Friday, 22 July 2011

Barn Owls have a mixed season

It is great to be out ringing Barn Owls again. I have worked on Barn Owls for nearly 20 years and I have yet to get tired of the summer round of nest box checks and ringing sessions. Admittedly, it can be a messy business. Barn Owl nests are not the most hygienic of places; the young have a tendency to poo on you and the birds sometimes carry flat-flies, unpleasant parasites that seem ever willing to jump ship and lodge themselves somewhere about your person! I’ll never forget one summer evening where, sitting in a pub with friends after a day of ringing Barn Owls, a flat-fly emerged from a friend’s hair, buzzed around the table and then landed on the blonde sat at the neighbouring table. She didn’t notice it and my friend was too embarrassed to tell her!

Barn Owl site in Lincolnshire
It appears to have been something of a mixed season for the Barn owls this year, with many pairs on two chicks and others just scraping the three that is the national average for brood size at fledging. Conditions elsewhere are better, with fours and a single five but without any of the larger broods that you tend to see most years. Some indication of food availability can be seen from the cache of small mammal prey items that is often present at the nest. The best of those seen the other day had seven Field Voles and three Water Voles, the latter an uncommon food item these days (primarily because the Water Vole population is not what it once was). The presence of a slowly meandering river within fifty metres of the nest box explained why these particular owls had been able to find the Water Voles so readily.

Prey caches across the sites were not too bad, perhaps suggesting that it might have been earlier in the season that the owls were struggling to find food. We know that the condition of the adults going through the winter will influence their chances of breeding the following year and it is possible that clutches sizes were below average this spring.
Young Barn Owl
Barn Owls have a strategy for coping with the unpredictable nature of their food supply. They incubate the eggs from when the first is laid. This means that each egg hatches roughly two days before the next one, producing a brood of chicks whose ages can vary by up to two weeks. Since the young compete for food, it is only when the oldest chicks have had their fill that the youngest get theirs. If times are hard then the youngest chicks will die, leaving the remaining chicks with a greater chance of survival. Nature can be cruel but is always resourceful.

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