Gressenhall’s Barn Owls – Success at last!

After two seasons without any successful breeding we finally have a pair of Barn Owls back at Gressenhall Farm! The pair have been occupying one of the nest boxes overlooking the wet meadows; using the box as a roost for some time. Our female has been sitting on five eggs which is a good clutch size for this particular species; if the pair can successfully hatch and raise all five chicks it will be quite an achievement since these birds do not fare particularly well in the wider countryside and are not expected to live much more than two or three years as adults.

5 eggs have been laid in total, between 9 and 19 April

5 eggs have been laid in total, between 9 and 19 April

The very wet summer of 2012, followed by an extremely cold winter into 2013 has undoubtedly had an impact on the breeding rate for Barn Owls because like much of the animal kingdom they will not attempt to breed if they cannot secure an adequate food supply. The wet summer of 2012 made hunting difficult and the cold and prolonged winter of 2013 led to a marked decrease in the abundance of key prey species such as Short-Tailed Vole, Wood Mouse and Common Shrew. This has meant that many adult birds have also faced starvation, and higher than usual mortality rates have been evident across the countryside with fewer birds being spotted and even fewer juvenile birds recorded during the 2012-13 period.

Barn Owls hunt by sound, using their excellent asymmetrical ears to locate prey in tall grass as it scurries through self-made tunnels in the soft, dead material below the new growth. They use stealth; gliding through the air making almost no sound at all as they have hollow cartilage in their wings and specially designed feathers with tiny barbs that trap air and prevent the typical ‘clap’ that bird’s wings make when in flight. Unfortunately, these special adaptations also have a key flaw- Barn Owls will not hunt if it is too wet not only because their prey are often holed up in the dry but also because the birds do not preen or produce any oils to ensure their feathers are waterproof; meaning that to get them soaked could prove fatal.

Barn-Owl-hunting-sequence

The good news is that we have had a very mild winter (the first in a few years!) and a fantastic early spring in 2014; many prey species survived in good numbers over the winter and have begun breeding around two weeks earlier than last year. At Gressenhall we are fortunate to have a perfect habitat for mice and voles in our wet meadow grasslands beside the river Whitewater (a tributary of the river Wensum) so it is no surprise that the pair of Barn Owls which had been occupying a nesting box overlooking the meadow were found to be on eggs in early April. In total 5 eggs have been laid, with the close pair continuing to mate whilst incubating and exhibiting wonderful signs of bonding behaviour during this time; it’s fantastic to have the ‘naturewatch’ cameras on hand to capture these moments!

Parent birds perform regular bonding rituals including this greeting gesture when the male returns to the nest.

Parent birds perform regular bonding rituals including this greeting gesture when the male returns to the nest.

The first chick duly arrived on 11th May, followed up by the second and third on the 13th and 15th respectively. It is usual for the eggs to hatch over a staggered period of a couple of days for each egg, given that they are generally laid over a week or so from first to last, with no more than two eggs laid in a 24-hour period. The female has put a great deal of effort into egg laying and it uses an immense amount of her energy to produce the required nutrients in the eggs. Studies have shown that there is a direct relationship between the bodyweight of the female and successful laying, including final clutch size. She will rely heavily on the male being able to supply her with enough food during laying and incubation to ensure that a good clutch is laid and incubated to hatching.

1st chick hatching 11.5.14

1st chick hatching 11.5.14

Another good omen for these chicks has been the sheer amount of food being brought in by the male bird. We have observed up to six fresh kills being brought into the nestbox in a 24-hour period. These early arrivals are now approaching two weeks old, and have had a great start; already they are proving quite a handful for the female bird as she tries to keep them warm under her brood patch. This is an area of bare skin that is packed with blood vessels and carries a lot of heat to that particular part of her body where she can press down onto the young and incubate their tiny featherless bodies. It will still be another week or two before they begin to grow the white ‘down’ that will make them less dependent on her warmth inside the nest.

2nd & 3rd chicks arrive 13 & 15 May. Barn Owls typically lay their eggs a day or two apart so that hatching is staggered over a period of days.

2nd & 3rd chicks arrive 13 & 15 May. Barn Owls typically lay their eggs a day or two apart so that hatching is staggered over a period of days.

Our fourth chick hatched on the 18th May; the first three are already twice the size of the newborn but if the plentiful food supply continues it shouldn’t be long before this chick catches up. Although the imagery from the ‘naturewatch’ cameras is grainy at times, it has still been possible to watch some of the chicks hatching. The chicks are born with what is sometimes called an ‘egg tooth’; a small protrusion from the beak which the chick uses to break through the eggshell over a number of hours. The egg tooth will fall off the beak of the young bird within a few days.

Mother with 4 chicks and one egg still to hatch. No4 arrived on the 17/18 May.

Mother with 4 chicks and one egg still to hatch. No4 arrived on the 17/18 May.

A possible fifth hatching may have taken place by the 21st; it certainly appears from the images we have been able to take that hatching began around the 19th or early on the 20th. This would tie in with the other egg hatching times. The problem we have is that the female bird is more or less constantly incubating and only moves off the chicks once or twice every 24-hours. This, coupled with the size of the earlier chicks now crowd out the smaller two; although it is clear to make out four chicks it remains impossible to determine whether the fifth has survived. Hopefully we will have a better idea in the coming days.

Are there 5 chicks in this shot? 23.5.14

Are there 5 chicks in this shot? 23.5.14

Barn Owl chicks will compete more and more fiercely for the scraps of food dispensed by the female as they grow, and the smaller, later chicks are at a distinct disadvantage. It is not uncommon for them to starve or be bullied and pecked to death by their older siblings; sometimes even eaten by them! Although it seems cruel, there is a natural advantage in that these stronger birds will certainly have a better chance of survival, whereas if the food was evenly distributed then all chicks would have the same chance of survival but also the same susceptibility to starvation in leaner times.

Barn Owls have a typical incubation period of 32 days. This is quite a considerable period of time and compared to other Owl species it is lengthy. In comparison to average female body weight, incubation is 10% longer than might be expected. Barn Owl eggs are prone to breaking before hatching; indicating that they might be thinner than other owl’s eggs. This might explain the longer incubation period or it might point to the fact that the female must rely on whatever the male is able to provide for nutrition whilst she is on the eggs. She certainly spends most of her time incubating and this practice continues once the chicks have hatched. They cannot regulate their own body temperatures for the first three weeks of their lives so it is up to the female to brood them until they have acquired the downy feathers that will enable them to stay warm.

Mother with her chicks

Mother with her chicks

In total the chicks will be in the nest for around 60 days. Though the reason is unclear, it is suggested that the Barn Owl has evolved in this way to compensate for when food is less abundant; a longer period of time spent in the nest should in theory support young birds in their quest to reach an optimum body weight before leaving the security of the nest. It certainly reflects the symbiotic nature of the relationship between predator and prey. Another item of interest is the significance of the male’s role in bringing food to the female whilst she is incubating the eggs. There is again a clear relationship between the amount of food he is able to provide to the female and the final clutch size.

There are six distinct small mammal carcasses in this shot. The male is doing a great job keeping his young family fed.

There are six distinct small mammal carcasses in this shot. The male is doing a great job keeping his young family fed.

60+ days in the nest will make for some fantastic images though so we shouldn’t complain! We have already captured some images of the female feeding the young chicks; with the male bringing as many as six rodents every night so far she will tear the meat apart and feed strips to the chicks. Until they are big enough to eat an animal whole she will continue to do this, and they will produce miniature pellets of their own when swallowing indigestible items such as bones and fur.

Owl chicks are renowned for their mobility and curiosity so we can expect plenty of movement in the nest box over the coming weeks as the chicks grow and begin to explore their surroundings. Already the largest of the chicks is becoming difficult to keep under the female’s belly! These amazing birds are quite unlike many other raptors in that the young will readily play and interact with one another and make frequent attempts to explore outside the nest as soon as they are big enough to get around.

A face only a mother could love..?

A face only a mother could love..?

We will continue to monitor the cameras and make regular updates on the chick’s development. Hopefully the weather will continue to be mild and not too wet; with plenty of food being brought in by the male every night these birds will continue to grow and we will have some fascinating images to show you over the coming weeks!

Daniel Johnson

Landscape Management Trainee

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