Vesa Koivunen

Experimental increase of predation risk induces breeding dispersal of Tengmalm's owl

Nest predation and its avoidance are critical components of an individuals fitness and play an important role in life history evolution. Almost all studies on this topic have been observational, and thus have not been able to separate the effects of individual quality, habitat selection and predation risk of given nest sites from each other. More experimental studies on nest predation and breeding dispersal, therefore, are needed to avoid confusing interpretations of the results. In western Finland, pine marten (Martes martes) predation risk was experimentally simulated at the nests of Tengmalms owls (Aegolius funereus) by using a caged American mink (Mustela vison) as a predator. Nests without exposure to a mink served as controls. In accordance with our predictions and earlier observational studies, males exposed to simulated predation risk increased nest-hole shift and breeding dispersal distances compared to control males. Nest-hole shift and long breeding dispersal distances probably decrease the risk of nest predation, because pine martens are known to revisit nest-holes they have found.

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 Tengmalms owls (Aegolius funereus) from 1993 to 1995. In a poor vole year (1993), almost all breeding Tengmalms 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.

Reproductive success and parental effort of Tengmalm’s owls: Effects of spatial and temporal variation in habitat quality

Spatial and temporal variation in habitat quality may affect reproductive success and survival of animals. We examined habitat quality of vole-eating Tengmalms owls Aegolius funereus for five years (1989-1992 and 1994) under cyclic food conditions in western Finland. We estimated food abundance (the most important component of habitat quality in Tengmalms owl) by snap-trapping small mammals on individual territories and by censusing birds (an important alternative prey of owls). We classified territories according to the owl breeding frequency in an earlier 10-year period (mean: 2 breeding attempts per territory) as poor (0-2 breeding attempts) or good ones (> 3 breeding attempts). More Microtus and Clethrionomys voles were snap-trapped on good territories than on poor ones in the increase and peak vole years, but not in the low vole years. Fledgling production, but not egg production, increased with the number of voles snap-trapped on indi- vidual territories. The abundance of small birds was higher on good than on poor territories. In the low vole year, more small birds were censused near occupied than unoccupied nest-boxes of owls, which suggests that in poor vole years owls may shift to sites with abundant alternative prey within their territories. In the peak vole year, the size of food caches, food provi- sioning rate of males and nest defence intensity of females were higher on good than on poor territories. These results suggest that main and alternative food sources may have different importance over time in determining habitat quality and that the abundance of different prey types may affect parental effort and reproductive success of owls. Therefore, studies on species subsisting on cyclically fluctuating food sources should incorporate both temporal and spatial variation in resource abundance.

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

Abstract. We analysed whether annual survival of male Tengmalms 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 Tengmalms 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.

Do radio-collars increase the predation risk of small rodents?

Small radio-collars are useful for the to study of movements and predation risk in cryptic small rodents. The results may be biased, however, if collars alter the behaviour or vulnerability of tagged individuals. We measured the effects of radio-collars (1.5 g) on the behaviour of voles [Microtus agrestis (body mass 33 g), M. epiroticus (27 g) and Clethrionomys glareolus (18 g)] in two aviary experiments. First, the mobility of collared and non-collared voles was studied 1 to 4 days after the attachment. On the 1st day after attachment but not later, collared voles moved less than non-collared voles. Second, the risk of predation by Tengmalms owls (Aegolius funereus) was estimated on the 4th day after attachment. Owls captured 5 of 13 collared voles and 8 of 13 non-collared voles. The results suggest that radio-collars do not markedly alter the behaviour or increase the risk from avian predation in these small rodents.

Clear-cut areas and breeding success of Tengmalm's owls Aegolius funereus

The effect of mature forest fragmentation on breeding success of vole-eating Tengmalms owls Aegolius funereus was studied in western Finland during 1981–1995. The owl pairs produced more fledglings in territories with high (>30%) proportions of clear-cut and plantation areas than in territories with low (<30%) proportions of such areas. Territory occupancy, clutch size, laying date, and parental characteristics such as wing length, body mass and age were not associated with the proportion of clearcut areas within territories. Snap-trapping data from the ‘increase’ vole year 1994 revealed that the Microtus vole density tended to be higher in large clear-cuttings than in small ones. The increasing amount of sapling and clear-cut areas in coniferous forests during the last three decades has created suitable grass habitats for Microtus voles, and simultaneously new hunting habitats for Tengmalms owls. Therefore, forest fragmentation due to clear-cuttings may benefit Tengmalms owls at the present scale. However, nest-boxes should be erected and old groves should be protected, because forest harvesting apparently reduces the number of suitable natural cavities for owls in the long run.

Are mature female voles more susceptible than immature ones to avian predation

It is well known that a number of activities related to reproduction can increase the predation risk for individuals. Both in the aviary and in the field, we studied whether maturity of female field voles (Microtus agrestis) affected their behaviour and consequently their risk of predation by Tengmalms owls (Aegolius funereus). In an aviary, we recorded the behaviour of mature and immature voles in control (owl absent) and predator (owl present) treatments, but did not find obvious differences in behaviour or vulnerability between mature and immature female voles. In the field, we compared maturity status of female field voles snap-trapped in owl territories with those caught by breeding owls in 1992 and 1994. In accordance with the results from an aviary experiment, there were no obvious differences in vulnerability to Tengmalms owls between mature and immature individuals. This suggests that mature and immature female field voles are equally exposed to avian predation.