The Thetford-based British Trust for Ornithology detects a sign of summer coming out of Africa.

It might seem a long way off just yet but summer will arrive - in fact it can already be seen coming our way from more than 4,000 away.

Researchers at the Thetford-based British Trust for Ornithology (BTO) have detected the first of their satellite-tracked cuckoos - a Suffolk-tagged individual known as PJ - starting his long return journey north.

PJ was fitted with the satellite-tracking device in June 2016 and has since been providing the BTO with important knowledge about cuckoo migration. He has now begun his spring migration, travelling from Angola to Gabon on his way back to Britain.

Last year PJ returned to Suffolk on April 29 and remained in the county throughout his breeding season. On June 24 he began his migration to Africa for the winter, taking a more central route from the UK to his most southern point – Angola – avoiding route further to the west where researchers have detected more fatalities in other tracked cuckoos.

PJ is one of only two BTO cuckoos to have gone as far south as Angola - the information received from the tag identifying the country as a new wintering location.

In the last 20 years the number of cuckoos in the UK has decreased by half. In 2009, the cuckoo was added to the Red List of Birds of Conservation Concern. Much was known about the species’ breeding behaviour in the UK, but little was known about their migratory behaviour.

Since 2011, the BTO has been using satellite-tracking devices to understand their migration routes and gain information on the causes of their population declines.

BTO chief executive Dr Andy Clements said: “BTO’s migration tracking research is pioneering. Six years into the programme we continue to discover new facts. For example, some of our cuckoos winter in Angola, previously not well known as a wintering location for UK migrants.”

From Angola, PJ has travelled 760km north but there has been little movement from him since December 27. However, researchers say this may indicate he is preparing for his journey back to the UK. It is likely he will soon head west towards Ivory Coast, following his previous migration routes.

The BTO says satellite-tracking enabled researchers to compare journeys from year to year. It could also help highlight differences, such as changes in the weather, and therefore offer an understanding about why individual birds may change their routes.

PJ’s journey - and those of seven other satellite-tracked cuckoos - can be followed at www.bto.org/science/migration/tracking-studies/cuckoo-tracking