Treesearch

During 1976-1995 we monitored 114 Barred Owl (Strix varia) breeding attempts in northern Michigan. We describe nest sites and report reproductive success for different types of nest sites. Most natural nest sites were tree cavities caused by decay at places where a limb or tree top had broken off. The mean d.b.h. of cavity trees (N = 18) was 48 cm and mean cavity floor area (N = 19) 508 cm². Overall, 75 percent of breeding attempts were successful with a mean brood size (N = 75) of 1.97 young/productive nest. Owls nesting in tree cavities (N = 49) and in boxes (N = 52) showed similar...

From 1991 through 1995, extensive owl surveys were conducted in late March and early April in Manitoba. Prior to these surveys, distribution records of owls covered only 16-71 per cent of their expected range in Manitoba. The degree to which the survey increased the documented range varied from no increase (6 of 12 species) up to an 88 per cent increase for the Northern Saw-whet Owl (Aegolius acadicus). The Boreal Owl (Aegolius funereus) and Great Gray Owl (Strix nebulosa) had the second and third largest increases respectively, a result possibly enhanced by using male territorial call...

Long Point Bird Observatory ran pilot surveys in 1995 and 1996 to monitor boreal forest owls in Ontario using roadside surveys with tape playback of calls. A minimum of 791 owls on 84 routes in 1995, and 392 owls on 88 routes in 1996; nine different species were detected. Playback improved the response rate for Barred (Strix varia), Boreal (Aegolius funereus), Northern Saw-whet (Aegolius acadicus) and possibly Great Gray (Strix nebulosa) Owls, and reduced variance among surveys for Barred Owls. Relatively few, long stops produced the most efficient survey for Barred Owls, while more...

We compared habitat information obtained from tracking 12 radio-tagged Ferruginous Pygmy-owls (Glaucidium brasilianum) (hereafter referred to as pygmy-owls) in southern Texas during 1995 and similar information from pygmy-owl response points to evaluate the effectiveness of broadcast surveys in determining pygmy-owl habitat use.

Nocturnal netting operations have been conducted at the Hawk Ridge Nature Reserve since 1972. From 1988 to 1992 a recording of human whistles simulating the calls of fall migrant Northern Saw-whet Owls (Aegolius acadicus) was broadcast on a random, on or off, half-night basis. Mist net captures of Saw-whet Owls increased about fourfold during the broadcast sessions, while captures of Long-eared Owls (Asio otus) decreased as much as 24 percent.

Research on Tawny Owls (Strix aluco) in Kielder Forest, northern England, since 1981 demonstrated that field voles (Microtus agrestis) were their most important food. Here, field voles exhibited a 3-4 year cycle of abundance, and mean clutch size in Tawny Owls was significantly related to vole abundance in March. In this analysis we use variations in clutch size as a surrogate to explore whether vole abundance was synchronized over a larger spatial scale, in this case between Kielder Forest and another forest (Kershope) in an adjacent valley system. We show that mean clutch sizes were...

During auditory surveys with tape playback between 13 February and 27 April during 1986-1996, our detection of calling by Northern Saw-whet Owls (Aegolius acadicus) varied dramatically and regularly in an apparent 4-year cycle: 1986, 1990, and 1994 were significantly high calling years; 1987-1989, 1992-1993, and 1995- 1996 were significantly low; and 1991 was intermediate. Calling was also significantly affected positively by increasing time since sunset and negatively by date. Results from daytime searches during 1986- 1990 for saw-whet owl individuals, roosts, and pellets did not show...

Black fly feeding and infection with the blood parasite Leucocytozoon spp. caused mortality in juvenile Great Horned Owls (Bubo virginianus) in the Yukon, Canada during 1989-1990. The mortality occurred during a year of food shortage corresponding with the crash in snowshoe hare (Lepus americanus) populations. We postulate that the occurrence of disease was mediated by reduced food availability.

We investigated landscape characteristics around 41 Northern Spotted Owl (Strix occidentalis caurina) nest sites to assess habitat proportions and patterns on this highly fragmented landscape in the central Coast Ranges of Oregon. We compared the proportion of seven forest cover-types between nest sites and random sites at plot sizes of 112 ha, 456 ha, 1,037 ha and 1,844 ha, and fragmentation patterns at the 1,844-ha plot size.

We studied survival of the endangered Burrowing Owl (Speotyto cunicularia) using radio-telemetry in both Alberta and Saskatchewan. Adult females exhibited the highest mean survival (0.83) whereas adult male (0.46) and juvenile (0.48) rates were similar. Most mortality occurred during the post-fledging period when there was a peak of activity around the nest. The greatest mortality from vehicle collisions occurred in a fragmented agricultural landscape where predation was also the lowest. Our survival estimates, along with other considerations, suggest that low annual survival is potentially...