Giving a hoot about owl populations

International Owl Day – who gives a hoot?  Dr Sarah Hoy sure does!  Sarah completed her PhD at the University of Aberdeen, which studied how the demography and dynamics of tawny owls have changed over the last 30 years in response to changing environmental conditions.  She is now a researcher in the School of Forest Resources and Environmental Science at Michigan Technological University, working with the Isle Royale Wolf-Moose Project and Yellowstone Wolf Project.  To mark International Owl Day (4th August), Sarah will be talon us owl we need to know about these remarkable birds.

Tawny owls (Strix aluco) are a medium-size, nocturnal, highly territorial, cavity nesting species of owl, which mostly hunt small mammals and can commonly be found in woodlands across most of Eurasia. Tawny owls have been continuously and intensively studied in Kielder Forest, northern England since 1985 (the year before I was born!), by both ecologists working at the Forestry Commission and by Professor Xavier Lambin’s research team at the University of Aberdeen. Owl habitat is constantly changing in this study site because Kielder is a working plantation forest, managed for timber production, yet tawny owls seem to be able to cope well with the continuously changing forest structure. For example, if foresters fell the tree or entire block of trees where a pair of owls were nesting, the pair will move to another nest box nearby. However, in addition to these ongoing changes in habitat structure, several other important aspects of the owl’s environment have changed radically over the last 30 years.

Tawny Owl (Photo: Sarah Hoy)

Firstly, there has been an important change in the abundance of their main prey, field voles (Microtus agrestis). Field vole populations show cyclical dynamics. That is, in one year there will be a huge increase in vole numbers, but then the following year the population crashes and remains low for a year or more, before there is another big increase year. This means that owls have to contend with years of plenty, with enough food available for most pairs to breed and parents to be able to successfully rear 4 demanding chicks (which is no mean feat!), but also years of foods scarcity where many pairs do not even attempt to breed or fail to fledge more than 1 or 2 chicks. However, like elsewhere in Europe, vole population dynamics in Kielder Forest have switched from high-amplitude to low-amplitude cycles in the mid 90’s (Cornulier et al. 2013). This means that although vole populations still fluctuate up and down, the increases in abundance are much less dramatic. The lack of such big vole increase years has ultimately lead to an overall decline in food availability for owls (and other vole-eating raptors) in more recent years. This has had a negative effect on the number of owlets surviving each year and has also influenced owl breeding behaviour.

Voles represent the main prey of tawny owls (Photo: Xavier Lambin)

A second important change for owls in Kielder Forest is that life has become more dangerous because there has been a dramatic increase in the abundance of northern goshawks (Accipiter gentilis), an important predator of tawny owls. In addition to the increase in the number of goshawk pairs in the forest there has also been a substantial increase in the number of tawny owl being killed per goshawk pair (Hoy et al. 2017).

Goshawks predate upon tawny owls (Photo: Sarah Hoy)

Together these changes mean that nowadays tawny owls have to not only contend with catching a less plentiful prey resource, but have to also avoid becoming prey themselves to the larger northern goshawk. However, despite all these environmental challenges, the owl population has seemingly remained stable over the study period, at least in terms of the number of owl territories occupied. Part of the reason for this apparent stability is thought to be because goshawks are mainly killing juveniles and old owls (Hoy et al. 2015), which have a lower probability of surviving and reproducing than prime-aged adults (Millon, Petty & Lambin 2010; Millon et al. 2011). Consequently, although the increase in goshawk abundance has had a negative effect on the survival of old owls and affected the recruitment of new breeders into the population, goshawks have only had a limited impact on owl population dynamics because the survival of prime-aged breeders has not been strongly affected (Hoy et al. 2015).

An increase in the number of immigrant owls entering the Kielder population in recent years is another factor which may be compensating for the increase in goshawk predation on tawny owls, and thereby contributing towards the population’s stability (Millon et al. 2014). Although we don’t know where most of these immigrant owls are coming from, we presume that they are born in areas with fewer goshawks and/or where they have better access to alternative prey. Nevertheless, goshawk predation appears to be contributing to Kielder becoming a sink habitat for tawny owls.

Young tawny owl (Photo: Sarah Hoy)

The third, and to me the most interesting factor which may be contributing to the apparent stability of the owl population is that tawny owl breeding behaviour has changed in response to deteriorating environmental conditions (fewer voles/food and more goshawks/predators). In the early years of the study period, when goshawks were relatively scarce (hence low predation risk) and vole populations had high amplitude cycles (e.g. years of very high vole abundance followed by years of very low abundance), owls exhibited an intermittent breeding strategy. That is, rather than attempting to breed every year, many pairs did not breed in years of low vole abundance/food availability. However, in more recent years owls have started to breed more frequently than before (i.e. more pairs breed now compared to earlier years with similar vole densities), but they are producing smaller clutches (Hoy et al. 2016). In theory, spreading breeding effort more evenly across years, i.e. a ‘bet-hedging’ breeding strategy can actually increase an individual’s fitness in certain situations. For example, whilst it may be beneficial to forego breeding when environmental conditions are only poor for a short period of time, it becomes less advantageous, even inauspicious to forgo breeding if environmental conditions remain poor for an extended time period, because the possibility of surviving to breed in the future under better conditions is low. Thus one interpretation of the observed change in owl breeding behaviour in response to the consistently high risk of being killed by goshawks and relatively low availability of field voles over the last decade is that owls have switched from an intermittent, to a more consistent ‘bet-hedging’ breeding strategy. Such an ability to adaptively respond to changes in environmental conditions may help to buffer the negative impact of declining vole densities and increased predation risk on owl population dynamics and is also likely to have important implications for the persistence of the Kielder owl population.

I’m owl you need! (Photo: Sarah Hoy)

More Information:

Cornulier et al. (2013) Europe-wide dampening of population cycles in keystone herbivores. Science, 340, 63–6.

Hoy et al. (2016) Food availability and predation risk, rather than intrinsic attributes, are the main factors shaping the reproductive decisions of a long-lived predator. Journal of Animal Ecology, 85, 892–902.

Hoy et al. (2017) Density-dependent increase in superpredation linked to food limitation in a recovering population of northern goshawks Accipiter gentilis. Journal of Avian Biology, doi:10.1111/jav.01387.

Hoy et al. (2015) Age and sex-selective predation as moderators of the overall impact of predation. Journal of Animal Ecology, 84, 692–701.

Millon et al. (2010) Pulsed resources affect the timing of first breeding and lifetime reproductive success of tawny owls. Journal of Animal Ecology, 79, 426–435.

Millon et al. (2011) Natal conditions alter age-specific reproduction but not survival or senescence in a long-lived bird of prey. Journal of Animal Ecology, 80, 968–975.




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