Thursday, 24 July 2014

Is 2014 really a good year?

There seems to be a general impression that this is a good year for Swifts in the UK, however, the Reporting Rate recorded by BirdTrack across the UK shows 2014 to be lower than 2013 and 2012, indeed, it seems to be the lowest ever! [Reporting Rate is the percentage of BirdTrack lists that record at least 1 Swift].

Written by Dick

BirdTrack Reporting Rate for Swifts, 2006-2008
(click on graphs to see them larger)

BirdTrack Reporting Rate for Swifts, 2012-2014
(click on graphs to see them larger)
Athough not designed for this purpose, an advantage of BirdTrack for an early indication of trends is that the data is available up to 2 years ahead of BBS. In fact it is available in near realtime!

The graphs, left, include the Reporting Rates for 2006 and for 2014.

The BirdTrack Reporting Rate for 2006 through June to mid-July (the peak season) averages 47.2%. In 2014, the reporting rate for this period averages 37.6%.

This is a decline in reporting rate of 20% in 8 years. Given that changes in BirdTrack Reporting Rate underestimate changes in abundance, this is quite a drop.

[Older graphs than this on the BTO website were calculated incorrectly - so we cannot go back further than 2006]

There could be a number of explanations for the apparent contradiction between numbers at colonies and numbers recorded by BirdTrack:

With the fine weather, it could mean that Swifts are finding plenty of food near their colonies, giving colony watchers an impression of abundance, but, as Swifts do not need to forage further afield, maybe they are seen less frequently by BirdTrack listers.

It could just be that they fly higher in good weather, so are again less likely to be seen.

Alternatively, it could be that those birds that still have their nesting places intact have had a good year, but there may be fewer intact nesting places, so birds recorded away from colonies may appear scarcer.

Another explanation is that birds from destroyed colonies are prospecting those colonies that still exist. This ties in with anecdotes from Poland, where survey data indicates increasing Swift numbers at a time when large numbers of colonies are being lost - the birds may be in the air, and visible, rather than sitting on their nests.

Whatever the explanation, it is a situation that needs watching.

Friday, 18 July 2014

Swifts do prefer boxes with concaves

There is little in the way of statistics that support what Swifts might prefer in their breeding location, so I was pleased to get this result.

by Dick
Feathers added by a pair of Swifts to a concave
Photo Judith Wakelam
Some years ago, we put 24 nest boxes in St Mary's Ely, with a concave in every other box. So 12 boxes with and 12 without a concave. When we examined the boxes a couple of years later, 10 were occupied, 7 with a concave and 3 without. So, it appeared that Swifts prefer a box with a concave. However, the probability of this being a chance result was 10.7%

This year we put 18 new nest boxes in Worlington church, again with a concave in every other box. So 9 with and 9 without a concave. We have checked the boxes and 6 boxes were occupied, 5 with a concave and just 1 without. The probability of this result by chance is lower at 6.6%.

Neither of these experiments passed the statisticians test for 'confidence' of 5%, but together, they do.

The probability of both of these results occurring by chance is 10.7% x 6.6% = 0.7% 4.18% (I used Fisher's Method to combine the results)

So now we can be sure that Swifts do prefer boxes with concaves.

2 chicks on a concave. Photo Rob Mungovan

Not only that, but 2 of the Worlington boxes contained chicks, (as did 2 first time occupants in Landbeach church) supporting the assertion that breeding in the first year of occupancy is more likely in a nest-box with a concave.

So, on our next visits to St Mary's and Worlington we will insert a concave in all unoccupied nestboxes.

We need more experiments like this e.g.:
Dark interior versus light interior
Oval entrance versus rectangular entrance
Rough exterior versus smooth exterior
Large box versus small box

[For the technically minded, for the statistics I used the Excel HYPGEOM.DIST() function.]

Postscript 2016: Since the above, George Candelin sent me the results after putting 12 concaves into 52 boxes in the Oxford Museum Tower. After the first season 6 boxes were occupied, 3 with concaves and 3 without. The probability of this result occurring by chance, assuming that Swifts have no preference is  12.73%. Combining this with the 2 results above (10.7% and 6.6%) using Fisher's Method gives a significance level of 2.92%

Thursday, 10 July 2014

Neat internal nest boxes in Nijmegen

We thought it worth reblogging 3 pictures from Jochem Kühnen's website, as they show just how unobtrusive, simple and effective built in nest-boxes can be.  Swifts are already occupying these nest-boxes.

Facade with virtually invisible entrances
Jochem says:
"I suggested several suitable nest bricks to the architects, and they went for this one. They placed 40 in the project in total, 10 in the facade where a Swift went in last week. I'm very happy with this development, this is a very busy square in the middle of the city centre, so a nice place to show people Swifts. Who knows, one day I'll go and stand there with a table with info to show passers by!"

The entrances were made by simply cutting a piece out of a brick and then positioning the Schwegler nest-box behind it

Entrance close-up
Schwegler internal nestbox