Molt stage, wing bar patterns and digital photography as tools for assessing age distribution and recognizing individuals of Great Grey and Snowy Owls

The world is heating up. The climate is changing, with increasing temperature changes towards the Arctic. Northern ecosystems of tundra and taiga are subject to changes, even in the most remote areas void of human presence. One of the most profound characteristics of these northern ecosystems are the cyclic changes in population size of mammals, birds and insects, small microtine rodents being the central species in the dynamics. Several studies have demonstrated that the cyclicity of lemmings and voles have changed during the recent decades, with consequences for many other species of the food webs. Changes in the cyclicity of lemmings and voles are especially expected to influence their predators. All arctic and boreal owl species hunt microtine rodents, and species like the Snowy Owl and the Great Grey Owl are totally dependent on such prey animals to breed. The Snowy Owl is listed as a vulnerable species worldwide, while the Great Grey Owl is considered to have a stable world population. The population of Snowy Owls breeding in Fennoscandia has declined while the Great Grey Owl recently has expanded its breeding distribution. Small mammal hunters like the Snowy Owl and the Great Grey Owl are directly influenced by changes in the cyclicity of microtines. The difference in population development of these two vole hunters in Fennoscandia enhance the importance of monitoring both species under a regime of expected future changes of ecosystem cyclisity. Knowledge of population size, reproduction and survival, and the age structure of populations are paramount information in such monitoring because reproduction and mortality varies with age. Moult patterns are essential for aging many birds. In species where juvenile and adult feathers look different, such differences can be used for aging a bird when it is captured for banding. In this thesis I present a method for aging Great Grey Owls and Snowy Owls based on the moult patterns in their wings. I have demonstrated the difference between the first juvenile wing feathers and later adult wing feathers, and that it takes three to four years for an owl to replace all juvenile wing feathers. As long as at least one juvenile wing feather is left in the wing, the number of moults and the age of the owl can usually be determined. I have further developed a method for individual identification of Great Grey Owls and Snowy Owls based on bar patterns of their wing feathers. I have shown that it is possible to use photographs of free flying owls for aging and identification of individuals. Identification based on visible characters is a long established and used technique in studies on mammals, but has hitherto not been used on free flying birds. Using photographs in such studies is a non-invasive technique which may enhance the amount of data available in population studies of birds. I have used these techniques in three studies on Snowy Owls and Great Grey Owls. Snowy Owls were photographed along transect routes during a summer invasion on Beliy Island north of the Yamal Peninsula in Russia in July 2015. A minimum of 25 owls were identified, sexed and aged by studying the moult and bar patterns of their wings from photos of owls in flight. The moult patterns showed that 80% of the owls were hatched in 2012-14. Results from Norwegian satellite studies of Snowy owls showed that 12 adult owls which bred in Norway in 2011 spent the summers 2012-14 along the Russian Arctic. This implies that Snowy Owls must have bred successfully in these parts of the Russian Arctic in 2012-14. An expanding population of Great Grey Owls was studied in Hedmark county, south-eastern Norway in 2009-2018, when the number of recorded nestings rose from 1 to 119. Adult breeding owls were captured for banding or control, and their wings were photographed for moult analyses and aging. Birds which could not be captured were photographed in flight. The photographic method increased the amount of birds which could be aged, and especially the amount of males which may be more reluctant to approach intruders at the nest site, and thus avoid capture. In 2011 no less than 77% of the recorded breeding Great Grey Owls were young birds hatched the previous year. Control of two nesting Great Grey Owls banded as nestlings in Sweden in 2010 indicated that good reproduction in mid-central Sweden in 2010 followed by natal dispersal of young birds was the foundation of the breeding population in Hedmark. I have demonstrated that data gathered through public sciences can be used to age Great Grey Owls when they hunt in open landscapes outside the breeding range during a vole depression year. More than 4000 reports of Great Grey Owls were received through the national species archives in Norway and Sweden in 2012, the majority of data derived from Sweden. Slightly more than 800 of these reports were accompanied by photo(s) of the owl, and of these 323 could be used to age the bird. The number of individuals was reduced to 144 by the ID number of localities, because one owl may be reported by different observers. Wing and tail feathers showed that at least 76% of all the Great Grey Owls reported in 2012 were juvenile birds hatched in 2011. This demonstrated that also 2011 was a good reproduction year of Great Grey owls in south Scandinavia