Relative abundances of birds of prey in different forest habitats in the Western Newfoundland Model Forest

Gosse, John W. (1997) Relative abundances of birds of prey in different forest habitats in the Western Newfoundland Model Forest. Masters thesis, Memorial University of Newfoundland.

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Abstract

Balsam fir forests in western Newfoundland are intensively managed for pulpwood production resulting in a fragmented landscape of different-aged forests and clearcuts. Prior to this research, the ecology of woodland birds of prey had been unstudied on insular Newfoundland, and knowledge regarding consequences of forestry practices on aspects of their biology was lacking. As top-level carnivores, raptors are susceptible to both natural and human induced perturbation and may effectively be used as indicator species of environmental health in boreal forest ecosystems. -- During the breeding seasons of 1993 and 1994, the diversity and relative abundances of birds of prey were investigated in the Western Newfoundland Model Forest (WNMF). The primary research objectives were to: 1) develop reliable, standardized techniques with which to census birds of prey in different-aged forests and clearcuts in the WNMF, 2) determine the species diversity and relative abundances of birds of prey in uncut old growth, second growth forest, clearcuts and pre-commercially thinned areas, 3) develop density estimates of selected species of birds of prey, 4) identify nest-sites of birds of prey and quantify habitat characteristics at these sites, 5) identify knowledge gaps with respect to Newfoundland birds of prey, and in doing so to develop research objectives and management strategies for future studies. -- Surveys using conspecific vocalization playbacks were conducted along forest access roads and lake shorelines that transected different forest habitats. Ground searches were made for raptor activity sites (i.e. nests, roosts, prey-plucking-sites), and habitat measurements at these sites and unused control sites were obtained. The vocalization playback method used was reasonably effective for locating owls but less effective for other woodland raptors. -- Nine species of birds of prey were recorded in the area: Merlin, American Kestrel, Osprey Rough-legged Hawk, Sharp-shinned Hawk, Northern Goshawk, Boreal Owl, Great Horned Owl, and Northern Hawk-Owl. Uncut old growth forests contained more species than second growth forests and clearcuts, however abundances in all forest types were low. Boreal Owls and Northern Goshawks were found exclusively in uncut old growth, and Sharp-shinned Hawks mainly in this habitat type. Three species of birds of prey were recorded in second growth forests in the study area, one of which, the Great Horned Owl, was found only in this habitat type. Clearcut areas were utilized by four species of raptors and Northern Hawk-Owls and American Kestrels used clearcuts extensively. Merlins were the most commonly detected bird of prey and unlike any other species were found in all forest habitats. -- Predator-prey relationships influence the occurrences of birds of prey in different habitats. A comparison of avian-dependent birds of prey (i.e. Sharp-shinned Hawks, Merlins) and small mammal specialists (i.e. Boreal Owls, Northern Hawk-Owls, Rough-legged Hawks) indicated that between 1993 and 1994, the numbers of avian predators remained identical whereas the numbers of rodent predators declined significantly. This population decrease is likely a result of the simultaneous decline in small mammals in this region and reflects the importance of food resources in influencing population dynamics of birds of prey in different habitats. -- Density comparisons made between western Newfoundland and a range of locations throughout boreal forest systems in North America and Scandinavia indicate that raptor populations are extremely variable. Although lacking rigorous empirical analysis, such comparisons provide an index of raptor populations at different spatial and temporal scales. In western Newfoundland, the low diversity and numbers of small mammals likely limits the densities of birds of prey dependent on this food resource, (Boreal Owl, Northern Hawk-Owl, Rough-legged Hawk). Great Homed Owls and Northern Goshawks may also have been food limited as their major prey species (Snowshoe Hare, Ruffed Grouse) were uncommon in this region. These predator-prey relationships may be responsible for the low densities of birds of prey found in the boreal forests of western Newfoundland. -- Habitat selection theory postulates that species are preferentially associated with particular habitats in which they can optimally survive and reproduce. For birds of prey, the mechanisms that elicit selection of habitats are not well understood yet this information is needed in order to effectively manage forests to maintain the natural biodiversity and abundance of birds of prey. Sample sizes of raptor activity sites were small, however trends of habitat selection were apparent and consistent with research conducted elsewhere. -- In recent years, the management of non-game wildlife has become an important issue in light of approaches to preserving forest ecosystems that go beyond the primary objective of fibre and timber production. In North America, birds of prey are protected under governmental legislation, and conservation measures for them have been implemented in forest management strategies. In Newfoundland, however, a previous lack of knowledge regarding the distributions and abundances of woodland raptors has resulted in minimal attention with respect to forestry planning. Based on census results of this present study, I suggest that large tracts of remaining uncut old growth forests be preserved from timber harvesting and that the existence of this habitat type be ensured at a landscape level in future years. Furthermore, research and systematic monitoring of woodland raptor populations should continue in an attempt to better understand impacts of forest harvesting operations.

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