About the Barn Owl
Causes of mortality
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Mortality is a really important subject to study. If we could increase Barn Owl life expectancy we'd be well on the way to securing the future of this wonderful species. In fact, the life-cycle parameter that influences overall population level more than any other is first-year survival. Reducing the main causes of juvenile mortality would (in theory) be a very effective way of increasing the population level.
Mortality is a difficult area of Barn Owl ecology to study because only a small proportion of the owls that die are ever found. Compared to other bird species, ringed Barn Owls have a very high recovery rate (one in seven) and the finding circumstances are often recorded so you may be forgiven for thinking that there's loads of information on why Barn Owls die. The various causes of death have indeed been identified but unfortunately results based on ring recoveries are always biased towards birds that die in conspicuous places. For example, a bird lying dead on a road is much more likely to be reported than a bird that's dead in the middle of a field.
In spite of its limitations, ring recovery data gives a valuable insight into mortality (amongst other things). Here's a table showing the main finding circumstances reported by researchers using ring-recovery data and a column showing an estimate of the true frequency of each death-cause.
British Trust for Ornithology (BTO) (Percival 1990)
Barn Owl Trust (Ramsden 2003)
BTO unpublished data (1989 -2010)
Estimated true value
|Flew into overhead wire||-||3%||1%||3%|
|Trapped in building||-||2%||7%||2%|
As you can see, road deaths are the most frequently recorded cause of mortality accounting for about half of all recoveries. In reality around 30% of all deaths probably occur on roads. Research by the Barn Owl Trust discovered that most road deaths occur on major roads (motorways, dual carriageways etc.). Considering the number of road deaths per 100km per year, 96% are on major roads and only 4% on minor roads.
Even though it is not often recorded, starvation is thought to be the main cause of mortality. It occurs most often amongst inexperienced young birds soon after independence, during deteriorating weather and prey abundance in the autumn, and in periods of severe winter weather.
Barn Owl weights
Healthy male Barn Owl: 240 - 320g.
Healthy female Barn Owl: 245 - 360g.
Starved individuals: < 220g.
Birds lying dead in remote places or hidden in isolated roost sites are most unlikely to be found. Therefore, along with starvation, causes of death such as flying into overhead wires, poisoning, drowning, electrocution, predation, and rail casualties are under-recorded and may be much more frequent than ring recoveries suggest.
Research into mortality tends to concentrate on the deaths of first-year birds (because most mortality occurs during juvenile dispersal) and deaths of adult birds (beacause the loss of birds old enough to breed is obviously important). Whereas mortality that occurs in the nest is often overlooked. With an average hatching success of 4.8, average brood size of 3, and mean number of young fledging only 2.6, it is obvious that there's (roughly) as much mortality happening in nests as there is amongst the fledged population.
Nestling survival is controlled largely by food supply and this in turn is determined by habitat quality, prey abundance, the weather, and the abilities and behaviour of the individual breeding adults. It can also be affected by the suitability of nesting places because some nests are much safer than others.
Since the mid 90's one particular aspect of nestling mortality has come to light - the frequency with which nestling Barn Owls fall from nests (whilst they are still too young to fly back up) and as a consequence they starve or are taken by ground-dwelling predators such as foxes. Because the Barn Owl Trust is involved in owl rehabilitation as well as conservation/monitoring we are particularly aware of this with numerous "fallen owlet" emergency calls being received every year.
Unfortunately many of the nestboxes that have been erected for Barn Owls (about 25,000 in the UK) are poorly designed having little or no internal depth and insufficient exercise area for emerging young. (for more details, click on this slideshow link). The Barn Owl Trust has developed safer nestbox designs which help to minimise nestling mortality. See "Choosing the best nestbox design".
There is more information about this topic in the Barn Owl Conservation Handbook.