Population status and recent trends in Canadian raptors: a review

摘要

We examined population trends and status in 37 taxa of Canadian raptors using Breeding Bird Survey (BBS) data (1966-1994 and 1985-1994) for the whole of Canada and seven ecozones separately, continent-wide Christmas Bird Counts (CBC) (1959-1988), migration counts from 12 hawkwatches (various dates) and specific research projects. According to the primary sources reviewed, many raptor populations increased since the early 1970s, coinciding with reductions in use of organochlorine pesticides (OCs). Species that increased included osprey Pandion haliaetus, bald eagle Haliaeetus leucocephalus (BBS and CBC) and peregrine falcon Falco peregrinus anatum (1995 peregrine falcon survey). Redtailed hawk Buteo jamaicensis, American kestrel Falco sparverius and merlin Falco columbarius also generally increased or were stable, but results differed according to source, time period and ecozone (both red-tailed hawk and American kestrel showed short-term decreases in the Mixedwood Plains BBS). Significant declines were found for burrowing owl Speotyto cunicularia (BBS), shorteared owl Asio flammeus and long-eared owl Asio otus populations (BBS and CBC). Golden eagle Aquila chrysaetos may be declining in some areas (CBC, four migration counts) but results were equivocal; there was some evidence to suggest that broad-winged hawk Buteo platypterus may also be declining. Long-term BBS data indicated that Swainson's hawk Buteo swainsoni populations were stable, but significant short-term declines were found in the Prairies ecozone, corresponding with decreases in productivity measured in long-term breeding studies. Sharp-shinned hawk Accipiter striatus apparently increased in number according to the overall long-term BBS, but may be declining according to some eastern migration count data. While no evidence of declines was found for prairie falcon Falco mexicanus, the Canadian population is small and vulnerable to habitat change that could reduce prey populations. Four species that reach their greatest abundance in, or are restricted to, the Prairies and Boreal Plains ecozones are declining there (northern harrier Circus cyaneus, Swainson's hawk, burrowing owl and short-eared owl). Although causal factors vary, our analysis suggests the possibility that large-scale phenomena such as human activity, (pesticide use or habitat modification) and perhaps climate are affecting populations of some Prairie raptors. Research on endangered species is being conducted under the auspices of national recovery plans. We recommend that research be instigated to examine critically productivity failures in Swainson's hawks, and possible declines in golden eagle breeding populations. In view of the increasingly widespread impact of forestry on northern forests in Canada, we further recommend that special surveys be set up, and breeding densities, productivity and ,occupancy rates be determined in different forest types for accipiters, broadwinged hawk, as well as forest owls (particularly barred owl Strix varia, boreal owl Aegolius funereus and northern saw-whet owl Aegolius acadicus brooksi). We caution that the BBS and other national surveys may be inadequate for monitoring populations of many raptor species in Canada, particularly forest-dwelling accipiters, some owls and buteos. Special monitoring or sampling programs are needed for these species. However, taken together, multiple sources can provide inferential evidence for population trends and status.