Vole Years Research Articles

Returning for more prey? Foraging in provisioning male Boreal Owls (Aegolius funereus)

Birds providing prey with a clumped distribution often return to the previous capture site after having delivered a prey item at the nest. However, details of this foraging tactic are still poorly known, in particular for birds of prey, which often travel far from their nest. We radio-tracked four provisioning male Boreal Owls (Aegolius funereus) during the nighttime and simultaneously recorded their prey deliveries by a video camera positioned at the nest in an increase year of the vole cycle. The camera allowed prey identification and made it possible to assign prey deliveries to fixes of the foraging owls, i.e. the last recorded fix before the owl returned to the nest for a prey delivery. Within a single night, the owls returned more often than randomly expected to the area where they had captured the previous prey item. A prey item delivered was more likely than randomly expected to be of the same species as the previous prey item delivered. However, the probability of a prey item delivered being of the same species as the previous prey item delivered was independent of whether the owl had returned to the area where it captured the previous prey and did not decrease with longer time elapsed since the previous delivery. The owls did not shift hunting area from one night to the next to a larger extent than they did over a longer time span, and overall tended to shift their hunting area gradually over more nights. Our study should be replicated in a peak vole year with higher prey abundance as well as in a low vole year, which should improve the study results by enabling a higher spatial and temporal resolution of the data on the owls’ movements.

Factors determining the frequency and productivity of double brooding of Barn Owls Tyto alba

ABSTRACTCapsule: Early nesting Barn Owls Tyto alba and those that switched nest sites fledged most chicks overall because they could fit two, more productive, nesting attempts into a breeding season.Aims: To determine the frequency and productivity of double broods in Barn Owls, and for double brooders, to determine what affects the probability of nest switching and how that affects productivity.Methods: We monitored the first egg date of each nesting attempt, whether it was in a ‘vole year’, whether a breeding attempt was first or a second annual attempt, the number of chicks fledged from each attempt, and whether a pair switched nest sites, if breeding twice, from 602 Barn Owl breeding attempts in an area of lowland England from 1996 to 2007. General linear models were used to determine predictors of the probability that a pair had a second brood and the number of chicks fledged in each nesting attempt, and then for those owls that double brooded, which variables best predicted the probability of switching, and the number of chicks fledged from the second nest. Finally, we tested whether switching resulted in a shorter laying interval and higher annual productivity.Results: Early nesting birds were more likely to double brood, although this was relaxed in vole years when later nesting birds also double brooded. Productivity (through increased numbers of chicks fledged or reduced chick loss) was higher the earlier a nest occurred, and there were more chicks fledged in good vole years and in second nesting attempts. Productivity, brood depletion, first clutch date and vole years did not determine whether a double brooding pair switched nesting sites. Productivity in the second nest did not change with a switch but productivity increased for early first nests and second nests with a shorter interval between the first and second nest. Switching however decreased nesting interval and nesting interval was also shorter if there were fewer fledglings from the first nest. Overall productivity was higher for pairs that switched.Conclusions: Double brooding in Barn Owls increased seasonal productivity substantially and its occurrence depended on vole abundance or early nesting. Nest switching between broods may be a strategy for earlier laying of the second brood. Provision of alternative nest sites, close together in a Barn Owl’s home range, may allow earlier re-nesting and so increase productivity.

Breeding dispersal of Eurasian kestrels Falco tinnunculus under temporally fluctuating food abundance

Costs and benefits of dispersal can vary in space and time, depending on environmental factors and individual state. Plastic, condition-dependent dispersal strategies, in which individuals rely on external cues such as food abundance to adjust their dispersal distances, are therefore expected to evolve in temporally fluctuating environments. We examined factors affecting breeding dispersal distances in Eurasian kestrels Falco tinnunculus subsisting on multi-annually and cyclically fluctuating voles as their main food. We attempted to avoid traditional bias in dispersal studies by having large study areas and by taking detection probabilities into account. We observed 320 dispersal events of male and 215 events of female kestrels from our study areas in western Finland during 24 yr. After correcting for distance-specific detection probability, the estimates of mean dispersal distances increased two-fold being still clearly higher for females than males. Vole abundance in the spring of settlement was more important in determining average dispersal distances than vole abundance in the previous autumn. At the population level (cross-sectional model), both males and females dispersed longer distances when the spring abundance of their main food (voles) was low compared to when it was abundant, as predicted by the food depletion hypothesis. At the individual level (longitudinal model), only females responded to the food situation by dispersing more when food abundance was low in the spring of settlement. Females also dispersed longer when vole abundance in the previous autumn had been high. Individual males did not respond to vole abundance, which implies that the population level response in males might have been caused by long-distance dispersers, which breed in the study area only in good vole years. Our results show that the dispersal distances of kestrels at northern latitudes depend both on individual properties (gender, age, and possibly individual tendency to disperse) and environmental conditions (temporal variation in main food abundance).

Sex roles during post-fledging care in birds: female Tengmalm’s Owls contribute little to food provisioning

Post-fledging care constitutes a large proportion of the total costs of parental care in many bird species. Despite being recognized as of critical importance to the survival of the offspring and their recruitment into the breeding population, post-fledging care, including the relative contribution by male and female parents, is under-studied. In this study, we quantified food provisioning (prey deliveries) by male and female Tengmalm’s Owl (Aegolius funereus) parents to their offspring both in the nestling and the post-fledging stages, in years of differing natural prey abundance. Parents and at least one offspring in 26 families were fitted with radio-transmitters. Male parents exhibited higher delivery rates than did females throughout the late nestling and post-fledging stages, but the intersexual difference was smaller in broods that were not deserted by the female at any stage. The female deserted her mate and offspring at some stage in 63% of the broods. Overall, deserted males delivered more prey to their offspring than did non-deserted males. Delivery rates were generally higher post-fledging. Prey delivery rates differed among years, and were highest in low vole years (probably because of smaller prey items), intermediate in peak vole years, and lowest in years of vole increase. Prey delivery rates increased with increasing brood size for both sexes, but the response was stronger in females. We suggest that female Tengmalm’s Owls contribute less than males because they are in a position to decide on their level of provisioning effort first, and because of the potential for re-mating after deserting the first brood.

Could fluctuating prey availability change protective nesting associations in forest birds? A hypothesis

One of the most striking behavioural adaptations for reducing nest predation is nesting near a top predator. However, in spite of increasing number of studies in forested landscapes reporting reproductive success benefits when nesting near protective associates, studies examining whether protective effects vary with food availability for the top predator are lacking. We asked whether the nest predation on ground nests of birds in Ural Owl (Strix uralensis) territories is dependent on varying food conditions for the owl. This was studied using dummy nests placed at different distances from the Ural Owl nests and repeating the study in different food conditions for the owls (low, increase and peak phases of the vole cycle). The proportion of predated dummy nests differed between different food conditions for Ural Owls being lowest in a low vole year. The distance from the owl nest affected the predation of dummy nests, but only in intermediate food conditions. Due to the lack of control nests located outside the owl territories, we are not able to draw conclusions about the background variation in the nest predation in different food conditions. We hypothesise that fluctuating prey availability for the top predator species under concern may have significant effects on the outcome of species interactions. This currently unexplored idea should be tested further, e.g. by including multiple breeding seasons in the future studies on protective nesting associations.

Decline of flying and red squirrels in boreal forests revealed by long‐term diet analyses of avian predators

AbstractConservation of species requires knowledge on population changes in time, but achieving such data in proper spatio‐temporal scales can often be difficult for rare and vulnerable species. We used long‐term (1950–2005) diet data of three avian predators (the nocturnal Ural and eagle owls and the diurnal goshawk) from four landscapes to study population changes of two forest‐dwelling species, the Siberian flying squirrel Pteromys volans and the red squirrel Sciurus vulgaris in Finland. We also determined the role of vulnerable flying squirrel in the diet of these predators. Both squirrel species were relatively rare in the diets of the three avian predators, with the exception of red squirrel in the diet of goshawks. The Siberian flying squirrel also appears to be a less important prey for avian predators than flying squirrel species (Glaucomys spp.) are in North American forest communities. Numbers of consumed flying squirrels, but not of red squirrels, increased as the abundances of voles increased in the diets of eagle and Ural owls, that is more flying squirrels were consumed during the good vole years than the poor ones. The declining trends in the diets of three avian predators appeared to be quite similar for both studied squirrel species, possibly indicating that the process behind the decline has been the same.

The Recovery of the Barn OwlTyto albain Friesland, Northern Netherlands: Population Growth in Relation to Landscape Features

By 1979, the Barn Owl Tyto alba had almost disappeared from the province of Friesland, northern Netherlands. With the placement of nest boxes and by specific forms of landscape restoration, the minimum breeding population of 8 pairs increased to 573 pairs in 2007. In this paper I compare the increase of the breeding Barn Owls across the three landscape types in Friesland, based on soil type sand, peat and clay. Each of these three soil types was divided into 15 categories of openness: open (1–5), partly-open (6–10) and closed landscapes with many trees and bushes (11–15). Each year, a large group of volunteers ensured that all nest boxes (up to 1470 in 2007) were inspected. The increase first became apparent in the sandy soil region with the breeding density exceeding the 1 pair/100 km2 threshold in 1982. This threshold was crossed on peat soils in 1985 and on clay soils in 1990. During the population incline, increasing numbers of nest boxes were made available so that nest sites were never in short supply. The population increase on all soil types stabilized as of 2000. In the peak vole year of 2004 the population density was 25.5 pair/100 km2 on sand, 20.1 pair/100 km2 on peat, and 10.9 pair/100 km2 on clay. To our surprise during the population increase Barn Owls did not show clear preferences for a particular degree of landscape openness on any soil type. To explain the highest densities of pairs that eventually established on sandy soils, the habitat type that is least open, I suggest that food conditions are more constant in the most heterogeneous landscape (sandy soils) which leads to the highest reproductive success. Only in good vole years did reproduction in the clay and peat areas exceeds that of sandy soils.

Population dynamics in a cyclic environment: consequences of cyclic food abundance on tawny owl reproduction and survival

1. Understanding which factors regulate population dynamics may help us to understand how a population would respond to environmental change, and why some populations are declining. 2. In southern Finland, vole abundance shows a three-phased cycle of low, increase and decrease phases, but these have been fading out in recent years. During five such cycles (1981-1995), all tawny owls Strix aluco were censused in a 250-km(2) study area, and their reproduction and survival were monitored. 3. Males and females showed similar dynamics, but experienced breeders recruited more offspring and had higher survival than first breeders. Offspring recruitment, but not survival of breeding individuals varied in accordance with vole abundance. 4. The population's numerical response to prey abundance was primarily due to first-breeding individuals entering the population in the increase phase when immigration was the highest. First-breeding birds were younger, but experienced breeders were older in more favourable vole years. 5. A stage-specific matrix population model integrating survival and fecundity showed that, despite obvious variation in fecundity between vole cycle phases, this variation had limited importance for overall tawny owl population dynamics, but that the survival of experienced breeders during the low phase is most important for population growth. 6. Model and data agreed that the vole cycle drives the dynamics of this avian predator by limiting the recruitment of new breeders during the low phase. Population dynamics hence differ not only from the classic example of the species in a more temperate region in the UK where the number of territories is stable across years, but also from the dynamics of other avian vole predators in Fennoscandia where the recurring crash in vole abundance drastically lowers adult survival thereby creating vacancies.

Maternal effects on offspring Igs and egg size in relation to natural and experimentally improved food supply

1. Maternal effects have been suggested to function as a mechanism for transgenerational plasticity, in which the environment experienced by the mother is translated into the phenotype of the offspring. In birds and other oviparous vertebrates where early development is within the egg, mothers may be able to improve the viability prospects of their offspring at hatching by priming eggs with immunological and nutritional components. 2. We studied how resource availability affects maternal investment in offspring by feeding Ural owl (Strix uralensis, Pall.) females prior to egg-laying in 3 years of dramatically different natural food conditions. 3. Supplementary feeding prior to laying increased body mass and the level of Igs of females measured at clutch completion. Supplementary fed Ural owl females laid larger eggs than control females, and had offspring with higher levels of Igs at hatching compared to offspring of control females. 4. We found variation in maternal allocation of resources to the eggs in response to environmental conditions: during a year of rapidly declining food abundance, maternal Igs in hatchlings were higher, whereas egg size was smaller compared to years with a more stable food supply. 5. Egg size had a positive effect on offspring body mass at fledging, whereas Igs at hatching did not affect Igs at fledging. 6. We conclude that maternal body condition and maternal Igs, as well as hatchling Igs and egg size are limited by food resources during egg production. Hatchlings rely on maternally derived Igs and, hence, our results suggest that mothers with high levels of Igs passively transfer higher Igs levels to their eggs instead of active manipulation of Igs levels in eggs. Ural owl egg size appears to be highly sensitive to short-term changes in food abundance, with important consequences for nestling growth.

Increased hunting effort buffers against vole scarcity in an urban Kestrel Falco tinnunculus population

Capsule In years with low vole abundance birds visited hunting grounds more frequently and for longer. Aims To describe diet composition, hunting behaviour, habitat choice and reproductive success of urban Kestrels during changing vole abundance. Methods For five years, we studied the hunting effort of Kestrels in a medium-sized city during the breeding season. Pitfall traps were used for determining vole abundance. Kestrel diet composition was determined from pellet analyses. The number of eggs and offspring was recorded during at least two consecutive visits for each nest and each breeding stage. Results In contrast to larger European cities, the Common Vole Microtus arvalis was a key part of the Kestrels' diet and did not fluctuate significantly according to vole availability. Reproductive success was quite high and stable throughout years with different vole abundance. In years of low vole abundance, the arrival frequency at hunting grounds and time spent there increased. During years with a low vole population, Kestrels had less hunting success and the rate of successful visits decreased. Therefore, Kestrels probably had to change hunting grounds more frequently. During low vole years Kestrels used less demanding techniques, e.g. perching, despite the lower success of these hunting techniques, to avoid extremely high energetic costs. Conclusions An increase in hunting helps to maintain a proper diet and consequently reproductive success. Vole abundance did not change dramatically during the study period, as reported by studies from western and northern Europe. The proportion of ruderal habitats on the city periphery is higher than in more monotonous farmland habitats. Ruderal habitats can be important when Kestrels look for mammals other than voles, especially during vole scarcity.

Effects of vole fluctuations on the population dynamics of the barn owl Tyto alba

Many predator species feed on prey that fluctuates in abundance from year to year. Birds of prey can face large fluctuations in food abundance i.e. small mammals, especially voles. These annual changes in prey abundance strongly affect the reproductive success and mortality of the individual predators and thus can be expected to influence their population dynamics and persistence. The barn owl, for example, shows large fluctuations in breeding success that correlate with the dynamics in voles, their main prey species. Analysis of the impact of fluctuations in vole abundance (their amplitude, peaks and lows, cycle length and regularity) with a simple predator prey model parameterized with literature data indicates population persistence is especially affected by years with low vole abundance. In these years the population can decline to low owl numbers such that the ensuing peak vole years cannot be exploited. This result is independent of the length and regularity of vole fluctuations. The relevance of this result for conservation of the barn owl and other birds of prey that show a numerical response to fluctuating prey species is discussed.

Do voles make agricultural habitat attractive to Montagu's HarrierCircus pygargus?

Loss and degradation of habitat threatens many bird populations. Recent rural land‐use changes in the Netherlands have led to a shift in habitat use by breeding Montagu's HarriersCircus pygargus. Since the 1990s, unprecedented numbers of this species have bred in farmland compared with numbers in natural habitat. Destruction of nests by agricultural operations, however, compromises breeding success. Between 1992 and 2005, the number of breeding pairs in the northeastern Netherlands was positively, though weakly, correlated with previous‐year estimated abundance of voles, mostlyMicrotus arvalis. In good vole years, the onset of laying was earlier and mean clutch size was larger. Vole abundance was relatively higher in set‐aside land and in high and dense vegetation. We suggest that agri‐environmental schemes aimed at increasing the availability of voles in agricultural breeding areas may be an effective management tool for the conservation of Montagu's Harriers in the northeastern Netherlands.

Vulnerability of Black Grouse (Tetrao tetrix) hens to Goshawk (Accipiter gentilis) predation in relation to vole cycles

In southern Norway from 1989 to 1996, the annual proportion of hens among Black Grouse killed by breeding Goshawks was not correlated with that of those shot in the previous autumn. The proportion of hens among Black Grouse killed by hawks was usually higher than that among Black Grouse shot, but in three out of four peak vole years, all with an early thaw, there was no such difference. In a stepwise regression model, both vole number (high or low), snow depth in spring and the proportion of hens among Black Grouse shot contributed significantly to explain the proportion of hens among Black Grouse killed by hawks. This was because predation on Black Grouse hens, but not that on cocks, was positively related to snow depth and negatively related to vole number. Vole populations peaked in post-mast years of Bilberry (Vaccinium myrtillus), possibly because the content of chemical defence compounds in Bilberry plants was low after a year with high seed production. Since the Bilberry is an important food plant for both voles and grouse, one possible explanation for the lower number of Black Grouse hens killed by hawks in peak vole years is higher food quality. If the ground is not covered by snow, the higher quality of Bilberry plants may reduce the time needed for foraging during egg formation and incubation, improve the body condition of the hens, and thus reduce their vulnerability to predation.

Habitat composition as a determinant of reproductive success of Tengmalm's owls under fluctuating food conditions

The effect of landscape composition on the breeding success of vole‐eating Tengmalm's owl (Aegolius funereus) was studied in western Finland at five different spatial scales (250–4000 m) around the nests during two consecutive three‐year population cycles of voles. Landscape composition had strongest effects on owl breeding in the decrease phase of vole cycles. Significant variation in owl breeding occurred along the productivity gradient from farmland predominated areas to barren hinterland. Owls tended to produce earlier clutches on territories predominated by agricultural areas in increasing vole years. A similar trend was observed in the decreasing phase of the vole cycle; owls breeding on barren hinterland seemed to delay breeding compared to owls breeding near agricultural areas. Surprisingly, nestling survival and fledgling production in the decreasing phase declined steeply with increasing proportion of farmland. Clutch size was not significantly related to landscape composition. The number of fledglings decreased with increases in clear‐cut and sapling areas in the decrease phase. During the declining years of vole abundance nestling survival increased from western farmland areas towards the eastern outlying district. These results indicate sudden summer decline of vole populations on farmland predominated habitats. This is probably due to that the number of vole‐eating predators, and hence their impact on vole populations is apparently higher in farmland areas than on forested hinterland. This finding gives support for the ‘spill‐over’ hypothesis, which states that predators and their exploitation tends to ‘spill over’ from luxuriant habitats to the barren habitats.

Survival of male Tengmalm's owls under temporally varying food conditions.

We analysed whether annual survival of male Tengmalm's owls in western Finland varies according to changes in the abundance of their main prey, Microtus voles. Our analyses were based on capture-recapture data over five 3-year population cycles of voles from 1981to 1995, each cycle consisting of consecutive years of low, increasing and decreasing vole abundance. Survival estimates of males in the increasing vole years (ca. 55%) were higher than in the decreasing vole years. In the latter case, males faced a drastic crash in Microtus vole numbers, and only about one-third of males survived over this crash. After the crash of vole populations, male survival increased rapidly (up to 76%) according to the recovery of Microtus vole populations. These results show that temporal variation in the abundance of their main prey modifies the survival of male Tengmalm's owls. In addition to survival, recapture rates also varied, largely due to the fact that in poor vole years a majority of males skipped breeding. The large among-cycle phase variation in survival (25-76%) probably creates selection for phenotypic plasticity in life-history traits related to survival and reproduction.

Parental investment and parent–offspring conflicts during the postfledging period in Montagu's harriers

We studied factors influencing the length of the postfledging period in the Montagu's harrier,Circus pygargus . Fledging date influenced the date of departure, for both wild birds and captive-reared birds (for which food supply was constant throughout the postfledging period), suggesting that departure time was constrained by migration. However, this relationship was absent in good food years, when some birds had short periods of dependency even if fledged early in the season. The end of the postfledging period was associated with a decrease in the food provided by the parents, coinciding with an increase in the fledglings' flying and hunting abilities and in aggressive food solicitation. The postfledging period was shortest when food was scarcest (when controlling for fledging date), and was also shorter for younger siblings (when controlling for brood effects), even though the parents provided less food in poor food years, and for nestlings in larger broods, particularly the younger ones. These results suggest that departure time is an adult's choice. However, we also predicted that if fledglings control parental investment, they should attempt to prolong the period of dependency when they receive little food. Length of the postfledging period was affected by food abundance, with shorter periods in better food conditions for equivalent hatching dates. In addition, the postfledging period was longer for birds from larger broods (when controlling for hatching date and food abundance), and for wild birds than captive-reared birds except in peak vole years. These results suggest that fledglings try to compensate for a deficiency in the food supply with longer periods of dependence, provided there is enough time before migration.

Predation on reptiles and birds by the common buzzard,Buteo buteo, in relation to changes in its main prey, voles

It has commonly been assumed that predators whose main prey are voles reduce their predation on other prey when voles are abundant. This assumption may, however, not be valid for slow-moving and ground-dwelling prey species with habitat demands similar to those of voles. In southern Norway, the field vole, Microtus agrestis, is an important prey for the common buzzard, Buteo buteo. Two other important groups of prey are birds and reptiles. In a forested study area with cyclic vole populations, prey remains and pellets were collected from buzzard nest sites in 2 peak vole years and 3 low vole years. Birds were most important as prey when vole populations were low. Reptiles, on the other hand, were most common as prey in peak vole years, possibly because the buzzards then concentrated on hunting ground-dwelling prey in habitats with high vole populations. In the study area, this was mainly clearcuts, which were commonly used also by basking reptiles. The vole-hunting adder, Vipera berus, may be especially vulnerable to buzzard predation in peak vole years because it is probably attracted to the vole-rich patches utilized by the buzzard.

Hatching asynchrony in the Eurasian kestrel Falco tinnunculus: an experimental test of the brood reduction hypothesis

Summary 1. We tested the brood reduction hypothesis by manipulating hatching spans of Eurasian kestrel Falco tinnunculus (Linnaeus) broods during two low vole years (1996–97) in western Finland. In addition, half the broods in 1997 were food supplemented during the entire nestling period. 2. Nestling mortality was high, occurring in 55% of nests, and the effect of manipulated hatching span on fledging success was more pronounced than either the effects of year or supplementary food. In contrast to the predictions tested, fledging success was higher in synchronous than asynchronous nests, both when food was limiting and when food conditions were substantially improved by food supplements. 3. We found no evidence that brood reduction resulted in higher quality (body mass) of surviving young in low vole years. On average, fledglings were heavier at synchronous than asynchronous nests. This was only the case, however, when food was limiting. 4. Most nestling mortality occurred when the chicks were 2 weeks old, and age of death did not differ between the brood types. Parents at synchronous nests fed their broods at a higher rate than at asynchronous broods, this divergence arising only in the latter part of nestling period. 5. This is the first hatching span experiment where synchronous hatching seemed more profitable than asynchronous during conditions of both food scarcity and food abundance. The higher parental workload at synchronous nests could, however, counterbalance the benefits of synchronous hatching. 6. Hatching in natural broods of Eurasian kestrel is more synchronous in low vole years compared to high vole years. The fact that synchronous broods out‐performed asynchronous broods in both ‘low’ years of the study is consistent with the facultative manipulations. The benefit of asynchrony in high vole years is still unclear.

Parental Effort and Blood Parasitism in Tengmalm's Owl: Effects of Natural and Experimental Variation in Food Abundance

We investigated the association between parental effort and susceptibility to haematozoan parasites in vole-eating Tengmalm's owls (Aegolius funereus) from 1993 to 1995. In a poor vole year (1993), almost all breeding Tengmalm's owls were infected with Trypanosoma avium, whereas in a good vole year (1994), only a few owls were infected. In a moderate vole year (1995), we found an intermediate prevalence of trypanosomes. In the moderate vole year, trypanosome-infected females were in poorer condition than were uninfected females. In the same moderate vole year, high parental effort was associated with increased susceptibility to haematozoan parasites for both genders, whereas in a good vole year no such association was found. In two breeding seasons (1996-1997) of relatively low vole abundance we tested whether supplementary food decreased parasite loads. In accordance with correlative data, trypanosome prevalence was lower among supplemented than control females. Our results support a hypothesis of a trade-off between parental effort and immunocompetence, and emphasize the importance of varying environmental conditions and physical condition of individuals on susceptibility to haematozoan infections.

Haematozoan Infections in the Eurasian kestrel: Effects of Fluctuating Food Supply and Experimental Manipulation of Parental Effort

The influence of parental effort on susceptibility to parasitism was investigated experimentally in the Eurasian kestrel (Falco tinnunculus) in Finland. Parental effort was manipulated by either enlarging or reducing broods by 1-2 young, while unmanipulated broods served as controls. This was done during 3 breeding seasons, during which the densities of the major prey of kestrels (voles) were relatively low, high and intermediate, respectively. Two, taxon specific, methods were applied for determining the prevalence (per cent individuals infected) of extracellular Trypanosoma spp. and intracellular Haemoproteus spp. infection. Blood samples were taken from females during the incubation phase, and from both parents during the mid-nestling phase. Trypanosoma was more prevalent when food was less available. Furthermore, the increase in the prevalence of Trypanosoma among females was most pronounced during the poor vole year. In contrast, Haemoproteus infection seemed not closely related to annual supply of main prey. Manipulations of parental effort were related to parental infection, but the effect differed between genders. The prevalence of Trypanosoma in males (main provider of young) increased with experimental brood size, and there was an interaction between food supply and brood size manipulation so that the difference in prevalences between reduced and enlarged broods increased with decreasing food supply. Among females, no such an interaction was found. Instead, the effect of brood enlargement on Trypanosoma prevalence of females was apparent in the pear of relatively high vole densities only. Manipulation of brood size did not have clear effects on Haemoproteus infection in either gender. These results support the idea that increased parental effort may make hosts susceptible to haematozoan infection, and are the first to suggest that resource levels (food supply) can modify the susceptibility, indicating that the magnitude of reproductive costs can be attributed to Variation in environmental conditions.