14 August 2018

Cornish Barn Owls in 2018

With all of our Barn Owl sites now visited for the year, it's a good time to crunch the numbers and see how they've fared this year. This is our eighth year of monitoring boxes in Cornwall, but we'll just compare 2018 to the last five years for ease:

20132014 2015 2016 2017 2018
Sites visited 32 44 41 47 64 85
Unoccupied 12
Occupied but no breeding 7
Average clutch size
(where observed)
3.64.6 5.2
Average brood size
(where observed)
2.43.5 3.3
Number of chicks ringed 1963 704790 126

The larger number of boxes visited is due to the funding from Paradise Park which has allowed us to really expand the scope of the project. There are several more new sites for 2018 as well which we hope will be occupied in the coming years. The high rate of unoccupied boxes is also due to the large number of new sites that birds have yet to move into.

However, we did see several regular sites unoccupied for the first time which may well be due to the mortality over the hard late winter weather. These two periods of snow cover won't have made it easy for birds to get in condition for breeding and it does seem that some regular sites weren't occupied, but those that did breed made the most of the good spring weather by laying quite big clutches (including a clutch of seven at one site). But we then saw one of the hottest and driest periods on record, which seriously stunted grass growth, and no grass means no voles. So most sites then saw significant brood reductions, though the average of 3.3 chicks still compared well to previous years.

We also think the 'Beast from the East' has had a greater impact on the breeding population than you might think, apparent by the number of birds breeding in their first year. This summer we've caught 15 adults already carrying rings, of which six were ringed as chicks in 2017. Compare this to the totals from the previous few years:

Year Ringed adults
Ringed as chick
previous year
2018  15 6
2017 11 0
2016 9 2
2015 8 2
2014 13 0

We can only assume that these birds (some at regular sites) were new recruits following the death of an 'original' adult, but if this has happened across the whole population then that my lead to quite a drop in the average breeding age. But that's why we do what we do and what the project is all about; understanding these changes in the county's Barn Owls.