A look back: Was the winter 2016 – 2017 challenging for barred owls in Quebec?

Several cases of primary inanition

From April 2016 to March 2017 a total of 56 barred owls (Strix varia) were submitted for post-mortem examination at the CWHC Quebec Regional Centre. This represents the highest number of submissions of this species of Strigiform to our laboratory for a given year. Most of these owls (42) were submitted during the winter (November to March). The two most common causes of death were primary inanition (23) and trauma (30). Deaths associated with inanition (starvation) are not uncommon in birds of prey, especially in young individuals with suboptimal hunting abilities. It is believed that these birds are unable to capture sufficient prey to fulfill their caloric needs and therefore progressively starve to death. These deaths are mainly observed during the fall and the winter.

Interestingly, the numbers of mortality caused by primary inanition in barred owls vary from one year to the other, following a two to three years cycles (Figure 1).

Figure 1: Inter-annual variations of the number of cases of fatal primary inanition documented in barred owls in the province of Quebec.

Even if the reason for these inter-annual variations remains unclear, it can be hypothesized that these variations are, at least in part, due to changes in the abundance of this species on the territory. An increase in the number of barred owls in a given year could obviously increase the probability of detection of a dead or sick barred owls. In addition, it is possible that the competition for food resources (such as rodents) between owls increases during winter with high number of owls, which would increase the risk of inanition in less experienced birds.

Citizen science: analytic tools

Wildlife agencies regularly perform surveys in order to determine the abundance of a species. Since these surveys are costly and demanding in human resources, they are usually directed toward harvested species and threatened populations. Consequently, the quantity of scientific data available on abundance is suboptimal for several species. Some wildlife conservation organisations use “citizen science” in order to follow changes in populations of some wildlife species. For example, Bird Studies Canada / Études d’Oiseaux Canada, a non-governmental organism which has for mission to promote avian wildlife conservation in Canada, manages an annual survey of nocturnal owls in Quebec. Since 2008, this scientific activity have relied on the voluntary participation of citizens that perform this survey using a well define protocol. These participants conduct a normalized survey during the month of April during which they count all owls observed visually or through auditory surveys during a pre-determined time and on a set territory (for more details on this program see the following web site: http://www.birdscanada.org/volunteer/qchiboux/). These normalized surveys enable scientists to determine relative indices of abundance for the studied species of owls, including the barred owl. These indices can be compared year after year. Since the presence of birds is mainly documented through auditory surveys of territorial calls, these indices also represent an estimate of the reproductive effort of the species.

It is interesting to note that there is a correlation between the abundance indices of barred owls in April and the number of documented barred owl that died of primary inanition the subsequent year (Figure 2).

Figure 2: Correlation between primary inanition during winter and abundance indices of barred owl in the preceding month of April.
Source of the abundance indices: Andrew Coughlan, Études d’Oiseaux Canada / Bird Studies Canada

This observation supports the hypothesis that the abundance of this species on the territory is an important determinant in the occurrence of documented primary inanition in this population. Indeed, the determination coefficient (R²) of 0.37 suggests that 37% of the variations in the occurrence of the cases of primary inanition can be explained by the variations in the abundance of the species. If we look at the figure 2 we can propose that the abundance of this species in April 2016 was a driving factor to explain the high number of cases of primary inanition in this species during the winter of 2016 – 2017. Actually, the highest abundance index over the past nine years, recorded in April 2016, is associated with the highest occurrence of cases of primary inanition during the winter 2016 – 2017. However, the variation in abundance of the species could only partially explain the variation in the frequency of inanition. Consequently, other factors most likely drive the frequency of these cases on inanition. One could ask, for example, how weather parameters could affect the hunting successes of barred owls.

Historical weather data

The government of Canada stores diverse weather data in an easily accessible web site (http://climat.meteo.gc.ca/historical_data/search_historic_data_f.html). After a little bit of work it is possible to extract some meteorological data from this site, such as variations in snowfall. Figure 3 presents the correlation between the number of cases of primary inanition documented in barred owl and the average daily snowfall during the winter (between November 1^st and April 30^th).[1]

Figure 3: Correlation between primary inanition during the winter and daily average of snowfall.
Source: Data for Ottawa (Government of Canada)

The correlation observed between the occurrence of primary inanition and the snowfall suggests that the quantity of snow have a negative impact on winter survival of barred owl. Actually, this meteorological variable could explain 36% of the variations in the frequency of this syndrome in barred owl. This suggests that this species of owls is potentially less adapted to winter conditions relative to northern species such as great grey owl and hawk owl. Having said that, when we look at figure 3 we can see that even if the mortality of barred owls was especially high during the winter of 2016-2017, the average daily snowfall (1.3 cm) was only slightly higher than the last 12 years average (1.0 cm). Consequently, this suggests that snowfall was not a determining factor in the mortality observed during the winter 2016-2017.

Other factors, such as prey availability, could also potentially contribute to the variation in the occurrence of primary inanition in this species.

Stéphane Lair, CWHC Quebec

[1] Snowfall data are for the Ottawa region and therefore represent a good estimate of the relative snowfall for Southern Quebec.