Slender-billed Barn Owl

Slender-billed Barn Owl Tyto gracilirostris is endemic to the Eastern Canary Islands: Fuerteventura, Lanzarote and associated islets. It has short wings and tail (12% and 14% shorter than alba: Cramp 1985) and a ‘gracile’ bill, combined with the strong tarsus and claws typical of an insular owl. Bannerman (1963) noted the dark grey colour of the upperparts and described the underparts as “mainly cinnamon-brown or deep rusty-buff (rarely light buff), the whole more or less heavily and regularly spotted with dark brown from the breast to the thighs, as well as the under wing-coverts.” A lucky observer might see broad irregular bars of grey-brown across both webs of the primaries, or three well-marked bars on the tail. 

Two discoveries from the Western Canary Islands suggest a wider distribution in the past. Jaume et al (1993) reported remains on La Gomera, more than 250 km west of the current breeding range, commenting that Slender-billed Owl is osteologically distinct enough to be regarded as a separate species. The layer containing the remains was dated to the Upper Pleistocene, ie, somewhere between about 126,000 and 12,000 years ago. On Gran Canaria in the centre of the archipelago, Alcover & Florit (1989) reported Slender-billed bones at an archaeological site approximately 2000 years old. This suggests that it became confined to the eastern isles only quite recently. 

In the Western Canary Islands today there are only Common Barn Owls T alba, the same white birds as in Western Europe and North Africa. How they came to be there is a mystery waiting to be solved. Felipe Siverio has surveyed barn owls throughout the archipelago. I asked him whether he thought Common Barn might have arrived in prehistoric times, given that they breed in cliffs and not in buildings. Felipe replied that barn owls elsewhere started to use human structures only because of a lack of natural ones. In the Canary Islands there is no such problem, so cliff nesting tell us nothing about when they arrived. Felipe suggested that in ancient times, any larger Common Barn arriving from the continent would have had trouble surviving without suitable mammals to prey on. Perhaps with the introduction of non-native rodents, their moment arrived. 

Barn owls make great collaborators for palaeontologists investigating the past distribution of small mammals (Rando et al 2011, Rando et al 2012). In the Eastern Canary Islands, Slender- billed Barn Owls have been using the same roosts and nesting sites for millennia, often volcanic tubes where the extinct Lava Shearwater Puffinus olsoni once bred. Some contain huge accumulations of owl pellets, broken down into layer upon layer of bones and dust. With carbon dating, we can learn the approximate age of the bones. 

From one layer to the next, House Mouse Mus musculus becomes an important prey item, having been completely absent before. The oldest House Mouse remains come from Lanzarote, and have been dated to 128-333 AD. Pellets dated to 756 BC are House Mouse free, so they must have arrived in the intervening time. Before their arrival, the commonest prey in fossil owl pellets was the endemic Lava Mouse Malpaisomys insularis. The two mice species coexisted for about 1000 years, until Lava disappeared around 1271-1394 AD, roughly the same time as the Lava Shearwater. From then onwards, Black Rats Rattus rattus appear in the pellets. What happened? House Mice probably arrived with the Guanche, the first human inhabitants of the Canary Islands, somewhere between 1000 and 100 BC. Lava Mice and Lava Shearwaters disappeared shortly after the arrival of Europeans who brought with them their nemesis, the Black Rat.

Genetic studies currently underway may help to answer several questions. Is there any gene flow between Common and Slender-billed Barn Owl? How long have they been evolving separately? Which continental population gave rise to each? I hope we find out sooner rather than later, as Slender-billed Barn Owl is critically endangered. Felipe estimates the minimum population at between 53 and 105 pairs (Siverio 2008), putting it in the same league as Zino’s Petrel Pterodroma madeira. 

I first heard Slender-billed Barn Owl in March 2001. Having made my way towards the mountainous interior of Fuerteventura in the late afternoon, I decided to stay there for the night. When I first heard a perennial screech ringing out, I was surprised and delighted. I had not known where to start looking for this rare taxon. Further screeches rang out at a rate of about three per minute, all of them in flight (CD1-13). The caller remained unseen, but hearing it in that resonant valley was an experience I will never forget.

In January 2010, René Pop and I visited several sites on Fuerteventura, recommended by Felipe, to try to record more Slender-billed Barn Owls. At most sites we were unlucky. I did hear one in a volcanic crater in the north of the island which had wonderful acoustics but it was distant, high up on the crater walls. So I ended up making most of my recordings at exactly the site I discovered by accident in 2001. Nine years on, there were two nests about 650 m apart on the same rocky slope.

Our visit coincided with the peak of courtship behaviour, so there was a great deal of activity. In CD1-14, a male is perched on a cliff top just above his nest in a tall vertical crevice. He gives courtship screeches about once every seven seconds until 0:51 when his distant neighbour, who has been courtship screeching faintly in the background, gives a rivalry scream. At this he becomes livid, taking off and calling more frequently while flying around the nest area. His calls are shriller and higher-pitched than before. In the meantime, his neighbour flies closer and utters another rivalry scream. In CD1-15, one of these males, perhaps the neighbour, gives three rivalry screams in a solo performance, echoing off the steep rocky slope where they have their nests. At the end of the recording, René is talking to me in the distance, complaining about the futility of photographing barn owls on cliffs in the dark.

Calls of the few Slender-billed Barn Owls that we have studied differed only slightly from those of Common Barn Owl, if at all. We have only three or four males to go by, not enough to draw hard conclusions. The perennial screeches may be slightly higher-pitched than in Common Barn from Portugal and elsewhere, but the courtship screeches seem to be lower-pitched, a surprise considering the smaller size of Slender-billed. Calls evolve slowly in barn owls despite dramatic differences in ecology. The world’s barn owls sound much less diverse than the world’s scops owls Otus, for example. Isolated from Common Barn by the sea, Slender-billed may not have needed to evolve different calls. As a parallel case, Tawny Pipit Anthus campestris and the 20% smaller Berthelot’s Pipit A berthelotii of Macaronesia (including Fuerteventura) also sound virtually identical. 

With its much smaller size, different structure and plumage, and its isolation, Slender-billed Barn Owl may well be a species. Without the genetic part of the story we are left in suspense. At the very least, it is a unique and very rare taxon, finely tuned to life in a rather special environment. How tragic it would be if it went the way of the Lava Mouse and the Lava Shearwater, which were only missed centuries after their demise.

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