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A Big Owl Nestbox Project in Switzerland

I was really excited when Dani Studler wrote in about his Tawny Owl nestbox project. Not only is it large, covering nearly 100 sq km with 60 boxes, but Dani is investigating the use of plastic cannisters as nesting sites. If these can be shown to be attractive to tawnies, they offer a simple and cheap way of giving these owls safe breeding sites now that there are so few tree holes available for them.

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Websitelink

Dani now has his own website (home page here or click on picture above), where he reports on his project (for project start here). Its in German, but Google translators make a semi-reasonable job of translating German into English, as shown in my screen grab. (May 2011)

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Project area

The project area covers a 12 x 8 km rectangle of woods and fields centred on Henggart, which is about 30 km (~20 miles) northeast of Zurich. At a rough guess the woods — which are a mix of conifer and deciduous — make up about 25% of the area, forming a straggly patchwork between the larger areas of fields. Crops grown in the fields are maize and other cereals, sugar beet, potatoes, and rape. There is also grassland.

Dani does a fantastic Google map of the area that shows what's going on in all the nest boxes. Click here to see (comes up in a new tab).

 

History of the project

Dani's interest in nestboxes began early, starting with 60 boxes he made for songbirds when he was a child. The idea to put up owl boxes came after he went on a bird excursion and saw Little Owls, which made a deep impression. Hoping to attract owls, he put up six larger boxes near his home, but for many years they remained unused and he nearly gave up hope of ever having an owl use one of his boxes.

Then Dani's luck turned. He writes:

"... by chance, one day I examined one box and found a female tawny owl inside! That was like a jump-start for me. I studied some books and spoke with several people to get as much information as I could about this bird. The first new box I built was made of wood. But I found that attaching it to a tree was tiring — not to say dangerous as well!

"So I looked for a new approach. I knew an owl expert who was trying plastic boxes. He was testing boxes made of wood and plastic on the same tree to find out which the owls preferred. He told me that in 10 places where the owls were offered a choice, in eight cases it was the plastic boxes that were occupied. That was why I decided to use plastic boxes. There was a ready supply because we use plastic cannisters in our factory.

"In summer 2006 I converted about 60 of the blue plastic cannisters we use. I cleaned them out several times and cut an entrance 13 cm in diameter in the front. Two approx 5 mm holes were bored through the back to fix the box to a tree. The bottoms have six small holes for drainage (needed after heavy rain). I distributed the nest boxes over an area of 96 sq km so that they are more or less evenly spaced. If a wood is less than 0.5 sq km I tend to ignore it because it's too small for the owls. And some other woodland areas I may give a miss if they don't provide a good habitat for the owls.

A very cute owlet photographed in the project area. See more of Dani's pics on his photo site here.

Choosing the nestbox sites

"The area has a mix of coniferous and deciduous woods or forest. I usually choose old forest with big trees so the owls can fly through the trees easily at night. Nesting sites are kept away from main roads and railways. All the boxes face east, away from the rainy side. And I look for sites that will allow the owls to fly in and land with minimal obstructions, like branches, in the way. The boxes are usually attached about 4 metres up the tree."

DaniOwl

Top left: Google regional map showing the project area (blue rectangle) in northeast Switzerland.Top right: On this Google satellite image the project area is outlined by the white rectangle. It's about 12 km x 8 km. (Click on thumbnails for large versions.) Right: Dani ringing a female.

There are two types of wood in the area — type 1 (left) and type 2 (right).

Breeding data

 

2007

In 2007, the first year of the project, there were four breeding attempts with a total of 15 eggs (3 broods of 4 eggs and one of 3 eggs). One box was raided by a marten. The only brood successfully raised to fledging was the one with 3 eggs. The other two attempts were abandoned.

 

2008

The next year, 2008, nine boxes were used, with a total of 21 eggs laid. Three eggs were infertile, 2 eggs were predated, 3 eggs were abandoned and 13 hatched.

Of the 13 owl chicks, 4 died, 2 were predated, and 7 fledged. 2008 was a poor year, with a small rodent population. Most of the clutches consisted of just 2 eggs, with other broods of 1 egg (one brood), 3 eggs (two broods) and 4 eggs (one brood).

So there was a modest increase in box usage from 2007 to 2008. Most owls in the second season laid small clutches, and it's possible that the reason for this was the limited food supply. Owl feathers were found in some boxes not used for breeding attempts, showing that owls are present in the area but not breeding.

 

2009

Winter 2008/09 saw long periods of cold temperatures, with more than 20 cm snow in some places. So Dani wondered whether he would find more than a few owls breeding this spring.

But the first inspection of the nest boxes, completed on 15 April, produced pleasing results.

The number of boxes used for breeding increased to 10 (from nine in 2008). The total number of eggs laid was 26 (up from 21 in 2008), with six broods of 3 eggs and four broods of 2 eggs. A total of 14 eggs hatched, and 11 chicks made it to the fledging stage.

In three cases, the same box is being used as last year. The other seven boxes are being used for the first time. Because the females using boxes in 2008 were ringed, it may be possible at the next inspection to find out whether some are using the same boxes or whether some have moved to new boxes.

For a Google map showing locations and data for all ten boxes, click here.

The 2009 data are tabulated on the next page.

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