31 July 2015

Interrupted Barn Owl brood

A couple of days ago we ringed the last of our first-brood Barn Owls, this time out at a National Trust farm in Penwith. This is a great example of the value of monitoring, with the male of this pair being a first-year bird ringed a few miles up the coast at another National Trust farm. As a young breeding pair, they had a faltering start... Their first nesting attempt was just three eggs, two of which looked like they were never incubated, with the female sitting on a single egg at the front of the box. They soon decided to start over, moving to another box on the farm, laying three eggs in quick succession but then waiting a week to lay a further two! It was presumably these late two that never made it beyond hatching, but three did survive big enough to look like fledging (below being ringed).

With all the first broods now ringed, we can tally up the year and see how it compares to the last few...

2011 2012 2013 2014 2015
Sites visited 32 34 32 44 40
Unoccupied 2 7 12 12 12
Occupied but no breeding 13 2 7 8 4
Occupied but no breeding 13 2 7 8 4
Failures 0 0 3 0 4
Average brood size (where observed) 3.1 3.1 2.4 3.5 3.5
Number of chicks ringed 33 46 19 63 70

So whilst we checked fewer sites than last year, we saw good rates of occupation and similar clutch and brood sizes to 2014, but managed to ring more chicks than any other year. What was different this year was the high number of failed attempts. Of these, two failed at the egg stage (the young pair above and another where the female was found dead mid-incubation) and two at the chicks stage.

One of the failures saw five large chicks found decapitated in the box. We've seen this once before and presumed it was predation by a Stoat, but we've since heard that adults under stress can sometimes kill their own chicks in this manner, so it was perhaps food stress that caused this particular loss. This happened overnight, being a grisly find for the barn owners when they turned on their nestbox camera in the morning.

But with more boxes being put up and monitored, we hope to have more interesting and enlightening stories to share over the coming years. Thanks as ever to the many landowners who let us into their barns and houses to monitor these birds and in particular to Shaun Boyns at the National Trust for coordinating all of the monitoring in Penwith.

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