Scientists studying the migratory patterns of birds have expressed "great concern" for one of their most popular cuckoos they fear is missing.
Chris, named after wildlife presenter Chris Packham, has provided satellite tag data for the Norfolk-based British Trust for Ornithology (BTO) since 2011.
It left the UK on 4 July but poor signals have led the BTO to conclude the bird is "missing in action".
Mr Packham said Chris's data was vital to help understand cuckoos' decline.
The UK cuckoo population has declined by 65% in the last 25 years, the trust said.
Chris is one of 16 being tracked as they make the 5,000-mile journey south to Africa for the winter.
However, the five-year-old had been the only one to have consistently provided UK scientists with migration data since being fitted with a solar-powered satellite-tracking tag.
"Chris has given pioneering insights into cuckoo wintering grounds, migration routes and speed of travel," a spokesman for the trust said.
Drought conditions in northern Italy may be to blame for the lack of data being sent back and a loss of signal from Chris's backpack, he said.
"The lack of rain is likely to have limited vegetation growth and reduced the availability of caterpillars, the preferred food of cuckoos.
"This is bad news for those British birds that use this area to fatten up, providing them with the energy to make a successful crossing of the Sahara Desert."
The last "good quality" signal from the bird was received on 3 August in northern Italy. Signals described as "sketchy" suggest he may have made it to Northern Chad by 8 August.