Pertti Saurola

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.

Large-scale change in the sex ratio of a declining eider Somateria mollissima population

Abstract The breeding potential of a monogamous animal population should be maximal during equal operational sex ratio, and empirical evidence suggests that the population-wide sex ratio may be linked to population density. We studied the sex ratio of eiders Somateria mollissima migrating into the Gulf of Finland, the Baltic Sea, in nine years during 1979–2005 (1979–1980, 1982–1983 and 2001–2005), and the sex ratio of birds collected by Danish hunters during 1982–2004. In two decades, the sex ratio during peak migration has reversed from female bias to male bias, and hunting statistics have shown a significantly increasing adult male bias. Also the proportion of juvenile males has shown a significant increase (Danish hunting statistics 1982–2004), which indicates either that the primary sex ratio of ducklings is exceedingly male biased, or that the mortality of female ducklings has increased. This shift in sex ratio is paralleled by a dramatic decrease in the Baltic eider population which started in the early 1990s. The proportion of juveniles in the hunting bag, an indicator of breeding success in the Baltic, significantly decreased during our study period. The sex ratio of migrating eiders showed seasonal fluctuations, the pattern of which has changed during the study period. Particularly the proportion of late-migrating females has decreased dramatically since the early 1980s, suggesting a declining influx of subadult females. Both the increased male bias and the decreased breeding success are likely to be linked with the population decline. A primary contributor to the shift in sex ratio and the declining trend in breeding success and population size is possibly differential mortality of the sexes during breeding, as the mortality of breeding females has increased sharply in the western Gulf of Finland, mainly due to predation by white-tailed sea eagles Haliaeetus albicilla and American minks Mustela vison, the former of which has recently increased in numbers. It is unlikely that differential winter mortality of the sexes can explain our results, as the wintering area of eiders from the Gulf of Finland has remained the same, and the Danish hunting bag reflects the existing sex ratio. Our study highlights the need for future empirical and theoretical work on the relationship between population sex ratio and population density.

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.

Estimating age-specific survival rates of tawny owls--recaptures versus recoveries

We compared estimates of annual survival rates of tawny owls ( Strix aluco ) ringed in southern Finland from several different sampling methods: recoveries of birds ringed as young; recaptures of birds ringed as young; recoveries of birds ringed as adults as well as young; combined recoveries and recaptures of birds ringed as young, and combined recoveries and recaptures of birds ringed as adults and young. From 1979 to 1998, 18 040 young owls were ringed, of which 983 were recaptured as breeders in subsequent years during this period, and 1764 were recovered dead at various locations. In addition, 1751 owls were ringed as adults, of which 612 were later recaptured and 199 were recovered dead. First-year survival rates estimated using only recoveries of birds ringed as young averaged 48%, while apparent survival rates estimated using only recaptures from birds ringed as young averaged 10-13%. Use of combined recapture-recovery models, or supplementary information from recoveries of birds ringed as adults, produced survival estimates of 30-37%. Survival estimates from young-recoveries-only models were biased high, because of violation of the assumption of constant recovery rates with age: birds dying in their first-year were one-third less likely to be found and reported than older birds. In contrast, recaptures-only models confounded emigration with mortality. Despite these differences in mean values, annual fluctuations in estimated first-year survival rates were similar with all models. Estimates of adult survival rates were similar with all models, while those for second-year birds were similar for all models except recaptures-only. These results highlight the potential biases associated with analysing either recaptures or recoveries alone of birds ringed as young, and the benefits of using combined data.

Estimating Demographic Parameters from Complex Data Sets: A Comparison of Bayesian Hierarchical and Maximum-Likelihood Methods for Estimating Survival Probabilities of Tawny Owls, Strix aluco in Finland

We compared a method of moments approach using estimates from a maximum likelihood framework, ultrastructural models within a maximum likelihood framework, and hierarchical models estimated using Markov chain Monte Carlo within a Bayesian framework for estimating survival and recapture probabilities and their variance components for a large, complex 20 year data set consisting of both live recaptures and recoveries. Estimates of mean age-specific survival and recapture probabilities for four age classes (young, second-year, third-year and adult) were similar with all approaches, but the maximum likelihood approach with year-specific parameters estimated some recovery and recapture probabilities on boundaries, leading to overestimates of some individual adult survival probabilities and hence overestimates of adult variance components. All approaches estimated similar coefficients for the relationships between winter temperature and survival probabilities, but the maximum likelihood approaches appeared to exaggerate variation in relation to prey abundance. Annual estimates from the Bayesian hierarchical models were sensitive to the choice of the hierarchical structure; modelling the difference between second-year, third-year and adults in survival and recapture probabilities as random effects better estimated the patterns of annual variation than treating all age classes as independent. Our comparisons suggest that Bayesian hierarchical models may be more likely to produce reliable estimates than maximum likelihood methods, even for large data sets, especially if there are many parameters and considerable annual variation in sample sizes.