Northern saw-whet owl (Aegolius acadicus brooksi) COSEWIC update and status report: chapter 6

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Biology

Information on the biology of the Haida Gwaii subspecies of Northern Saw-whet Owls comes from various sources, including past and ongoing research on habitat (Gill and Cannings 1996; Holschuh 2004a, Holschuh 2004b.), diet and related life history characters (Sealy 1998; Sealy 1999; Hobson and Sealy, 1991).  General knowledge on the biology of Northern Saw-whet Owls in general is mainly derived from Cannings (1993). As well, relevant information gathered through the personal observations of researchers and naturalists is also presented.

Life cycle and reproduction

Like the nominate subspecies of Northern Saw-whet Owl, these owls are secondary cavity nesters, requiring cavities with an opening no less than 75 mm in diameter. Although cavities are often described as being in the lower third of the canopy (e.g. Terrill 1931, Johns et al. 1978), nest cavities may also be found higher in the canopy.  On Haida Gwaii, most cavities are probably excavated by Northern Flickers (Colaptes auratus) or Hairy Woodpeckers (Picoides villosus).  Four nests of the Haida Gwaii Saw-whet Owl have been found and described; two were in western hemlock and two in Sitka spruce snags that ranged from 61 to 130 cm in diameter at breast height (Tarver, 2001; S. Charest, C. Epners, pers. comm.; Holschuh 2004a).

Saw-whet owls are highly territorial of the area near potential nests during the spring breeding season. Males defend core areas that appear to be around 70-100 ha in size. Their home ranges, however, are often much larger than this, as movements up to 2 km from the territory core being defended have been noted with telemetry monitoring (Holschuh and Otter, unpubl. data). Much of this activity was also pre-dusk, and the owls are thought to be foraging during this time. Haida Gwaii’s latitude leads to short summer nights, and hence it may not be surprising that nocturnal species may also hunt during the day during the critical spring breeding time, in order to meet energetic demands.

Cavities are thought to be limited across the Haida Gwaii landscape (Doyle, pers. comm.), and found more often in Sitka Spruce and Western Hemlock  snags, than in Western Redcedar snags (F. Doyle, unpubl. data).  As cavities are limited and primary cavity excavators are known keystone species (Martin et al. 2004), woodpecker habitat needs may provide an important link to Northern Saw-whet Owl population dynamics on Haida Gwaii.

Northern Saw-whet Owls are solitary breeders, and the pair actively defends their nest territory. Observations of the nominate subspecies of Northern Saw-whet Owl indicate that females choose the nest sites but are guided by the male’s advertisement/territorial call (Cannings 1993). At both nests that have been found on Graham Island, the male was consistently observed calling from within 100 m of the nest tree, likely guiding the female to possible nest sites (CIH, pers. obs.).

Northern Saw-whet Owls start breeding once they are one year old and those of the migratory, nominate race are thought to survive between five and seven years (Cannings 1993); survivorship of brooksi individuals is unknown but may be longer due to their non-migratory habits. The onset of the breeding season seems to be highly asynchronous in Haida Gwaii Saw-whet Owls, with some birds defending their territories as early as late February, while others do not become territorial until mid-May (CIH pers. obs.). When Saw-whet Owls begin establishing their nests, the level of territorial vocalizations (and thus territorial defence) drops dramatically (R.J. Cannings, pers. comm.; CIH pers. obs.) suggesting that nesting is equally asynchronous, ranging from mid-March until the end of May.

The late dates of territorial activity may represent two possibilities; the owls at some territories may begin breeding later than most, or owls at some territories may begin breeding even earlier in February and establish a second nest later in the spring. Although some evidence of this has been collected in A. a. acadicus (Marks et al. 1989), there has been no evidence of second broods or polygyny in the Haida Gwaii subspecies.

Activity at the nest has been observed in detail at one nest site on East Limestone Island (ELI) (Tarver 2001). At this nest, calling activity began on 5 May, a full month before nesting activity began on 7 June. Juveniles were first detected begging from within the nest cavity July 7. Incubation is done solely by the females, and the period of incubation is 27-29 days (Cannings 1993).

The young are altricial upon hatching and are brooded exclusively by the female, while the male delivers food to the nest for the female and young (Tarver 2001, Cannings 1993). The young in the ELI nest apparently fledged approximately one month after hatching (between 27 July and 1 August).

Hence, evidence from the ELI nest (Tarver 2001) as well as from observations in the nominate subspecies (Cannings 1993) suggest that the nesting phase from egg laying to fledging lasts approximately 2 months. Overall, as breeding activity on Haida Gwaii may begin as early as March and as late as June, the sensitive breeding period for these owls appears to be from early March until August.

Some level of territory site fidelity is apparent in this species. In a two-year study of A. a. brooksi, 7 of 13 sites that were monitored for both years were active in the second year (Holschuh 2004). Further, evidence from two of three test sites using recognition of individual calls suggested the territories were occupied by the same individual for both years (Holschuh and Otter 2005). Site fidelity is uncommon in the nominate subspecies (Cannings 1993). This possible behavioural difference between the two subspecies may be a result of A. a. brooksi being non-migratory, and hence less likely to be as nomadic as A. a. acadicus (Marks 1997).

No data on the fecundity or breeding success of A. a. brooksi are available. In the nominate subspecies, the average clutch size is 5-6 eggs (Cannings, 1993). These birds can feasibly breed yearly, but it is not known whether individuals establish nests annually. Possible limiting factors may be food shortage (rodents) or a lack of suitable cavities (for more detail, refer to section on limiting factors). Further, as individuals occupying areas with decreased old growth forest appear to be in lower condition, as was indicated by decreased expression of a condition dependent signal (Holschuh 2004a), their reproductive output may also be affected. 

Predation

Due to the general lack of nocturnal raptors, there are likely not many avian predators on adult Northern Saw-whet Owls on Haida Gwaii. Nonetheless, Northern Goshawks (Accipiter gentilis) occasionally take Northern Saw-whet Owls (Frank Doyle, pers. comm.).

Rates of nest predation have not been studied directly; however, Northern Saw-whet Owls are probably vulnerable to mammalian nest predators such as the American Marten (Martes americana). Further, introduced species including Red Squirrel (Tamiasciurus hudsonicus) (Martin and Joron 2003) and Common Raccoon (Procyon lotor) likely pose an even greater predatory threat on the Saw-whet Owls, as both are well known to be effective nest predators (Eder and Pattie 2001).

Physiology

Studies on the energetic food requirements of captive birds found that a female Northern Saw-whet Owl has to consume approximately 18% of her body weight daily to maintain her body weight. The daily energetic intake is around 95 kcal/day for male birds and 125 kcal/day for female birds (Cannings 1993 and references therein). The normal body temperature for Saw-whet Owls is approximately 38-39°C, but increases when the ambient temperature exceeds 33°C (Cannings 1993 and references therein), indicating these birds are adapted to temperate environments.

Dispersal and migration

The Northern Saw-whet Owl brooksi subspecies is a non-migratory resident of the Queen Charlotte Islands/Haida Gwaii. No vagrants have been detected outside of the archipelago, either in southern Alaskaor British Columbia (Sealy 1997). This is in contrast to the nominate subspecies, in which individuals are known to undergo large-scale movements both in eastern North America (e.g. Zusi 1958, Weir et al. 1980) and through British Columbia (Levesque 2002, Mackenzie Nature Observatory 2003).

On Haida Gwaii, there are no data on the dispersal of young in these owls, and because of their non-migratory nature, comparisons with the nominate subspecies cannot be made. Some evidence based on diet and behaviour (increased detections/collisions with cars along coastline in fall and winter) suggests that the birds may shift from more interior-island habitats to more coastal habitats for the winter, presumably to capitalize on the rich intertidal food source (Hobson and Sealy 1991, Sealy 1999).

As A. a. brooksi is non-migratory and endemic to the Haida Gwaii archipelago, the rescue effect does not apply.

Interspecific interactions

No other species of owls are known to nest on Haida Gwaii, although there are non-breeding records of Short-eared Owls (Asio flammeus) and Snowy Owls (Nyctea scandiaca) (Peter Hamel, pers. comm.). As there are no other nocturnal avian predators known to occupy Haida Gwaii, niche-specific competition is probably minor.

Saw-whet owls are secondary cavity nesters and their population may be affected by fluctuations in primary cavity excavators. Thus, indirectly, the owl population may interact with Northern Flickers, Hairy Woodpeckers and Red-breasted Sapsuckers. The Hairy Woodpeckers on Haida Gwaii are also an endemic subspecies (P. v. picoides) thought to exist at low densities across the landscape. Thus, it is likely that potential nesting cavities have been built up as a function of time, rather than a function of high densities of woodpeckers. Declines in nesting habitat for primary cavity nesters (suitable snags) may therefore have long-term effects on Saw-whet Owls.

Direct interactions with introduced species may also affect these owls. For instance, European Starlings (Sturnus vulgaris) may compete with the owls for nesting cavities and are known to harass cavity holders, although nest observations throughout the spring showed that the owls were not displaced (CIH, pers. obs.). Continued harassment by starlings may lead to the abandonment of active nest sites (K. Otter, pers. comm.).

Adaptability

Current evidence suggests that the Haida Gwaii subspecies has diverged quite recently, and thus current differences between the acadicus and the brooksi subspecies are likely related to adaptations to the local environment on the archipelago (Byun 1998). One important difference between the subspecies appears to be their over-winter movement towards the coastline, where there is an abundant and easily accessible food source, intertidal amphipods and isopods (Sealy 1999). Furthermore, in their foraging, the Haida Gwaii Saw-whet Owls seem to be more generalist that the nominate subspecies, taking locally available food items other than rodents, which are thought to be the major food source of Northern Saw-whet Owls elsewhere. For example, on East Limestone Island, which houses one of the few breeding colonies of Ancient Murrelets (Synthliboramphus antiquus), a Northern Saw-whet Owl has been observed taking downy murrelet chicks (Gaston 1992).  Overall, the adaptability to very localized differences in food availability is likely an important factor in the persistence of this subspecies, as is illustrated by the high mortality rate, apparently due to starvation, of other owl species when they move through Haida Gwaii (P. Hamel, pers. comm.).