples of breeding Kestrels (Falco tinnunculus) and Long-eared Owls (Asio otus) were collected in the peak and low phase of their preferred prey (Microtus voles) in western Finland. Diets of pairs that bred as neighbours (≤1 km) with interspecifics were compared with those of non-neighbours. In both species, neighbouring pairs fed less on Microtus voles and more on alternative prey than did non-neighbours. Competition theory predicts that diet overlap should be lower during prey shortage and that diet similarity should be especially reduced in neighbouring pairs. Observations were consistent with expectations: diet similarity was lower in the low vole years and neighbouring pairs showed less diet overlap that non-neighbours. Differences in habitat composition and prey availability at the sample sites should not confuse the results. In addition to the high diet similarity, hunting habitats and nest sites of the species overlapped almost completely; they only showed clear temporal segregation in hunting. Probably because of food competition, the neighbouring pairs of both species produced significantly fewer young than the non-neighbours. These results contrast with the view that the diet composition and dietary shift of rodent-feeding predatory birds can be interpreted in terms of simple opportunistic foraging. In the breeding season, interspecific competition for food seems to be an important factor that affects the niches of these species, especially in northern areas, where the seasonal low phase of voles in spring and the number of alternative prey are lower than in more southern areas.

Breeding of Say's Phoebe in Arctic Alaska

found in 1970. Harvest mice were present each year and were most numerous in 1970. Other items taken were two Dipodomys, one Perognathus, one bird (unidentified), and one grasshopper. Kites are dependent on a prey whose numbers fluctuate markedly from year to year. During periods of low vole density, kites must select other prey, move to areas of higher vole density, or continue to prey on voles by increasing their hunting effort (Stendell, unpubl. data). At the Hastings Reservation there is no other small, diurnal mammal to replace the vole during periods of low vole density. Trapping and observations of small mammal signs suggest that the vole populations fluctuate in synchrony over the entire Reservation. Furthermore, kites were observed hunting the same fields during all 3 years. The fact that voles constitute a high, relatively constant percentage of the pellet contents indicates that the kite strategy at the Hastings Reservation is to remain and to spend more time per mouse capture. Pearson (J. Mammal. 52:41, 1971) has suggested that carnivores are an important factor in generating microtine cycles by continuing to prey on declining vole populations even after the population has reached its lowest density. Our data suggest that White-Tailed Kite predation may operate on Microtus populations in this manner.