What Explains Forest Grouse Mortality: Predation Impacts of Raptors, Vole Abundance, or Weather Conditions?

We investigated predation rates of black grouse chicks during 1985–2007 in two localities in western Finland in light of three predation hypothesis: The Alternative Prey Hypothesis (APH) stating that vole-eating generalist predators cause a collapse in grouse reproduction after voles’ decline, the Main Prey Hypothesis (MPH), where grouse specialised predators by a lagged response cause an inversely density dependent predation for prey and the Predation Facilitation Hypothesis (PFH), where generalist and specialist predators act in concert. We also studied the effect of weather on grouse reproduction. We found that buzzard predation alone did not support APH, but did so when combined with goshawk predation. Kill rate by goshawks showed a linear response for black grouse chicks but was not density dependent. It, however, explained the losses of chicks but not their autumn density. Combined density of chicks with adults correlated with vole index in the latter study period (since 1994), thus, giving some support for APH. Weather seemed to have no effect on black grouse reproduction. Although buzzards and goshawks took, on average, only 10% of hatched grouse chicks we conclude that the among-year survival pattern of juvenile forest grouse may largely be determined by raptor predation

Poor reproductive success of polygynously mated female birds with obligatory bi-parental care: a result of deceptive behaviour of males?

Social polygyny usually benefits males by increasing the number of offspring, whereas it is detrimental for females as they must share the resources provided by their mate. An intersexual conflict may exist in animals with obligatory bi-parental care, such as birds of prey, in which females incubate and brood, whereas males provision food for their families. Long-term ringing data from Eurasian kestrels (Falco tinnunculus) breeding in nest-boxes and data on density indices of main prey animals (voles) were collected during 1985-2013 in western Finland to study polygynous behaviour. Of 1294 males, 54 (4.2%) were encountered at two (53) or three (1) nests during the same breeding season. Polygyny occurred more frequently during years of high vole abundance. The distances between nests of corresponding primary and secondary females were greater (median 1010 m) than the distances from nests of primary females to the nearest vacant nest-box (median 455 m). Twenty-eight (53%) of 53 secondary females had nearest available monogamous male within 2 km from their nest-boxes, indicating that mating options were available. Secondary females produced 30% less fledged offspring than simultaneously laying monogamously paired females. The abundance of prey animals is apparently alleviating the effort of males mating with multiple females. Spacing out the nests of primary and secondary females implies deceptive behaviour in the nest-site selection of polygynous males. Contradicting the polygyny threshold model, reproductive success of secondary females was significantly reduced in comparison to monogamous females laying simultaneously. These results show that secondary kestrel females apparently made a maladaptive choice, likely because they were deceived to accept polygynous mating status during the courtship feeding period.Significance statementAs dedicated parental effort of both the male and the female is vital to ensure the offspring survival amongst animals with obligatory bi-parental care, polygyny should be inherently a maladaptive mating strategy for females. However, regular social polygyny has been documented in at least 10% of bird species from ten orders. Previous studies on breeding success of polygynous birds of prey indicate reduced offspring production of secondary female partners with no apparent cause for females to choose polygynous males over other mating options. We showed that polygyny in Eurasian kestrels is frequent when food is abundant facilitating males to provision their two or more females during courtship feeding. Polygynous males space out their two nests thus attempting to hide their mating status from their secondary partners which suffer from their mate choice in form of poor reproductive success. Therefore, amongst the "cost of polygyny to females" hypotheses, the deceptive behaviour of males during courtship feeding appeared to be an apparent explanation for maladaptive mate choice of secondary females.</p

Reproductive responses of birds to experimental food supplementation: a meta-analysis

Introduction Food availability is an important environmental cue for animals for deciding how much to invest in reproduction, and it ultimately affects population size. The importance of food limitation has been extensively studied in terrestrial vertebrate populations, especially in birds, by experimentally manipulating food supply. However, the factors explaining variation in reproductive decisions in response to food supplementation remain unclear. By performing meta-analyses, we aim to quantify the extent to which supplementary feeding affects several reproductive parameters in birds, and identify the key factors (life-history traits, behavioural factors, environmental factors, and experimental design) that can induce variation in laying date, clutch size and breeding success (i.e., number of fledglings produced) in response to food supplementation. Results Food supplementation produced variable but mostly positive effects across reproductive parameters in a total of 201 experiments from 82 independent studies. The outcomes of the food effect were modulated by environmental factors, e.g., laying dates advanced more towards low latitudes, and food supplementation appeared not to produce any obvious effect on bird reproduction when the background level of food abundance in the environment was high. Moreover, the increase in clutch size following food addition was more pronounced in birds that cache food, as compared to birds that do not. Supplementation timing was identified as a major cause of variation in breeding success responses. We also document the absence of a detectable food effect on clutch size and breeding success when the target species had poor access to the feed due to competitive interactions with other animals. Conclusions Our findings indicate that, from the pool of bird species and environments reviewed, extra food is allocated to immediate reproduction in most cases. Our results also support the view that bird species have evolved different life-history strategies to cope with environmental variability in food supply. However, we encourage more research at low latitudes to gain knowledge on how resource allocation in birds changes along a latitudinal gradient. Our results also emphasize the importance of developing experimental designs that minimise competition for the supplemented food and the risk of reproductive bottle-necks due to inappropriate supplementation timings. &nbsp;</p

Predation risk in relation to brain size in alternative prey of pygmy owls varies depending on the abundance of main prey

Large brains in prey may select for adoption of anti-predator behavior that facilitates escape. Prey species with relatively large brains have been shown to be less likely to fall prey to predators. This results in the prediction that individuals that have been captured by predators on average should have smaller brains than sympatric conspecifics. We exploited the fact that Eurasian pygmy owlsGlaucidium passerinumhoard small mammals and birds in cavities and nest-boxes for over-winter survival, allowing for comparison of the phenotype of prey with that of live conspecifics. In Northern Europe, main prey of pygmy owls are voles of the generaMyodesandMicrotus, while forest birds and shrews are the most important alternative prey. Large fluctuations (amplitude 100-200-fold) in vole populations induce rapid numerical responses of pygmy owls to main prey populations, which in turn results in varying predation pressure on small birds. We found, weighed and measured 153 birds in food-stores of pygmy owls and mist-netted, weighed and measured 333 live birds of 12 species in central-western Finland during two autumns with low (2017) and high (2018) pygmy owl predation risk. In two autumns, individuals with large brains were captured later compared to individuals with small brains, consistent with the hypothesis that such individuals survived for longer. Avian prey of pygmy owls had smaller heads than live birds in autumn 2018 when predation risk by pygmy owls was high. This difference in head size was not significant in 2017 when predation risk by pygmy owls was reduced. Finally, avian survivors were in better body condition than avian prey individuals. These findings are consistent with the hypothesis that pygmy owls differentially prey on birds in poor condition with small brains. These findings are consistent with the hypothesis that predation risk imposed by pygmy owls on small birds in boreal forests varies depending on the abundance of the main prey (voles)

Age and sex differences in numerical responses, dietary shifts, and total responses of a generalist predator to population dynamics of main prey

Fluctuations in the abundance of main prey species might shape animal communities, by inducing numerical responses and dietary shifts in predators. Whether numerical responses and dietary shifts differ among individuals of different age and sex has so far gained little attention. These differences could affect how much predators consume main and alternative prey, thus causing variation in predation pressure on main and alternative prey species. We studied the effect of fluctuating main prey abundance (voles) in autumn on the age and sex composition of a food-hoarding population of Eurasian pygmy owls Glaucidium passerinum (327 individuals), and on the species composition of their food stores in western Finland during 2003-2017 (629 food stores). Numbers of yearlings (< 1-year old) of both sexes and adult (+ 1-year old) females increased with increasing vole abundance. During low vole abundance, adult owls stored more small birds and less small mammals than yearlings. Females stored more small mammals than males and showed a tendency to store less birds. The amount of consumed birds (the most important alternative prey), and in particular of crested, willow, great, and blue tits, increased with low vole densities. Our results show that numerical, functional, and total responses of pygmy owls, and probably also other vertebrate predators, to the availability of the main prey in winter are shaped by the age and sex composition of the predator population, which both show large spatio-temporal variation in boreal forests

Predation risk landscape modifies flying and red squirrel nest site occupancy independently of habitat amount

Habitat choice often entails trade-offs between food availability and predation risk. Understanding the distribution of individuals in space thus requires that both habitat characteristics and predation risk are considered simultaneously. Here, we studied the nest box use of two arboreal squirrels who share preferred habitat with their main predators. Nocturnal Ural owls (Strix uralensis) decreased occurrence of night-active flying squirrels (Pteromys volans) and diurnal goshawks (Accipiter gentilis) that of day-active red squirrels (Sciurus vulgaris). Unexpectedly, the amount of preferred habitat had no effect on nest box use, but, surprisingly, both squirrel species seemed to benefit from close proximity to agricultural fields and red squirrels to urban areas. We found no evidence of trade-off between settling in a high-quality habitat and avoiding predators. However, the amount of poor-quality young pine forests was lower in occupied sites where goshawks were present, possibly indicating habitat specific predation on red squirrels. The results suggest that erecting nest boxes for Ural owls should be avoided in the vicinity of flying squirrel territories in order to conserve the near threatened flying squirrels. Our results also suggest that flying squirrels do not always need continuous old forests, and hence the currently insufficient conservation practices could be improved with reasonable increases in the areas left untouched around their nests. The results of this study demonstrate the importance of taking into account both habitat requirements and predation risk as well as their interactive effects when modeling the occupancy of threatened animal species and planning their conservation

Food hoarding of an avian predator: sex- and age-related differences under fluctuating food conditions

Hoarding behaviour (storing food for a later use) has evolved to reduce starvation risk when resources are scarce. Different age and sex classes often show differences in foraging due to experience, skills or life history strategy, but such differences in hoarding under spatio-temporally varying environmental conditions have rarely been studied in the wild. We studied hoarding behaviour of Eurasian pygmy owls (Glaucidium passerinum) during 2003-2016 in western Finland, where the abundance of their main prey (voles) fluctuates in three-year population cycles. In 14years, 1056 food stores were found during the hoarding season (Oct-Dec) and 330 pygmy owls were trapped at these stores. The number of stores per individual did not vary in relation to age, sex or vole abundance. Adults (+1-year old) had their stores farther apart than yearlings. Both the number of stores per year and the biomass of stored prey items increased with vole abundance. Females and yearlings had larger and heavier stores than males and adults, respectively. The same individuals stored more food as yearlings than as adults. These sex- and age-differences in hoarding indicate that it is not constrained by experience or skills. It rather seems that less-experienced yearlings rely more on stored food than adults. Females may need more food due to their larger size and need to accumulate energy reserves before reproduction. A detailed knowledge of age- and sex-related differences in hoarding behaviour under fluctuating abundances of main foods is fundamental to better understand a population response to climate change and forest management.Significance statementThe hoarding behaviour of animals has evolved to cope with the problem of food limitation. On the basis of 14-year data from pygmy owls, we show that the number of stores per year and the biomass of prey items per store increased with vole abundance in the environment. Adults had stores farther apart than yearlings, and females and yearlings stored more prey items and biomass compared to males and adults, respectively. These results indicate that hoarding behaviour responds to the available main prey abundance and varies with traits such as age and sex. Because different age and sex classes might respond differently to variation in food abundance, due to habitat alterations or climate change, a detailed knowledge of hoarding behaviour can be of particular importance to understand changes in body condition, reproductive success and survival of pygmy owls under changing climate and management of boreal forest

Molecular Identification of Sarcocystis sp. (Apicomplexa, Sarcocystidae) in Offspring of Tengmalm's Owls, Aegolius funereus (Aves, Strigidae)

Background: Birds act as intermediate or definitive hosts of cyst-forming coccidia parasites of the genus Sarcocystis Lankester, 1882. However, the spectrum of species of Sarcocystis in birds and the role of the latter in the transmission of coccidia are still incomplete for many avian species, including the Tengmalm's owl Aegolius funereus (Linnaeus, 1758). During the research on Tengmalm's owls in Finland, some fledglings were found dead and subsequently parasitologically examined. Therefore, this study is focused on the morphological and molecular description of a Sarcocystis species found in the intestine of the Tengmalm's owl and its possible role as a definitive host.Methods: Eleven fledgling owls in the Kauhava region of west-central Finland were found dead and subsequently were submitted for necropsy and parasitologically examined through the flotation–centrifugation coprological technique for the presence of oocysts/sporocysts of the genus Sarcocystis by light microscopy. Wet mounts were used for the examination of muscle samples (breast, legs, and heart). Polymerase chain reaction (PCR) and nested-PCR were carried out using primers for 18S rRNA, 28S rRNA, ITS1 region, and CO1 genes.Results: All 11 examined owls were parasitized by numerous sporocysts and oocysts in the intestinal mucosa scrapings (prevalence, 100%). Sporulated oocysts and sporocysts measured 16.34–16.96 × 11.47–12.09 μm and 11.85–13.52 × 7.77–9.25 μm, respectively. The skeletal and heart muscles were negative for sarcocysts. Sarcocystis sp. ex Aegolius funereus (hereafter Sarcocystis sp. Af) is closely related to Sarcocystis strixi in the barred owl (Strix varia Barton, 1799) from the USA and Sarcocystis sp. isolate 5 in the European shrew (Sorex araneus Linnaeus, 1758) from the Czech Republic. Phylogenetic analysis allowed determining the relationship of the herein reported Sarcocystis sp. with its congeners.Conclusions: This work provided the first and most comprehensive record on Sarcocystis from owls obtained in Finland, thus highlighting the importance of molecular data in species identification.</p

Habitat use of coexisting Microtus vole species under competition and predation risk

Competing species and predators can alter the habitat use of animals, but both factors are rarely simultaneously controlled. We studied in experimental enclosures how closely related species, the sibling vole (Microtus levis Miller, 1908) and the field vole (Microtus agrestis (Linnaeus, 1761)), adjust their habitat use when facing either the competing species or simultaneously competition and predation risk. The species responded differently in their proportional use of two habitat types, a low cover (productive but riskier) and a high cover (safer but poorer). When alone, field voles used the low-cover habitat according to availability at low densities, but decreased its use with increasing density. Sibling voles, however, avoided the low-cover habitat in single-species populations. Under interspecific competition, the habitat-use patterns switched between species: sibling voles used the low-cover habitat according to availability, with decreasing use as densities increased. Sibling voles responded to predation risk by showing a stronger density-dependent decrease in the use of low-cover habitat. Field voles, initially using mostly high cover, did not change behaviour under risk of predation. Our results highlight the importance of considering both predation risk and interspecific competition when interpreting patterns of habitat selection among coexisting species.</p