Patrik Byholm

The impact of climate and cyclic food abundance on the timing of breeding and brood size in four boreal owl species

The ongoing climate change has improved our understanding of how climate affects the reproduction of animals. However, the interaction between food availability and climate on breeding has rarely been examined. While it has been shown that breeding of boreal birds of prey is first and foremost determined by prey abundance, little information exists on how climatic conditions influence this relationship. We studied the joint effects of main prey abundance and ambient weather on timing of breeding and reproductive success of two smaller (pygmy owl Glaucidium passerinum and Tengmalm’s owl Aegolius funereus) and two larger (tawny owl Strix aluco and Ural owl Strix uralensis) avian predator species using long-term nation-wide datasets during 1973–2004. We found no temporal trend either in vole abundance or in hatching date and brood size of any studied owl species. In the larger species, increasing late winter or early spring temperature advanced breeding at least as much as did high autumn abundance of prey (voles). Furthermore, increasing snow depth delayed breeding of the largest species (Ural owl), presumably by reducing the availability of voles. Brood size was strongly determined by spring vole abundance in all four owl species. These results show that climate directly affects the breeding performance of vole-eating boreal avian predators much more than previously thought. According to earlier studies, small-sized species should advance their breeding more than larger species in response to increasing temperature. However, we found an opposite pattern, with larger species being more sensitive to temperature. We argue that this pattern is caused by a difference in the breeding tactics of larger mostly capital breeding and smaller mostly income breeding owl species.

FOOD REGULATES REPRODUCTION DIFFERENTLY IN DIFFERENT HABITATS: EXPERIMENTAL EVIDENCE IN THE GOSHAWK

Food supplementation experiments have been widely used to get detailed insight into how food supply contributes to the reproductive performance of wild animals. Surprisingly, even though food seldom is distributed evenly in space, variation in local habitat quality has usually not been controlled for in food supplementation studies. With results from a two-year feeding experiment involving a habitat-sensitive avian top predator, the Northern Goshawk Accipiter gentilis, we show that treatment effects on goshawk reproductive performance are habitat dependent. Extra food reduced nestling mortality in low-quality territories where prime habitat (forest) is scarce, but not in high-quality territories where prime habitat is abundant. Consequently, brood size did not differ between treatment categories in heavily forested territories, but fledgling numbers differed between unfed and fed goshawk pairs breeding in territories where forest is scarce. However, because extra food was not superabundant, this artificial increase in offspring number induced a dramatic decrease in nestling condition in low-quality territories. Treatment effects were detected even after controlling statistically for other potentially confounding effects (year, territory identity) and strongly covaried with territory-specific abundances of the most important summer prey species. These results highlight the importance of acknowledging the effect that small-scale variation in habitat quality and availability of natural food may have on the results of food supplementation experiments. In order to assess the generality of food supplementation effects, the integration of habitat heterogeneity and variation in food abundance is thus needed, especially among species in which small-scale variation in habitat quality influences demographic patterns.

Causes of dispersal in northern Goshawks (Accipiter gentilis) in Finland

Abstract Environmental quality has the potential to influence dispersal if cost of dispersing is outweighed by cost of staying. In that scenario, individuals experiencing different conditions in their natal area are expected to differ in their dispersal. Even if there is wide agreement that reasons behind the dispersal decision are multiple, it is often less clear what conditions actually add to the observed dispersal behavior. The scale at which the dispersal behavior is analyzed can also be of crucial importance for a correct understanding of the dispersal process. Furthermore, in long-lived species factors influencing dispersal behavior of juveniles may differ from those adding to dispersal of adults. Using 12 years of banding data (1989–2000), we studied dispersal behavior of Northern Goshawks (Accipiter gentilis) hatched over a wide area in Finland in relation to local demographic and ecological conditions. Hatching rank and hatching date added to the probability of leaving in the first place; whereas hatching date, local prey availability, and sex of the disperser were related to dispersal distance. Among adult birds (+3 years), none of the analyzed variables were related to distance; whereas the probability of remaining locally was related to local grouse density in the hatching year (for males only). Results show that the combined effects of factors working at several levels act together on dispersal behavior in Northern Goshawks and highlight the importance of studying different age classes separately in long-lived species. In summary, our results suggest that goshawk individuals distribute themselves spatially in parallel with factors determining the costs and advantages of dispersing where philopatry seems to be connected to factors positively associated with survival; but to test the validity of that idea, more data on fitness consequences of dispersal are needed.

Raptors as surrogates of biodiversity along a landscape gradient

Summary With biodiversity facing threats, there is a need to improve reserve selection procedures. However, detailed information about different biodiversity measures (e.g. species richness) at potential sites is often lacking, and selecting areas that protect most biodiversity is difficult. To simplify matters, biodiversity surrogate species, that is, species associated with higher biodiversity than average, have been used for area selection. However, consensus about the performance of the surrogate species concept is lacking, and there are few studies investigating potential differences in the effectiveness of multiple predators as surrogates for biodiversity over large spatial scales. We evaluated two avian predators, the northern goshawk Accipiter gentilis and the Ural owl Strix uralensis, as surrogates for biodiversity in the boreal forest biome in Western Finland. We used a study design including nest sites and two reference sites for each nest, the diversity of birds and wood-decaying fungi (polypores). We assessed simultaneously whether surrogacy persisted at the landscape level while moving from one vegetation zone to another. We generally found more birds and polypores around the nest sites for both goshawks and Ural owls than at their respective reference sites. However, the goshawk outperformed the Ural owl. Additionally, although biodiversity was found to decrease at the landscape scale as a result of a decrease in vegetation complexity with increasing longitude, the surrogacy efficiency of the raptors remained unchanged. Synthesis and applications. These findings suggest that the surrogate species concept applied to raptors may be an efficient addition to methods for identifying areas of conservation priority, even across vegetation zones. We conclude that protecting areas around raptor nests is a method to consider in order to halt forest biodiversity loss. Finally, sampling biodiversity along diversity and landscape gradients can improve the necessary assessment of surrogate species.

Competitive exclusion within the predator community influences the distribution of a threatened prey species

While much effort has been made to quantify how landscape composition influences the distribution of species, the possibility that geographical differences in species interactions might affect species distributions has received less attention. Investigating a predator-prey setting in a boreal forest ecosystem, we empirically show that large-scale differences in the predator community structure and small-scale competitive exclusion among predators affect the local distribution of a threatened forest specialist more than does landscape composition. Consequently, even though the landscape parameters affecting Siberian flying squirrel (Pteromys volans) distribution (prey) did not differ between nest sites of the predators Northern Goshawks (Accipiter gentilis) and Ural Owls (Strix uralensis), flying squirrels were heterospecifically attracted by goshawks in a region where both predator species were present. No such effect was found in another region where Ural Owls were absent. These results provide evidence that differences in species interactions over large spatial scales may be a major force influencing the distribution and abundance patterns of species. On the basis of these findings, we suspect that subtle species interactions might be a central reason why landscape models constructed to predict species distributions often fail when applied to wider geographical scales.