JULY BIRD DIARY - The Common Cuckoo

caterpillerLast month I wrote about the joys of seeing summer migrants from Africa, the most iconic being the Cuckoo with its distinctive call. Over the last 30 years or so, however, their numbers in Britain have dropped by 65% but the reasons for this are not known.  A lot is known about their breeding behaviour whereby they parasitise other birds, notably the Dunnock, Meadow Pipit, Pied Wagtail and Reed Warbler. So could changes in the numbers and breeding patterns of these birds be an influence ?

Research using breeding and nest survey data suggests not very much. So other explanations focus on a reduced supply of the Cuckoos’ food (mainly caterpillars) during the breeding season and worsening conditions on their migration routes and in their wintering grounds in Sub-Saharan Africa. 

Click on the video to hear the cuckoo's call....

Whilst it was known that Cuckoos are doing better in some areas of the country, with a greater decline in England than in Scotland and Wales, little was known about where they actually wintered and how they got to and from the wintering grounds. In order to answer these fundamental questions, which could offer pointers to possible measures to help the birds, the British Trust for Ornithology (BTO) began a project in 2011 using satellite tags which had become small enough to attach to birds. It began by tagging a group of male Cuckoos in East Anglia and has been subsequently extended  to other areas. 

The results have shown that British Cuckoos winter in and around the Congo rainforest. They arrive in Britain towards the end of April and the beginning of May and over 50% depart before the end of June. They can spend less than 20% of their year in Britain. On their return to Africa some head across the North Sea to the Netherlands and Belgium whilst others head south over the Channel to France. Birds then either migrate southwest via Spain and Morocco or southeast via Italy and or the Balkans before converging on the Congo Basin. The northward, Spring migration has a single route over the Sahara to Europe. Stop-over sites are important and the eastern route birds have an important site for re-fuelling in the Po Delta in Italy whilst the western route birds have a site near Madrid. And previously unknown is the fact that northbound birds have stop-overs in West Africa during their two-month flight to Britain. 

The emerging conclusions from the on-going study suggest that conditions on these migration routes are a key influence on the decline in numbers. Droughts, fires and major land-use changes in Spain are one example. The decline in the Cuckoos’ main food source on their breeding grounds is another. And one mustn’t forget the hazards of crossing the Sahara Desert. 

Cuckoos can be sponsored by companies and individuals so here is your chance to help the icon of Summer in its fight for survival. Click on BTO.org and then “view our cuckoo tracking project”/ Get Involved/ Sponsor a Cuckoo.