Garden Warbler

Change in breeding distribution
(Click to enlarge)

If all we had to go on when assessing change in distribution was the presence/absence data that comes from species lists, then Garden Warbler would look like a success story, with colonisation of many areas of Scotland and new sites within Ireland. In England and Wales there seems to be no cause for concern, with the number of recent losses (black triangles) almost exactly balanced out by red triangles (gains), and that’s before taking account of ones where birds have reappeared (light pink areas).

The northward movement of the northern edge of the species’ range had already been apparent in the 1988-91 Atlas, as can be seen from the open-red triangles, sprinkled liberally across Scotland. This colonisation pattern within Scotland is seen across a range of species, whether migrants (House Martin is one example) or residents (e.g. Linnet), as you will see when you buy your copy of the new book.

The change in breeding distribution (right): red upward-pointing triangles are new gains, open triangles are gains between the first two breeding atlases and solid salmon areas have been occupied since at least 1968-72. Light salmon squares represent p/a/p – present in 1968-72, absent 1988-91 and present again this time around. There have been some losses, indicated by downward-pointing triangles.
Breeding season abundance of Garden Warbler
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Thanks to the people who volunteered to count all of the birds they saw and heard as part of Timed Tetrad Visits, we have data that can be used to model the relative abundance of a very wide range of species, including Garden Warblers. This is an interesting distribution, with hot-spots in a range of regions across Britain, although particularly in Wales, the far north-west of England and south-west Scotland. A quick look at the equivalent map in the New Atlas of Breeding Birds, published after the 1988-91 survey, gives a hint that there have been losses in many southern counties, particularly in Kent and Sussex, but there is a reassuringly similar general pattern across the country.

Using data from Timed Tetrad Visits, we can model the breeding-season abundance of Garden Warbler (left), with darker red indicating higher densities.
Garden Warbler - The change in abundance
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Instead of just ‘eye-balling’ the data on the 1988-91 and 2008-11 maps, we can use the TTV occupancy rates from both atlases to look for changes in abundance over the twenty-year period. The third map shows these changes in the relative abundance of Garden Warblers over this 20-year period, using information analysed at the 20-km square scale level. Gains are shown as shades of pink and losses by shades of brown, with the darkest, almost black, squares representing losses of up to 50%. As we saw by eye, there are particularly obvious black-spots in Kent and Sussex. There is an overall pattern to these changes with major losses in the south turning into gains in the north.

There are still lots of Garden Warblers in England but more sparsely distributed than there were, and losses are starting to become clear in South Wales too, despite the fact that there are still high densities, relative to most areas of the country.

Change in abundance (right) has been calculated using the timed counts from the breeding seasons of 1988-91 and 2008-11. Deeper pinks show gains and darker browns show losses. Gaps indicate areas where change was minimal, or the species does not occur in sufficient numbers.

BBS Change in abundance
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There isn’t another scheduled BTO-led Atlas until 2027–31 but we can use annual data from the Breeding Bird Survey (the joint project with RSPB and JNCC) to try to pick up change patterns for widely spread species that are found on a sufficiently high sample of the 3200 BBS squares. The map to the left shows the BBS equivalent of the change in abundance map from the Atlas. As you’ll see, the model produces a very similar story, over a somewhat shorter time-period, between 1994–96 and 2007–09.

While we wait for the next Atlas, with its wonderfully comprehensive data from every 10-km square in Britain & Ireland, we are becoming increasingly confident that we can monitor the changes taking place for about 50 species using trend analysis of BBS data. This is great news and really emphasises the contribution made by BBS volunteers, who count birds on their squares every year. The picture becomes better, of course, if the number of squares in less well-covered areas increases, so we will be working hard in Scotland, Wales and Northern Ireland to train more volunteers!

Mapping changes using BBS data from 1994-96 and 2007-09 (left). In this model, red dots are losses and blue dots are gains, with the size of the dot reflecting the abundance in each cell.

by Graham Appleton

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