By James Duncan
Woods Mill Learning and Engagement Officer
The ghostly silhouette of a barn owl (Tyto alba) has been a regular sight here at Woods Mill over the years, but worryingly they hadn't bred on the reserve since 2012. Last year the old nest box reached the end of its useful life and was replaced with a plush new one, situated at the edge of Hoe Wood, in the hope it would encourage them back. After consistent sightings of a hunting pair and upon hearing a riot of screeching from the box over several nights, favours were called in and the box was checked.
We're more than delighted to announce that there were five chicks inside! They were carefully weighed, aged and fitted with ID rings, by licensed owl ringers Stuart and Jake. Two of the young were female, three male. They were very well developed, with some larger than the parent birds and having already shown signs of venturing outside the box. Most birds tend to incubate only when their clutch is complete, but barn owls incubate from when the first egg is laid. This is known as 'asynchronous' hatching and it results in a hatching egg every 2-3 days. This can of course mean there may be a difference in age of the youngest and oldest nestlings of around 10-15 days. This difference seemingly helps to reduce the peak in food demand, aiding the survival chances of the whole brood.
The barn owl was until 2015 an amber listed species, showing significant declines in its UK range. The use of pesticides such as DDT, loss of suitable hunting habitat, harsh winters and collisions with traffic were all cited as possible reasons for their decline. However, there's been a recent upturn in numbers and the erection of nest boxes, amongst other conservation measures, has assisted them greatly. This is largely owing to barn owls not 'building' a nest. They tend to lay on an accumulation of last years' amalgamated nest debris - a compacted pile of their regurgitated pellets. Where there is none (in this case) the eggs may be laid directly onto pellets produced while roosting, or alternatively a nearby hard surface!
The fledging of six Woods Mill kestrels this year has shown the reserve to be a healthy habitat, alive with mammals. Rodent populations tend to be cyclical and raptor survival is directly tied in to their success. Short-tailed field vole usually determines almost half the barn owl diet, with common shrew and wood mouse making up more than a quarter. It's worth considering that each owl will typically require 3-4 prey items per night, with the same ideally required for each owlet. This equates to over 2000 prey items needed to feed both the adults and their offspring during the breeding season. They sure have their work cut out! This is made even more difficult considering their inability to hunt during unfavourable weather - heavy rain or strong winds.
If food is in good supply the owlets frequently overtake the weight of the adult birds by around 6 weeks old. Any premature deaths owing to food shortage will have occurred by 7 weeks and healthy owlets typically take around 10 weeks to fledge and become competent flyers. During the fledging period the young will regularly return to the nest box and roost together by day. Learning to hunt is an entirely instinctive process and amazingly they receive practically no 'training' from the parent birds. Many of the owlets will have captured their first prey item by 11 weeks and by week 12 they're receiving reduced amounts of prey from the adults. They will be venturing further and further from the nest site until around week 14, when 'dispersal' occurs - they leave to find their own home range. There is evidence to show that lingering fledglings are chased away by the parents, though conversely some are tolerated - for a while at least. The average dispersal is less than 10 miles, though it's an incredibly sad fact that many die during this period. As they leave Woods Mill they'll face a multitude of challenges. These include their hunting inexperience, the unfamiliarity of new landscapes during dispersal, the weather conditions during the shift from autumn to winter and their chances of encountering the man-made hazards that are part of our landscape. We certainly wish our five all the very best in their journey.