Many thanks to Andrew Martin for this piece:
We are very fortunate at our place to have a pair of barn owls which both roost and breed in a box we put up a few years ago. It’s fairly normal to find a few owl pellets near the box, and last winter we did collect and analyse 24 pellets to see what the barn owls had been eating.
24 pellets contained the remains of 31 field voles, 16 common shrews, 15 woodmice, 12 bank voles, 3 house mice, 2 yellow necked mice (although these can only be distinguished from woodmice by a jaw length of > 16mm, so any not fully grown will be recorded as woodmice) 2 rats and 1 pigmy shrew.
There is a good key to identifying small mammal bones from owl pellets here:
https://www.nhbs.com/the-analysis-of-owl-pellets-book
and there was some discussion of the owls’ prey selection on this website here:
https://carmarthenshiremeadows.com/2017/02/16/moles-owls-and-first-signs-of-spring/
Usually, once on the ground the pellets don’t last long if they get wet. In West Wales, where rain has been know to occur, this usually means they last only a few days before they disintegrate, in fact soaking them in water is the best way to get the bones out of the fur so you can identify them. But during the recent very cold weather, colder in fact that it has been since 2012 when we came here, there wasn’t much liquid water, it was frozen. And we noticed quite a few owl pellets on the ground not only under the box, but also out in the field. So we had a good look under all the trees in the treelines surround our fields and collected a large number of pellets in a bucket. Almost all of the pellets we found were under one tree – the left one of the two oaks in the foreground which is about 50 m from the nestbox. They might get analysed during any forthcoming wet weeks.
Since the weather has gone back to normal (i.e. lots of rain) they are not accumulating any more, but you can still make out little piles of bones surrounded by a pad of wet fur.
Andrew Martin
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Carmarthenshire Meadows Group - Grŵp Dolydd Sir Gaerfyrddin 
Really interesting Andrew… just out of interest, how many pellets will an owl typically produce per day, say. Or is that a very variable figure depending on hunting conditions?
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I’ve just looked it up, and apparently it’s a maximum of 4 in 24hrs but that’s exceptional, it’s usually 2. It’s a minimum of 6.5 hrs before the food is digested enough. Their stomach acid pH is higher than many diurnal birds of prey (i.e. less acidic) which is why the fur and bones are so intact. And they don’t have a crop like most birds, the food goes straight in the stomach. Studies on captive barn owls suggest they eject a pellet (it’s voluntary, not reflex, they choose whento do it) when the next meal is imminent.
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Thanks Andrew. That’s really interesting
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That’s a greater diversity of small mammals than we found in our barn owl pellets on Waun Las NNR. I presume the time of year makes a difference?
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I guess so! That and what habitats they are hunting in, I mean they like grassland and open fields but more/less hedges in their home range would affect the small mammal spp available. Probably the best time to be an owl is the autumn when small mammal numbers are maximum. Things must be a bit tough at the moment, after a hard spell of weather. There’s a good report on how they did last year just come out on the Barn Owl Trust website: https://www.barnowltrust.org.uk/wp/wp-content/uploads/State-of-the-UK-Barn-Owl-population-2017.pdf
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