The Birds in Your Garden – September

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Cuckoo

As everyone knows, Cuckoo numbers in the UK have dropped by more than half over the last 20 years or so, albeit with marked regional variations.

When did you last hear one calling? If it was this year, you were lucky. I haven’t heard one here on the edge of York for two years, which seems typical of the groups to whom I (used to!) talk all over N Yorkshire.

There is little evidence that this is due to reduced numbers of young Cuckoos being produced, so what is the problem?

Back in 2011 the BTO took advantage of what was then leading-edge technology and fitted a dozen Cuckoos with GPS location tags. These allowed them to be followed throughout their migration, to within a few hundred yards. Since then around 80 birds have been tagged giving a good idea of where UK Cuckoos migrate to, and how they get there and back. Equally important, is the knowledge of where they die, whether en route, here, or on their wintering grounds.

One of the first discoveries was that there are two main migration routes, one via Italy and the other via Spain, which was completely unexpected. Not only that, but Cuckoos migrating via Spain were more likely to die en route than those migrating via Italy. Further, there was good correlation to show that birds using the Spanish route came from the UK region where they are most in decline, England.

The tags have identified several stopover areas where birds can feed-up to gather energy for the next stage. These are absolutely vital, as without enough food to generate the necessary energy, migration cannot happen. One such is near the River Po in Italy, an important fattening site for British Cuckoos, allowing them to cross the Mediterranean, and then the Sahara.

UK Cuckoos spend the winter in Central Africa, mainly in and around the Congo rainforest, and in similar habitats as far south as Angola. These habitats have not yet been extensively cleared and, as the Cuckoos prefer forest edges rather than unbroken forest, a degree of opening up may benefit them. Without knowing which habitats they occupied in the past, however, it is difficult to be certain.

In spring, our Cuckoos use a different return route, crossing the Sahara from previously unknown stop-over sites in West Africa, where favourable weather patterns provide suitably rich food for them to fatten up. It was thought that they crossed the Sahara in one mammoth flight from their wintering locations all the way to North Africa, or southern Europe, so this stop-over, and the dogleg migration into West Africa that its use entails, were completely unexpected.

Understanding more about the location of stop-over sites is another crucial part of the conservation of this species. Cuckoos and Swifts – which make a similar spring migration –– are both undergoing severe population declines and are also failing to advance their spring arrival dates in the UK. Possibly because the weather pattern in their West African stopover is not advancing as is that in Europe.

The project continues, and the arrival of cheaper tags (the current ones cost well over £1000) will help. Short-term studies can give misleading results – for example the habitats Cuckoos used in the Congo basin during the first two years are not the same habitats used now.

Tagging more birds, over a longer time, will give the BTO enough data to examine how survival varies as environmental conditions change. Similarly, the sample of birds tracked on spring migration will soon be large enough to work out how they time their arrival back in the UK.

If you would like to know more, and follow the birds on their migration, try this BTO site.

If you find the lives of our garden birds to be of interest, and would like to join in and count the feathered occupants of your garden, please contact me or visit the BTO Garden BirdWatch website (www.bto.org/gbw)
Mike Gray gbwmike@gmail.com.

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