Scotland's changing bird populations

Blackcap - photo by Ben Darvill

Birds bring simple pleasure to our lives. We each have our favourite species and enjoy seeing or hearing them when we're out in the countryside. For many of us, the thought that they might be in decline is a concern. It's clearly important that we keep a close eye on how they are getting on.

In the UK, populations of our commoner species are monitored via the Breeding Bird Survey (BBS). This is organised by the BTO on behalf of a partnership which includes JNCC and RSPB. The survey has been running since 1994, following on from the Common Birds Census (CBC). By analysing records from these two schemes it is possible to chart bird population changes from the 1960's.

Cuckoo population change

Cuckoo population changes, 1994-96 to 2007-09

In Scotland, dedicated volunteers visit over 350 BBS squares. Their efforts monitoring the changing fortunes of Scotland's birds have helped to highlight both successes and concerns.

BBS volunteers visit a 1km square twice a year, recording the birds that they see and noting down how far away each bird was from their transect line. These records, combined with information on the 'detectability' of each species (PDF, 204.05 KB), allow estimates of the population size of each species to be made. These estimates can then be compared over time.

Taking cuckoos as an example (map to the right) we can see pronounced changes between 1994-96 and 2007-09. The blue dots indicate relative increases (more birds present) whereas red areas indicates a decline. The SE to NW shift across the UK is very clear. You will see similar trends for several other species if you explore them using the interactive maps, here. Willow and Garden Warbler, for example, seem to be doing relatively well in Scotland, against a backdrop of declines in England.

Why might this pattern be emerging? Well, you might well think 'climate change'. It is very likely that this is a contributing factor, with both temperature and rainfall patterns changing, in turn shifting the location of optimum conditions for breeding. Additionally, land use is changing, resulting in changes to bird habitats. For example, some areas are being forested and in other areas grazing pressures are changing.

So, can we unpick the relative importance of each of these factors in driving bird population changes?

BBS model uncertainty

The 'certainty' with which BBS models can predict population change. Red indicates low confidence.

At present, no we can't...

The population change maps are produced using computer models which combine BBS data with national habitat maps in order to estimate what is happening in the many squares that are not directly monitored.

Clearly, if an area or habitat is represented by very few BBS squares then the model will stuggle to produce accurate results. The map on the left shows how 'confident' the model is, with darker areas indicating higher uncertainty (read more). Readers who are familiar with Scotland's geography will see at a glance that the areas of lowest certainty are the uplands.

Comparing this 'uncertainty' map with the cuckoo map (above) highlights a concerning fact. Birds such as the cuckoo may be increasingly relying on areas that we are not monitoring adequately.

So, what should be done? Well, clearly we need to increase coverage of the uplands and other remote areas. A new project called "What's Up?" is aiming to do just this. So here is where you can help.

It would be fantastic if you could monitor an upland BBS square. Alternatively, could you mentor a less experienced bird-watcher while they get established on a BBS square? Your support and encouragement could make the difference.

It looks like Scotland is becoming increasingly important for some bird species. With your help, we can improve our understanding of these changes and ensure that Scotland is empowered with the evidence to make good conservation decisions.