Twit-twoodunnit? The curious case of the silent feathers*

On 6th February we stumbled upon an avian crime scene. Piles of feathers lying on the ground close to Kinramer Wood indicated that an unlucky bird – and clearly quite a large one – had recently met a sticky end here. We tried to examine the feathers from the road using binoculars. A goose, perhaps? No, that wasn’t right. So who was the victim, and what had happened to them? We went in closer for a stickybeak…

crime scene

Evidence at the scene. Left: two large piles of feathers on the grass. Right: a closer view.

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The velvety texture of the wing feathers

When we got closer, we could see lots of large wing feathers, orange, grey and white with bold black bars. In addition, these feathers were incredibly soft, with a fine downy texture. Immediately we knew they had to belong to an owl, and from the size and distinctive markings, it didn’t take long to conclude that they had to be from a Long-eared Owl, one of Rathlin’s most secretive residents.

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This clump of material contains the sharp, pointed front teeth of a mouse.

Mingled with the feathers, we found another piece of evidence – the tail and teeth of a mouse, which we can only assume the bird had been holding or eating when it died.

We gathered up the feathers (and the bits of mouse) and took them home for closer study.

Intriguingly, we found that these remains included an almost complete set of beautiful flight feathers from the left wing, but very few from the right wing. There was an assortment of fluffy body feathers, but not a single tail feather.  Not to mention the almost complete absence of any other parts of the unfortunate owl.

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We found nine out of ten primary feathers from the left wing of a Long-eared Owl. Only the outermost primary is missing.

LEO fimbriae

The comb-shaped fimbriae reduce air turbulence, and thereby noise, at the front edge of the flying owl’s wing.

Owl feathers really are worth a close look. One unique feature is this intricate barb structure along the outer web of the outermost couple of primaries. These feathers form the leading edge of the wing when the bird is in flight, and the distinctive saw-tooth shapes (technically known as fimbriae) along the front edge muffles the noise of the air rushing over the wing, allowing the bird to approach its prey silently. Another sound-muffling adaptation is the soft fringing on the trailing edge of the feather. These features and how they reduce noise are described in detail in this paper by Pulkit Sagar et al.

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The soft fringe on the trailing edge of each wing feather also help an owl fly silently

LEO feathers ageingWhat else can we tell from these feathers? Well, based on a fantastic information sheet by Javier Blasco-Zumeta & Gerd-Michael Heinze (available here), we think we can fairly confidently determine the bird’s gender and age:

  • The large pale area on the inner web of each secondary is plain white (A). This makes it a male, as females would have darker buff colouring in this area.
  • The pattern of barring on all these feathers is quite sparse, with just four strong black bars per feather (B), and the outermost bar is set well back from the feather tip (C). This tells us that the bird was an adult; i.e. at least three years old.

So, it’s an adult male Long-eared Owl, but how did it die, and why was most of one wing and a pile of body feathers left behind at the scene?

Our suspicions turned immediately to Ferrets, the feared feral foe of much of Rathlin’s wildlife. These non-native mustelids are ferocious predators of ground-nesting birds and many other animals, and consequently a huge conservation issue on Rathlin. Perhaps the owl had pounced on a mouse, and in the process of eating that was itself pounced upon by a fortuitous ferret. But the other evidence didn’t seem quite right for a ferret kill. The bird had clearly been plucked, and that would be more typical behaviour of a bird of prey.

owl eaters table

Predators of Long-eared Owls, and the number of records for each predator. Species occurring on Rathlin are in bold. Adapted from Mikkola (1976).

But would any birds of prey kill a Long-eared Owl, itself a top predator? We managed to dig up this eye-opening paper by Heimo Mikkola, who gathered together records of owls preying upon, and being preyed upon, by other owls and raptors. One of the findings was that, of all European birds of prey, the Long-eared Owl is by far most frequently preyed upon by other birds (46% of the 934 records, see table). Its main enemies are Northern Goshawks and European Eagle Owls, which we certainly don’t have here on Rathlin, but it has also been known to fall prey to Common Buzzards, Peregrine Falcons and even (on just a couple of occasions) Common Sparrowhawks, all of which do live here. From the data in that paper, the buzzard is the most likely of these three raptors to eat a Long-eared Owl, but it would still be a fairly rare occurrence.

One final theory is that the owl injured itself, perhaps by colliding with overhead power lines during the night, and was then found, plucked and carried away by a scavenger. This is possibly the most likely scenario – there were overhead wires nearby, and there are lots of Hooded Crows, Common Ravens and Common Buzzards around to take advantage of such a mishap.

We’ll never know for sure what happened to this bird. It is always a thrill to catch a glimpse of a live Long-eared Owl, but even finding these few remains was exciting. It gave us a rare opportunity to study its beautiful plumage, take a close look at its unique adaptations, and to learn more about one of Rathlin’s most elusive inhabitants.

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Boo!

*Ok, we know Long-eared Owls don’t really go twit twoo (that’s Tawny Owls), but this was the punniest title we could think of.

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It’s here: Rathlin Bird Report 2017

Hot off the press, here is our Rathlin Bird Report 2017.  This is a summary of everything we saw during a year of birding on the island, plus all the sightings we’ve heard about from others, and lots of pictures.

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Rathlin’s bird list for last year reached a satisfyingly round total of 150 species. The highlights of 2017 included a first for Northern Ireland and several local rarities, plus all Rathlin’s usual specialities like the Great Skua, Red-billed Chough, Corncrake, and of course squillions of nesting seabirds.

Click here for all the details.

Now we’re looking forward to all the birds that 2018 will bring to the island. We’re sure 150 species can be beaten!