The Country Owl, That Great Stranger

The field owl is a nocturnal bird of prey with a mainly holarctic distribution that, unlike other owls/owls, has very daytime habits and mainly occupies open spaces where it patrols in search of its main source of food, the peasant vole. They nest in the ground, laying between 4 and 8 eggs. They are birds that are fertile when they reach one year of age, this being a key factor for their survival strategy, which we will detail later.

It is of a nomadic nature, that is, its presence is closely linked to the explosion of unpredictable resources, in this case, food, and this makes its understanding and ecology complicated, as well as its conservation, leaving in question whether it is a stable or regressing species, although everything points to the latter.

And to delve a little deeper into this unknown nomadic species, we are going to give a brief review of an article published recently, in which several individuals of field owls from different European populations located in different latitudinal ranges (Iceland, Scotland and Spain) have been followed, thus covering most of the breeding populations of Western Europe.

Remote tracking unveils intercontinental movements of nomadic short-eared Owls (Asio flammeus) with implications for resource tracking by irruptive specialist predators

31 adult specimens (more than one year old) and 16 immature (specimens in their year of birth) were captured, totaling 47 birds, 34 females and 13 males, between 2017 and 2022.

The total sample size has been distributed as follows: 13 individuals marked in Iceland, 17 in Scotland and another 17 in Spain (Palencia, Valladolid and Huelva).

These captures have been made in the breeding areas close to the nest, with fog nets and trap nets with the use of fencing. The GPS is placed with a chest harness, weighing 2.6% of the total weight of the birds. This technology allows us to obtain information on the specific position of the bird at very short intervals (every minute) or much longer (every several hours). The average duration, until the last transmission obtained from GPS devices, was 184 days in Iceland, 231 in Scotland and 249 days in Spain. This is because these devices are equipped with small solar panels that nourish and charge them, so that, as we descend to the south, we find more hours of light and therefore, more data.

MOVEMENTS

With regard to nomadic movements, it has been found that these birds spend more than 60% in their “home-ranges”, that is, areas of regular stay and breeding areas, and less than 10% making long-distance movements (Sahel - Arctic).

In the most geographically isolated population, Iceland, it has been found that the propensity for long-distance travel was much lower (2%) than in the case of Scotland (8%) or Spain (19%), occurring mainly between the months of March-May and August and November).

Some birds from Scotland and Spain have moved through Europe and North Africa, and a specimen from Iceland made its way down to Great Britain.

By showing some data obtained, it has been possible to verify that the distances between breeding areas have ranged from 41 to 4216 kilometers, observing that there is very low fidelity or philopathy to the previous breeding areas. Most of these large scale movements were nocturnal.

In Scotland, there was a female marked in 2017, which in 2018 reared twice, once 41 km from the previous breeding area, and another in Norway, 926 km away from the latter.

In Spain, it has been the case that a female abandoned chickens once they were able to thermoregulate on their own, but before they had total independence, leaving the male with the role of nurturing until that absolute independence. This denotes a clear strategy, the female capable of breeding up to three times in the same year, optimizes the time in this way to start the same process again elsewhere, and to take out another flock of young field owls and thus ensure the survival and continuity of the species.

MORTALITY

Another factor that has been studied as a result of this follow-up has been the survival percentage of this species, which was quite unknown. The survival rate in the entire sample is only 0.47%, which is very low for a species of such dimensions. This mortality has been concentrated during long-distance travel and immediately after breeding. The adult specimens, once the breeding process is over, carry out a complete molt, that is, they completely change all their plumage. This means a very high energy cost, which, if overlapped with the moment of long-distance travel, poses a high risk of death.

The survival rate in the adult sample (31) was 0.45% (between 17 and 22), very close to the figure for the entire sample, which was 0.47%.

The final conclusion is that this species is closely linked to the exploitation of a very specific resource, food, and the explosion cycles of voles range from 3 to 5 years, forcing field owls to live in that constant uncertainty, and to obtain that nomadic character. This study also provides us with data in relation to the great distances they travel, both in dispersion and in sequential breeding places or areas. Studies like this bring us closer to these unknown species, and help us to understand or get an idea of their current state of conservation.

This was just a brief overview of everything contained in this article, which I invite you to consult for much more information about this unprecedented species.

Remote tracking unveils intercontinental movements of nomadic short-eared Owls (Asio flammeus) with implications for resource tracking by irruptive specialist predators

Alfonso Guío, Biodiversity